Can Wingtip Motors Improve Aircraft Efficiency? RCTestFlight Solar Collaboration Part 1

[@smartereveryday]

I wonder if the vortex interacting with the props make each individual prop get more lift/propulsion. Worded poorly but I think you know what I'm saying. You're forcing more air on each blade.

[@thinkflight]

I think perhaps making my own wind tunnel with multiple smoke streams starting in front of the wing and prop would be informative.

[@montithered4741]

Wingtip props counteract wingtip vortices by interrupting the flow of high pressure flow from the bottom of the wing into the low pressure air above the wing,

[@polomarknot17]

I was thinking along these lines, but considering that a vortex would have a different angle of attack for the props than freestream air which would change prop performance.

[@gustavgnoettgen]

@@polomarknot17 My thoughts too. The air seems to be 'screwing into' the prop already.

[@Sethgolas]

If this is the case, I wonder if you'd this would have the result of a less aggressive prop pitch angle being optimal.

[@smartereveryday]

I really enjoyed this thanks for making it.

[@thinkflight]

Thank you for all the wonderful content you keep putting out.

[@hectord27]

Hi Dustin

[@bolgron6371]

oh yooo hello, i find your videos very very interesting. =)

[@apersunthathasaridiculousl1890]

yus

[@SGTRhonin]

A wild Destin appeared

[@JosephHarner]

It would be interesting to see a comparison of wingtip motors in a push vs pull configuration.

[@jhughes2286]

Yes it would!

[@center4nerds]

more experiments more data!

[@13orangeleaf]

I want to use motors to launch from flat land,but without the trike, and not in the prone position, but just using a seat, so I wonder how realistic it is to think of having motors mounted to the wingtips of a hang glider without compromising safety and affecting the flight of the wing in a negative way. I would like to have more options as to where I can launch from other from a mountain, or paying a pilot to pull you and the wing to altitude from flat ground. The idea is to get you and the wing off the ground and to a soaring altitude then access the thermals. The motors can be turned off until they are needed. The wing that is used in the video Is a simple design and similar to a hang glider wing. I used to go to a place where they trained you to fly a hang glider, there you have two options, climb the mountain, or pay a pilot to pull you once you are fully trained. In the video his motor placement seems to be pushing which worked very well. But I was thinking another mounting option for the motor would be in front , then the motor would be pulling I think, so I was wondering which would be the more efficient way to use the motors, push, or pull. Lol, I am not a pilot, and have never flown on my own, but I picture it, and think about it all the time.

[@nathanlowery1141]

@@13orangeleaf pulling prop motors generally have more air to bite into, therefore a little more thrust. This comes at a cost of drag and reduced aerodynamics. Push style generally have less drag and doesn't affect aerodynamics as bad, but less air to bite into due to the body obstructing it. Of course this is all general statements due to the varying body shapes, arguably more important.

[@13orangeleaf]

@@nathanlowery1141 ok, then what would be a better choice to get a wing off the ground and into the air , in other words to launch a pilot and wing into the air and cut the engines. Sorta like a plane pulling a glider into the air to its designated release point. I am going to guess that a motor that pulls would be this choice for this type of task?

[@mithrandir1313]

Might have been interesting to reverse the rotation direction of the motors and see the implications.

[@thinkflight]

That would, unfortunately the spar was snapped and I decided to take the plane apart but maybe in the future?

[@Anon_jonn]

Yes that was my thought when he said the vortices were going opposite the normal direction. I would love to see more experimentation

[@VacuousCat]

twin prop planes usually has the opposite directions, but tiltrotors don't. I always wonder why.

[@mithrandir1313]

@@VacuousCat Tilt Rotors usually (always) have counter-rotating rotors... torque cancelling.

[@chrisdrake4692]

@@VacuousCat All tiltrotors must go opposite (e.g. @13:05) - they have to - if they both went the same way, the craft would spin in circles as soon as it left the ground. kzbin.info/www/bejne/oIOvhKaundunjK8

[@VinceSamios]

I think in practical GA use, the risks of differential thrust in the case of an engine out, vastly outweigh any efficiency gains.

[@sebastienl2140]

maybe it's possible with central propultion and lowering trust on thrusting wing

[@VinceSamios]

@@sebastienl2140 you're talking about essentially running on 1/3rd power to solve the differential thrust problem. Most aircraft would seriously struggle with 1/3rd power.

[@VinceSamios]

@Bobb Grimley If you had even minimal flight experience you would know you're not often within gliding distance of a runway. And if you had even minimal electronics experience you would know there are many points of failure, especially speed controllers, and engines also fail. Losing a propeller isn't unheard of either.

[@Skinflaps_Meatslapper]

@@VinceSamios If you were a decent pilot you'd know how to handle asymmetric thrust and how to handle an engine out below Vmc. You'd also know that runways aren't necessary to land a plane safely, and small aircraft may also have the luxury of BRS installations that completely negate controlled emergency landings in unsuitable terrain. And if you had minimal knowledge of electronics, you'd know that it would be simple to program an RPM delta between the two motors, so that if they got beyond a certain difference in RPM (as in the case of a complete failure) the ESC's would both shut down before the asymmetric thrust had the opportunity to adversely affect stability, way before your reflexes had time to counter with rudder. You could even program it as a function of airspeed, so that it wouldn't cut all power to the good motor unless you were close to Vmc, in which case you might be able to continue flying under power until a better landing solution was found. Unfortunately, you are lacking in all of those aspects, and rather than seeing solutions, you only see problems. Such a shame.

[@horse1-1]

@@Skinflaps_Meatslapper You’re essentially talking about a 2 engine aircraft without single engine operation? You’re saying if an engine fails then cut/reduce the good engine as well, yes? And you justify this with the existence of BRS installations and landing planes on non-runway surfaces? If it’s me buying a twin engine plane, I’ll pay for the extra gas to get OEI capability every single time. The moment arm for a wingtip motor is so large that countering with rudder is probably out of the question.

[@suprvideo]

What amazes me is how much work you put into this project. And that your wife comes along to support you. Great photography and narrative.

[@picknikbasket]

Fascinating! I'm going to show my STEM group this because it's such a good lesson, thanks for uploading it!

[@thinkflight]

Fantastic! Very glad it provided value.

[@toolbaggers]

Pure scientists and mathematicians think theoretically. Practical engineers and technologists/technicians know that that things fail and need maintenance. One wingtip motor failure = flat spin/cartwheel of death For safety, you would need more than just one motor on each wingtip. Possibly a dual motor, coaxial design per tip. The V-22 Osprey has a relatively bad safety record. Everybody dies if one motor fails!

[@TremereTT]

@@toolbaggers But is it's safety record bad because of the wingtip motors or because of Boing?

[@flappingflight8537]

@@toolbaggers there the props are connected by interconnection shaft en.wikipedia.org/wiki/Bell_Boeing_V-22_Osprey#/media/File:V-22_concept.jpg

[@piotrszalczynski83]

@@toolbaggers V22 motors are connected by common gearbox so if one motor fail no one dies cause of it.

[@chinookmech]

I'm pretty excited about this channel, I hope you continue to grow.

[@thinkflight]

That's the plan!

[@cornishcactus]

A one engine out emergency would be interesting, that's why they don't do it unless you're the bell and can run both props from one engine if needed

[@vattuvarg]

Having two electric motors per prop could help.

[@TommyApel]

I was thinking the exact same thing when he showed the left/right torque of the plane in the workshop

[@RCP-1136]

Thats how its done on the valor

[@Mike-oz4cv]

Yeah, you’d need a huge vertical stabilizer to counter the yaw torque.

[@linusa2996]

@@vattuvarg so the wing becomes heavier because of the additional motor and the extra structural strength required to mount the motors and keep the wing intact.

[@ancientbriton12345]

Check out the development of the Vought V-173. The propellers were arranged to rotate in the opposite direction to the tip vortices, allowing the aircraft to fly with a much smaller wing area, or in your situation should provide greater efficiency

[@spindash64]

To be cleae, the Flying Pancake had a pretty ridiculous amount of wing area: the goal was to get a plane with incredibly short takeoff and landing runs compared to conventional aircraft, without sacrificing high speed capabilities. It's still _correct_ to say allowed it to fly with a smaller wing area (for a given airspeed), it just doesn't 100% address the design intent The Flapjack and Pancake are kinda my current obsession (literally trying to build my own version in RC atm)...

[@leifhietala8074]

The noted improvement of efficiency is with the motors in pusher configuration; what improvement would you get with them in tractor? Put the props just ahead of the leading edge and rotating against the usual direction of wingtip vortices.

[@MrFadjule]

That's the interaction I was curious about. I would like to SEE the effects of changing the prop direction. What are the pro's and con's of spinning the same direction as the naturally induced vortices, or the opposite?

[@JHarris]

Loved the video. Also I’m sure one of the reasons companies have stayed away from the wingtip engine configuration is with engine failures. It may provide great numbers in efficiency, but the cost of one engine failing means the other engine is basically useless. There would probably be too much yaw the allow the plane to fly straight.

[@witext]

that too but mostly because engines are too heavy and big, electric engines are much smaller and allow for small ones to be placed at the wingtips

[@Skyfighter64]

Being an Aircraft mechanic, I'm going to have to doubt a number of statements made at the beginning of the video. Most specifically, I figure that While electric motors are improving all the time, batteries are still nowhere near being able to replace Gasoline or Kerosene (Jet A) as storage mediums for aviation. The simple fact is, for a given energy capacity, batteries still take much more volume and weight to store. The really nice thing about liquid fuels, especially in aviation is the fact that they get used up. By the midpoint of a flight, on even say a Cessna 172M, the airplane can be as much as 130lbs (approximately) lighter (22 gallons out of a maximum of 46). Batteries leave you dealing with a not insignificant chunk of dead weight once the battery is depleted. This is before you even start considering the impact of potential surprise condition changes during a given flight. When running on battery, your maximum range is directly dependent on the other uses of electricity, and if say you wind up finding a IFR condition by nasty surprise (it happens more than you think), suddenly your range figures could go down below the minimum safes. Liquid fueled engines do not deal with this the same way, since generating electricity through the alternator is generally treated as a byproduct, and range is almost unaffected by the condition of the aircraft's electrical system. Secondly, I'm curious as to why the sole choice of pusher configurations for this test. IIRC, one of the reasons that the pusher configuration is relatively rare in aviation is that clean airflow over airfoils generates more aerodynamic forces (i.e. lift OR thrust, depending on the aircraft), so putting pusher props into already turbulent airflow results in a more efficient wing , but worse output from the powerplant. I'm kind of curious how your results pan out if you include A: Pusher vs tractor configurations, and B: Ducted fan operations in similar and alternate setups, as again, IIRC, ducted fans are less affected by turbulent air, and act as small scale surrogates for turbine engines. But I'm just ranting at this point. Like I said, I'm a mechanic, so I prefer sticking to tried and true, and letting new technologies prove themselves instead of jumping on the early adoption train. Seriously, props on asking a question and trying it out.

[@andyb.1026]

40 years in Aircraft Industry, 16 as a Rigger ~ I concur with your views, Theory is Theory, but far from the harsh realities of practicality. Even the Wright flyer of1903 carried a payload

[@Skyfighter64]

@@andyb.1026 Aircraft rigging. That ranges from one of the more fun jobs on the aircraft I deal with (Ailerons, on earlier 152 and 172's) to one of my most hated (Friggin Elevator trim... 152, 172, and anything Piper). And that's without even considering trying to run new control cables... I can scarcely imagine trying to do that job on more complex/larger scale aircraft... Especially something like the central "spool" (it looks like a spool, kind of) that has the entirety of Douglas DC-8 family aircraft wing control cables go around it at one central point. I salute you for your longtime service in the industry.

[@emilelepissier7550]

Battery technology has improved by many folds since the days you were in school. And then battery is not the only power source for electric motor. Fuel cell, especially hydrogen fuel cell, is already used in active research. See wiki en.m.wikipedia.org/wiki/Fuel_cell

[@Skyfighter64]

​@@emilelepissier7550 I'm sorry, but I'm NOT flying with a hydrogen fuel cell. Too freakin dangerous, without even addressing some of the other issues I was concerned about.

[@emilelepissier7550]

@Skyfighter64, your risk assessment of hydrogen system may not be inline with state of the art now and in the future when hydrogen technology is fully matured, but you are entitled to your choice. Flying is inherently more risky than many other transport modes anyway, so to be consistent, may be you should reconsider about flying.

[@ezrarichardson279]

I found it interesting that it increased the efficiency at all. Thanks for the video! Can’t wait for the conclusion of the project!

[@zefkosta]

No need to wait, google Charles H. Zimmerman.

[@center4nerds]

i think for RC especially having the extra torque from the motors at the wing tips would be super beneficial as well as proved by the footage at the end.

[@AV-14]

What a cool concept. I like the way you construct your aircraft too; Looks really professional.

[@flyingrock6381]

Top notch stuff! love the scientific content, innovation and humour. One of the very (very) few channels on my 'must watch' list. Cheers!

[@thinkflight]

Wow, thank you!

[@blacklion79]

Shots of test mission (from quad?) are so beautiful!

[@thinkflight]

Yes they are!

[@sUASNews]

The two of you working together is off the charts. It's going to be real.

[@gjermundwestad3889]

Seems to me the pusher props at the tip is battling an already established vortex as they are placed behind the wingtip. Could there be additional gains from arranging the tip motors in a tractor config.? This way the prop would counteract the "leaking" air around the wingtip before the vortex is established. Or....?

[@thinkflight]

There was precedent for both in the literature, the answer is unknown to me. Wings and propellers do affect the air in front of them, but how much? I don't have an answer for you other than I'd love to try both.

[@ZagadkaTeg]

Yeah, as per Xf5U. As I understand it there are two main advantages to wingtip motors generally. Most obviously (provided they rotate in the correct directions!) they oppose the induced vortex, ie high pressure air beneath the wing spilling out and around the tip to lower pressure above and so reduce drag and lift losses. Also they 'see' cleaner air than pushers inboard. Tractor tip props should see free stream air only for maximum efficiency (and also co-related lower noise generation). Cool video and interesting to see your results!

[@manifold1476]

I'm reminded of something Richard Feynman said about experimental research. (~13:min into Messenger Lecture #1) He said: "The Key of modern science is this Idea; That to look at (a) thing, (time after time), and to record the details, and to hope that in the information thus obtained, (there) may lie a Clue - to one or another of a possible theoretical interpretation. It's only through such hard work that we can find out anything."

[@schipe]

@@thinkflight You are on the wrong track. Props and wings need clean, undisturbed air. You need less separation. If you want to recover energy from wingtip spillage or vortices, put something in their way. I am not an aero engineer, but i think you will get more benefit from vertical fins, in increasing size towards the wing, stopping outward motion of air. Or even better, introduce less dihedral, or anhedral angles...

[@gjermundwestad3889]

@@schipe I think he is on the right track. Because he is putting the propulsion system out at the wingtip where they perform yaw stability so that vertical stabs can be eliminated. That is a gain in efficiency is it not? Also at the wingtip you find the pesky vortices that can be diminished or eliminated with the propulsion system acting as a sort of aerodynamic winglet reducing drag and increasing the effective wingspan. That is in effect to " put something in their way". Unless you have researched this out and share the results showing how this approach is futile I think we should encourage his efforts the best we can.

[@edinfific2576]

Other KZbinrs could learn A LOT from you about good quality video and audio, good editing, and a fitting and tasteful choice of background tune/music. After so many exceedingly bad videos, yours stands out with its pro quality.

[@ammerudgrenda]

Great video, thank you! One safety concern I have is what happens if one engine fails? I’m a multi engine pilot and I know how severe an engine-out is on a twin engine aircraft. With the engine at the wingtips, it would be even more dangerous. Makes me wonder how they would be able to certify a people-carrying airplane.

[@miscbits6399]

I agree. The eviation triple setup looks ok from a control POV though - if you have an engine failure you can compensate by instantly shutting down the other side and moving to degraded control mode using the central pusher prop Electric aviation is problematic in many ways though - badly range limited and I can't see how that can be realistically addressed. My suspicion is that the longer term solution will be synthetic fuels produced from MSR nuclear sources to achieve carbon neutrality as being the only practical way to maintain long-haul flights, unless vac-trains (hyperloop) start eating aviation's lunch (and that's only really feasible on 1000mile+ legs)

[@ActionBOX]

I really enjoyed your production. Thank you for sharing 😃

[@loteknomad5032]

Great video. :) Really enjoyed it. I tend to think the safety advantage - specifically for aircraft carrying people - of having two or more engines as close to the fuselage as possible outweighs any potential efficiency value, even if one existed. A plane with two engines near center tends to have the ability to survive the loss of one and continue under powered flight to land safely when possible. Move those engines out to the wingtips and you introduce a tremendous amount of yaw to overcome while running on one engine. Wouldn't be surprised if you'd have to cut that second engine right away and more or less glide in, completely losing out on the redundancy benefit of the otherwise functional engine in the process.

[@nuchbutter]

Great point. On first inspection, the image of that massive moment arm out on the end of the wing made me gasp. However, I would tend to believe that the use of electric motors mitigates most of the risk of asymmetrical thrust. Interesting dilemma...

[@jameswalker269]

Very cool experiment, and great job on the fabrication work 👍. It's always a bummer watching "your baby" plummet to a painful encounter with the ground, but I love that you remained focussed on the silver linings of what you learned from it.

[@devinpauley6183]

As a multiengine pilot I could see significant issues with P factor, accelerated slip stream, and torque issues if one engine ever failed. While a very curious video and very interesting idea, I’m not sure that a twin engine aircraft would ever be safe enough in that configuration to carry passengers.

[@coolbean777]

What if it had an engine in the front as well? Or directly behind it going backwards? Like 3 engines in total.

[@osakanone]

"Serenity has been through a lot lately" - proceeds to plummet into the dirt. That is more relatable than it deserves to be.

[@alexgor7346]

Great idea, but I think propellers should be installed before the wing, but not after the wing! So that the swirls created by the propellers compensate for the swirls created by the wingtips. Because it makes almost no sense to unroll the vortex after the wing. Unless, perhaps, the propellers after the wing will work a little more efficiently. The direction of rotation of the propeller of the left wing should be counterclockwise (if you look in the direction of flight), and the right wing should be clockwise. Pay attention to the plane Vought XF5U.

[@peceed]

Yes and no. Actually puszer propeller loses efficiency, but only small fraction of arc is disturbed by wing, so efficiency loss due to turbulent flow is very small. What seems to be important is final effect - no vortex = no energy loss for vortex formation, regardless where the propeller is located. In pusher variant there should be higher thrust (energy of vortex is converted), in dragger wing should have higher lift.

[@JeremyCoppin]

Videos like this are what makes KZbin invaluable. Thanks.

[@sakarrc5001]

Wow, outstanding engineering and testing. The model rocks.

[@leGringoo]

Great work you did there. I’m looking forward to the solar plane!

[@river1711]

What design software do you use? And would you consider doing a tutorial on using it?

[@johnbergamini3567]

Yes! Please!

[@cosmicHalArizona]

That CW- CCW prop rotation is something that never occured to me. GREAT idea. Experimentation: the proofs in the pudding. Look at that bad boy go!

[@nathank7989]

"Reducing the size of the vortex at the wingtip indicates that you have reduced the induced drag of the aircraft." *Uh, no.* An easy way to understand what's going on is to think about the wing in terms of Prandtl's lifting line theory. The trailing vortices shed by a wing are caused by changes in the strength of the bound vortex (and local lift) across the span of the wing. Conservation of vorticity states that vorticity can neither be created nor destroyed, so any change in the strength of the bound vortex has to go somewhere: the trailing vortices. A larger change in bound vorticity (or local lift) between two spanwise locations results in a larger amount of vorticity being shed into the wake between those locations. If you decrease the size of the tip vortex, it just means that vorticity is being shed somewhere else on the wing. Shifting where the vorticity is shed changes the lift distribution along the wing. Here's where it starts to get interesting. If Serenity were a conventional (main wing and tail) aircraft, then the induced drag of Serenity would be minimized when the main wing's lift distribution was elliptical. However, Serenity is a (statically stable) flying wing and requires positive lift near the centerline and negative lift near the wingtips to keep it trimmed in steady level flight. The region of negative lift just inboard of the wingtip means that at the wingtip the lift coefficient is increasing with spanwise position, and so the vortex shed at the very tip of a flying wing will rotate outwards. Slightly further inboard - where the lift coefficient is decreasing with spanwise position - the trailing vortices rotate inwards. The interactions between these vortices cause them to roll up, which has the effect of reversing the direction of the vorticity downstream of the wingtips, exactly as you observed! There have been some papers published about optimal lift distributions for flying wings, but 1) a disturbing number of those papers neglect to make sure their design is trimmable or aerodynamically stable, and 2) the optimal lift distribution for a flying wing depends on the planform - and in particular the sweep! In general it's shaped something like a bell with a large region of positive lift near the wing root and a small region of negative lift near the wingtip to keep the aircraft stable. So, back to your experiment with tip mounted motors. The tip motors impart additional vorticity on the air and alter the shape of the wing's lift distribution. It's possible that your motor and propeller placement did a better job pushing Serenity's lift distribution towards its optimal than the winglets did, which would account for the ~1% decrease in energy consumption. It is really cool that you were able to see a difference! As for companies claiming 10% to 15% increases in efficiency: don't believe everything you read. A lot of aerodynamic design is gaining 1% or 0.1% increases in efficiency. If it really were 10% more efficient to mount propellers on the wingtips then you would see a lot more aircraft with propellers on the wingtips. As I mentioned, it is possible to use the propulsion system to alter the lift distribution and push it closer to optimal but it's a lot easier to simply alter the wing design. P.S. About a third of the way through the video you discussed your placement of the motor wires. I believe you mixed your terminology. Transition is where the boundary layer changes from laminar flow to turbulent flow. Separation is where the boundary layer detaches from the surface of the wing. Separation almost always occurs after the boundary layer has transitioned to turbulent. You did a good job placing the motor wires because it is possible that the imperfection in the wing surface where the wires are buried could trip the boundary layer and cause it to become turbulent.

[@thinkflight]

I love comments like these. Thank you for taking the time, it helps more than just me I am sure. Generically that statement is true no? I can understand in the case of a Prandtl wing you are simply moving the vortices around to different locations but if you have a typical sailplane and extend its wingspan you didn't simply move the vortex somewhere else, you reduced it.

[@toolbaggers]

The V-22 Osprey has a relatively bad safety record. Everybody dies if one motor fails! A wingtip motor failure = uncontrollable yaw.

[@ca_pilot]

@@toolbaggers Not true, on the V-22 both motors are connected to a central gearbox which provides redundancy in an engine out.

[@leifhietala8074]

@@toolbaggers The rotors are interconnected by a mighty driveshaft and the ship can survive an engine-out scenario, presuming nothing else goes wrong. And one pilot reported that in the event of forward flight and losing both engines, the V-22 can be glided in for a safe landing, though I imagine the craft would be thoroughly grounded by the landing due to unavoidable rotor damage.

[@nathank7989]

​@@thinkflight Sometimes it's true but not always. If the wing planform does not change, then a change in the strength of the tip vortex (before roll-up) merely indicates that the lift distribution near the wingtip has changed. This is what is happening with Serenity. On the other hand if the wing geometry changes, then it is possible and even likely that your statement is correct (dependent on the two wings generating equal total lift and having identical or at least similar lift distributions, as I'll discuss in a second). Back to the sailplane. Let's say we have two aircraft that weigh the same and have identical spanwise lift distributions but have two different aspect ratios. The airplanes have equal weight, therefore the lift generated by their wings must also be equal. The lift generated by the wing equals the integral from y = -b/2 to y = b/2 of L'(y), where L'(y) is the lift per unit span. From Kutta-Joukowski, L'(y) = ρ * V_inf * Γ(y), where ρ is the air density, V_inf is the freestream velocity, and Γ(y) is the bound circulation. Therefore we can write L = ρ * V_inf * integral from y = -b/2 to y = b/2 of Γ(y). Now since the lift of the two aircraft is equal, we know that the integrals of the bound circulation for each wing must also be equal. However, the aircraft have different wingspans, and therefore Γ(y) must be different to maintain equal lift. If we take the case where Γ(y) = Γ is constant and the lift distribution across the wing is uniform like in the horseshoe vortex model (which would require infinite tip chord, but anyway...), then Γ1 * b1 = Γ2 * b2 where Γ1 and Γ2 are the bound vorticity of the two wings and b1 and b2 are the corresponding wingspans. Double the wingspan while keeping the same wing area and total lift and the bound vorticity is halved. Continuing with the horseshoe vortex model, all of that bound vorticity is shed into the wingtip vortices, so their strength is also halved! I should note that doubling wingspan while maintaining the same wing area actually quadruples the aspect ratio. I'm sure you've seen the result from finite wing theory that CD_induced = CL^2 / (π*AR*e) where e is the span efficiency factor. (Not Oswald's efficiency! Oswald's efficiency also includes the quadratic part of the airfoil's profile drag. People make that mistake a lot, even in research publications.) So why is induced drag dropping by a factor of 4 when we're only halving the strength of the wingtip vortices? The answer is that halving the strength of the wingtip vortices is only half of the answer (haha). Consider a comparison between the same relative spanwise position on each wing. The velocity induced by a vortex outside of the vortex core is equal to Γ/(2π*r), where r is the distance from the center of the vortex. By doubling the wingspan while maintaining the same wing area, not only have we halved Γ, we've also doubled r! This means that the induced angle of attack and the induced drag is one quarter of what it was before quadrupling the aspect ratio, which is consistent with the finite wing theory result. Wait, but we're integrating over a wingspan that is twice as long! Wouldn't that mean we're right back where we started with an induced drag that is only halved? Nope! The wing chord was halved, too. The local induced drag coefficient equals the induced angle of attack times the local lift coefficient (using small angle approximation). The local lift coefficient is unchanged because we said that the lift distributions of the two wings were equal, and the two wings have the same wing area. What has changed? The wing chord! So while the local induced drag coefficient is one quarter of what it was before because of the change in induced angle of attack, the local induced drag is *one eighth* of its previous value. Integrate over the new wingspan that is twice as long and you get that the induced drag is one quarter of what it was before. So the strength of the wingtip vortices is a relative indicator of how much induced drag there is if the two wings are generating the same amount of lift, have the same wing area, and have the same or similar lift distributions. What if the wing geometry is unchanged? Then we're right back to redistributing the lift and the bound vorticity. Remember that L = ρ * V_inf * integral from y = -b/2 to y = b/2 of Γ(y). The total lift remains constant, therefore the integral from y = -b/2 to y = b/2 of Γ(y) must also remain constant. If you decrease the bound (and shed) vorticity at the tips, that vorticity has to go somewhere else. Depending on where it goes and how your lift distribution changes it may either increase or decrease the induced drag.

[@TheBigCheeese85]

@ 1:26 I'm curious why you feel that lift is "quite the subject of debate" Low pressure airflow on top, High Pressure airflow underneath. There's no debating how lift is created on a wing.

[@mikecloud9828]

In the event of one engine going out, the adverse yaw would probably make the aircraft uncontrollable.

[@Videoctr]

I was thinking the same thing. I wonder if that is why we saw a third engine located in the tail of the Eviation Aircraft? To possibly mitigate adverse yaw effect and maintain better authority.

[@zed9zed]

@@Videoctr Unless both wingtip engines were shut down (leaving the center engine), it would still probably be uncontrollable or would require huge, stronger control surfaces (and would be much less efficient).

[@Boomchacle]

@@zed9zed If you had a system which could shut off both engines immediately in the case of one going out, it would be just as risky as having a single engine in the first place.

[@zed9zed]

@@Boomchacle I'm not advocating for a system to shut off the remaining engine. There is no solution to the problem faced when one engine fails on a plane like this, so the obvious answer is - don't build a plane like this, at least not for living occupants.

[@Boomchacle]

@@zed9zed Would the solution not just be to cut off the other engine immediately to prevent asymmetrical thrust? Again, how would that be any different than a plane with one engine having that engine fail.

[@firstnamelastname2298]

This is really good example of thinking about air flow in 3d. Most people think only about 2d cross section of the wing.

[@JathTech]

I think wing tip propellers in your use case make a ton of sense, even if it's just a 2% efficiency improvement with this tested design. 2% can add up when you also take into account all the other efficiency improvements you might make. Don't scoff at 2%. But as you learned, making the propellers bigger in relation, i bet you could in fact increase that number considerably.

[@lowerastral1963]

Cool aircraft design. 13:55 I noticed the deadly smog layer and I am so thankful that I moved my wife and daughter out of Southern California this October (2021) to Tennessee where the air is much, MUCH cleaner.

[@peterboy209]

I think this configuration makes sense on low aspect ratio wings (Vought XF5U) where i have a much higher induced drag component compared to a high aspect ratio wing. And low aspect ratio wings are better suited to resist heavy loads on the tips (like on an aircraftcarrier)

[@nustar1]

The flying flapjack indeed banked on the efficiency of the wing tips engines. My understanding it was based on NACA research from the late 30's early 40's.

[@marutiroma6295]

Thank you for sharing it. It is a great and very beautiful project. It thrilled me. A fundraiser is the right way. Go forward with courage and faith. I will pray for you. Good luck.

[@joshroberts9487]

I don't think the main reason for having the thrust closer to the centerline on a multi motor configuration has to do with efficiency. Just imagine an engine out scenario.

[@leifhietala8074]

My first thought. With the huge off-center leverage, you'd get an instant spin, esp. in a tailless design like Serenity.

[@Musikur]

It is also to do with efficiency though, for a traditional engine, the extra strengthening required to place them at the wing-tip completely outways the benefits... But also engine out operations

[@abdobakry3344]

my heart broke with the RC drone for the second time even I watched this vid twice. the drone is a nice story, but maaaan you're bringing me to the video mood every time I watch it. Thank you,

[@cloudpandarism2627]

coop with RCtestflight? that is an insta-sub! i love to see more projects like this. such coinsidence that i just finish and maiden fly a wing with the same T-motor you are using. what size props are on there and how many S lipo you use?

[@thinkflight]

6x4 apc 5S

[@cloudpandarism2627]

@@thinkflight cheers mate! i fly that motor on a 7x5 and phase wires gets so hot on 6s that i fear it will melt the styrofoam stuff inside

[@thinkflight]

@@cloudpandarism2627 Sounds quite a bit over propped!

[@cloudpandarism2627]

@@thinkflight if we talking about the F90 then specs say 6" prop or 7040/7050... sooo i am not sure. maiden flight was as i said ok but i also didnt punch it. just fly very gentle. i need to see what happens if i do some full throttle runs. but so far i cant. first try resulted almost in lost of the AR wing because it started to tild left and down without under 100% power. i couldnt do anything to catch it. pulled throttle to zero and that gave me control right before impact. strange things these wing planes...

[@thinkflight]

@@cloudpandarism2627 No I'm using a t-motor 2203.5

[@jeffmcvay]

I watch your video after seeing the Prandtl-D flights. It make me wonder what placing the motors at the cross over between the downdraft to updraft would do. Now I have to checkout your channel and get into this hobby myself. Thanks for that. :)

[@anjarwicaksono229]

there's something more important than efficiency: safety factor. Having thrust further from center of mass, would cause instability and difficulty to control WHEN one of them becomes malfunction

[@indyjons321]

Those areal shots.... it looks soooooooooooo stable! Its unreal!

[@nicholasthon973]

Wouldn’t you want the design for stall to be last at the wing tip? Airflow is forced with the propellers out there, thus you would have ability to control that surface at very low stall speed.

[@patbanta]

Great video. What caught my I is "I maybe totally wrong" I have not been here for 35 years but is that near the LZ for Crestline Hang Gliding site.

[@aeropeoplellc8160]

Awesome ! thank you for helping us learn in fun way. I would suggest using a smoke machine to create a smoke screen to better visualize the vortices. this way you just got to make sure altitude is enough so it doesn't enter ground effect. this also allows you to visualize the vortex interactions. because even the interactions that happen after the aircraft has past do effect the efficiency of the aircraft in the subsonic regime.

[@faizfarooqi9792]

If the motors and propellers were ducted would it be even more efficient? What are the best use cases for ducted propellers in terms of efficiency?

[@nickmuller6317]

Great video! Thanks. When talking about efficiency for real pax aircraft: Do not forget the yaw moment in case of an engine failure with wingtip mounted engines. In a passenger plane you would need a huge vertical stabilizer / rudder to compensate for the big yaw moment in case of engine failure. This vertical stabilizer / rudder would add a lot of form drag...

[@bergkongs]

Ever considered a Prandtl configuration for the lift distribution of the wing? To avoid tip vortexes?

[@mailboxr]

Shoutout to the drone shots to film serenity!

[@jamesomega]

That yaw turn was sharp! If you're not going with motors on the wing tips, you should run some sims on split-tip/scimitar winglet configurations. It's roughly double the surface area of a traditional sharklet but tuning the size and angle of the lower projection can reduce a lot of drag and lower stall speeds. The trade off is oscillations at high speed if they're too big and adverse yaw if they're too small but every swept wing shape has a sweet spot. Just some food for thought! Great work as always and looking forward to Part 2!

[@thinkflight]

I'd be curious to see what a pilot like yourself could do with wingtip motors. Get on it good sir....

[@basfinnis]

The drone flight footage looks lovely. Really interesting video with a great build and with some really good information 😉

[@hello235698741]

I remember reading somewhere that the reason engines are not at the wingtips for twin engine aircraft are safety. In the unlikely case of an engine failure, the yawing force of a single engine at a wingtip is enough force to induce a flatspin.

[@Raging.Geekazoid]

Apparently the guys in the video didn't get that memo. 🙄

[@rohansully584]

Wow, that was awesome! Thank you!

[@super-rc-geeks]

You should test setting the propellers in front of the wing tips，it would be much better.

[@JAMPROSOUND]

Holy crap. I am watching this in my studio and the mic rumble at around 7:25 scared the Hell out of me and made the neighbor dogs bark. Ha ha.

[@anil-vc1pd]

Awesome testing. Good preliminary research is the one that leads to many more questions to ponder. I think you've done that perfectly. It would be very interesting to see how things change between push and pull configuration, different rotation directions, different propeller sizes. The only crib I have with the work you've already done is the additional motor pods on wing that were there in your second configuration. You can't really compare the drags in a true sense especially because the final degree of improvement was very low and I'd expect those pods to add as much intereference drag.

[@ChoppersModelworks]

I often wondered if a ducted fan (fanjet) were built into the wing tip with the housing being tangent to the bottom wing surface if this would be efficient and reduce tip vortices drag.

[@SetUP1]

Fantastic. Love it. Very interesting implementation of the placement of motors

[@martinlagrange8821]

Its been done - over 60 years ago ! I'd have recommended trying out a scale model of the Vought XF-5U 'Flying Flapjack' in which the big propellers at the tips then interacted with the induced wing-tip vortex for 'cancelling' the drag induced in turn- permitting high performance despite high aspect ratio. Technology from the 1940's in WWII for you - there are videos demonstrating model flights from 1939 or so to show that not only do you get low drag, but you also get STOL !

[@gefginn3699]

Great post. Very exciting progress and lessons learned.

[@kitemanmusic]

The flight is extremely stable. Great experiments! The plane @13:07 is super sleek.

[@RobWhittlestone]

What insane skills you have. Very impressive. Thanks for sharing, Rob in Switzerland

[@lyndonthan4350]

Excellent video, beautiful visuals, and really fun topic. Good work. 2 things I find really notable on this test mule: rotation direction of wingtip props. I might have missed it in the video, but surely, the prop directions are important to the project, and I would expect the 2 props would have to spin in opposite directions - the question is whether the props are rotating with, or against the normal wingtip vortices that would develop? If against, the props would see more air, I would think. The other question is the dihedral. The plane has zero dihedral. I assume the normal wingtip vortices are minimized in this wing configuration, at least compared to dihedral wings. With negative dihedral, there may be further reductions in wingtip vortices. I assume dihedral is a trade-off - slightly reduced lift, increased wingtip vortices and drag, but greatly improved roll stability. The plane going nuts with the motors close-in was interesting. I assume again, both motors were spinning in the same direction, creating some level of net torque on the aircraft, and then, no dihedral, so low roll stability. I don't really see the wingtip motors compensating for roll, though they would for yaw. However having the weight of the motors at the tips, would greatly increase the moment of inertia, helping to make rolling more difficult.

[@faslanhafizha]

im wondering what software did you use for create the wing 3d at 2:33

[@thinkflight]

XFLR5, now rolled into Flow5

[@bryanseegers1928]

Back in the 70s, John Erwin, then at AiResearch, studied the use of TFE 731 turbofans on aircraft wingtips as vortex ingesters. He stated that drag reduction resulted in a net reduction in fuel flow. He had beautiful model in his cubicle showing this installation. I don't remember what aircraft it was.

[@elevvfr800]

Wonderful video! Whether or not the wingtip motors end up as a positive in aviation, your research is valuable. Thanks!

[@theecstatic9686]

Hey man, after about one minute of watching this you got yourself a new subscriber....keep it up....

[@guirich38]

Very intersting stuff. Major problem I can see is engine failure, especially with no rudder, but the further from the center, the more torque you get on the yaw axis. Also any difference in propulsion is enhanced by the distance from the center. Neat design

[@benneely6318]

Really cool! My guess for why we don't do that with airplanes is if you have an engine failure, the moment created by the operating engine exceed any amount of rudder you can put in to stay straight.

[@patman0250]

I think what you should do is instead of having the a wingtip motors going off at launch. Just have a regular motor on the back at launch, And once you stabilize and reach a good altitude and speed, then switch the launching motor off and turn on the wing tipped motors. I think it'd be nice to see how it work's once you get some speed and altitude.

[@thextremeking]

The serenity is a beautifull plane. Well done!

[@HutchinsonJC]

When you have the props on the middle of the wing, is there resistance added because of the air being pulled around the wing? I'm imagining like a large box fan with a piece of paper stuck on the back side of the fan... attempting to be sucked through the fan. When you put the props on the wing tip, there's less material in the way of what the fan is trying to do. I'm just curious how much that impacts things in addition with your notes in the video.

[@paulr449]

Idea throw back: Remembering the use of tip tanks from the 50's and 60's planes to reduce wing stress, would it be advantagus to put some of that battery cells in the tip wing motor pods ?

[@lorandhajos9483]

Could you please share what program did you use to design the aircraft?

[@dbfrobro]

Very cool! I did my senior project on wingtip vortices and made a model just like this, but was much smaller and only flew in a wind tunnel. PS Your craftsmanship is top class!

[@billwendt9571]

Thank you for detailed explanations of wingtip propulsion. Like Destin's comment, I wonder if you can vary the vortex at the wingtip. For example, what happens if you used a system to rotate and/or change the camber / castor of each wingtip motor? And if you got a positive result, what would happen if you could independently control the movement of yaw / camber / castor of each motor? I look forward to your next video. Again, thanks and keep up this great work!

[@thinkflight]

this would be optimal. Probably beyond what I can pull off at the moment but great comment, maybe in the future.

[@georgefrankpitt]

How about that massive drag coef. induced by those motor power cables you on the exterior at the very point where the wing experiences the most turbulent flow?

[@robertwalsh5461]

If the motors are counter-rotating, yes! It will definitely help efficiently and give additional lift but with the drawback of catastrophic loss of control if one goes out. The best setup would be a 3rd motor in the center line of thrust as a backup and for peak performance. This would be throttled down and feathered for endurance.

[@thinkflight]

Exactly, a practical application would have a third motor as backup.

[@francom6230]

I'm glad I read further.. a centre mounted motor would ensure control if a tip motor failed, but the added weight, complexity & electrical draw might kill any value added

[@robertwalsh5461]

@@francom6230 If the idea is just to prove concept, then Okay. But if you want to eventually get it carrying people both wing tip motors must be able to feather down…leaving you with a glider for any hope of landing. My understanding is that this was to prove concept so crashes are part of development. If you look at the V-22 osprey or Chinook both use a complex, heavy transmission & drive shafts for linked/shared power. What would weigh more? Electric changes some of that as high output motors can be very light and can tap into main battery power on demand with a little bit of cheap circuitry. I think if these guys are just trying to test efficiently: two counterrotating wing tip motors is all they need too. Everything changes when it has to be safe, though.

[@lucarosu]

Damm super interesting, and the drone shots of the plane flying are beautiful!

[@thinkflight]

Glad you enjoyed it

[@shuashuashua1]

I think cons of wingtip mounted engines is requiment for much more rigid structure to hold extra stress (same that made Your yaw axis controll better) - and that lead to incerease plane mass. Anyway I think Your idea is genius for RC planes and drones, where theres extra stresses are not an issue at all, because of minimum material thickness possible to manufacture. It also look awesome, cannot wait to see more!

[@AClark-gs5gl]

Your channel's content is greatly appreciated. We are currently developing what we hope/pray will be the most efficient stand alone EDFJ and EDFJ JET-A/DIESEL HYBRID. Test aircraft will be a Rutan Stagger-Ez, Velocity 4-place and possibly a Revelaero design. An original design is in the works, but not at this time a priority. Ultimate Goal: to fly faster, farther, more quietly and comfortably than any other E Aircraft on the Experimental and eventually Certified market. Thank you and keep up the innovation/thinking outside of the box.

[@fabriglas]

What program did you use for the design and any comments about it?

[@mokshpatel6986]

Superb , i wanted to know the larger wing = more efficient then right but hard to make rudder right ? Or turning side with stable condition

[@domni5138]

Well. . . I don't do RC planes or RC in general. Yet I still learned a lot from this video. Thank you.

[@Unboredplayer2005]

Can I get specifications like wing span , airfoil ,chord, what motor used

[@kenton6098]

One of the advantages of a dual-engine aircraft with motors near the centerline of the fuselage is that failure of one motor has less effect on yaw than a wingtip motor would. So from a safety standpoint, it seems to me that the rudder would have to have a large enough control surface to offset the larger amount of yaw produced by propulsion located at only one wingtip. Is that possible without introducing excessive drag?

[@Cozzma14]

What software did you use for design?

[@travisSimon365]

Here is what I learned from this video: you have an amazingly understanding, considerate and indulgent wife. Don't forget to cherish her :-)

[@htomerif]

I think a good way to explain the increased efficiency is to look at something like the CFM RISE engine. Its a turboprop that has a second set of blades behind the first, only they don't spin. They're an unducted stator. The stator, which sits behind the rotor, turns the rotational momentum of the air coming from the rotor into extra thrust. Wingtip vortices act like the stator in the CFM RISE engine, only the wing itself is the stator. Wingtip motors, if they were dimensioned appropriately, could push against that already rotating air, using less force than they would somewhere else on the aircraft to produce the same amount of thrust.

[@ZoonCrypticon]

I have studied the wrong subject. I would have loved to study engineering and creating specific models like this. Thank you for showing your design!

[@TKBTKB1122]

I was learning about induced drag yesterday, and I was thinking of the same idea.

[@GWAIHIRKV]

Can you use the wing tip motors for direction control. Rather than Elevons by rotating them?

[@GWAIHIRKV]

@@daviddou1408 : I was thinking that if the motors could also be rotated under control, they could provide vertical and horizontal control as well as yaw. No need for elevons. . .

[@GWAIHIRKV]

@@daviddou1408 I was thinking of just a small rotation of the motor body with servos so the change in thrust would replace the elevons, Resulting in a cleaner - more efficient - wing. As for rudder effect, differential propeller speeds will provide yaw.

[@ddegn]

I loved this. Thank you. If you try this again, it might be worth comparing how reversing the motor's rotation effects the flight characteristics. I assume reversing the rotation would reduce the efficiency but it would be interesting to see this tested. Does your flight controller have the capability of datalogging? If so, it would be really interesting if you could log the current to each motor. Something to keep in mind, we are all hearing your script for the first time. The background music makes it much harder to understand what you are saying. I think this video would have been even better if you had muted the music while you were talking. I look forward to your future videos.

[@stevecunningham6821]

The V-173 flying pancake explored this concept by having the props rotate in opposite directions to counter wingtip vortices and it worked quite well. However, one thing you need to consider is asymmetric thrust when one motor fails. Just off the top of my head, if you place a third motor at the centerline and shot off both wingtip motors if one fails, then you can eliminate the asymmetric thrust.

[@tomarmstrong1281]

You are either single or have a super supportive partner. There is another possibility but I am not going there. Knowing something about aerodynamics. I am very impressed.

[@princesfpvdrones]

Utter genius! I like that wing very much!