Flying Basics: Aircraft "Glide Ratios" (Testing L-39 & F-16C In DCS WORLD)

[@ChrisGordan]

I've been a glider pilot since '99... weight does not change the L/D (glide ratio)... the best L/D will simply occur at a higher speed when weight is added. This is why sailplanes often carry water ballast- to get the same glide ratio at a faster speed, and penetrate the wind better when flying upwind. The "minimum sink speed " is the speed at which you lose the least altitude over TIME. This occurs a few knots above the stall speed. Best L/D speed is the speed at which you get the most DISTANCE per foot of altitude lost, and is considerably faster than min sink speed. When weight is changed, both of those speeds change, which could easily account for the anomalies you observed. Flying the same speed, at different weights, will not give you the best L/D. You'd need to look at a "Speed Polar"- a graph of performance at different speeds and wing loadings.

[@MeronBridgeMR]

Exactly 👍

[@srky19]

this is the best explanation yet! I forgot that best L/D stays the same -it's only airspeed at which it is obtained that is changing.

[@grimreapers]

Thanks Sir

[@spookyNorbert]

More fuel mass means more momentum. That's why glider pilots fill up their machines with water to secure the desired speed during competition and long flights.

[@caolindennehy2553]

was going to say myself, though still another important thing is the fact that the vehicle/body has less mass means that it can achieve lift at a far slower speed, what happened was cap descend a tad too quickly and because he had less mass then gave a worse glide ratio(or at least in theory in game could be different) thus landing sooner.

[@Ripstop_pilot]

@@caolindennehy2553 glide angle is not changed by weight. It just changes speed at the given glide angle

[@caolindennehy2553]

@@Ripstop_pilot yes, I think that's fair to say, given cap was at the optimal glide angle he would have I higher lift ratio at higher speed and a nominal one at a lower speed

[@ottoottensen4836]

The reason your L-39 went shorter than the other one is most propably that it was still flying too fast for it‘s weight. Remember, two identical planes will always maintain the same glide angle, regardless of wing loading, but only at their respective airspeed for best glide. Flight manuals tend to show only „worst-case“-, or maximum Takeoff-weight figures for performance related topics such as best glide speed. So your buddy might have been perfectly on speed while you should have flown slower to maintain the same glide angle. Conversely in the f-16 you were most probably nearer to the best glide speed for you aircraft weight than your buddy, hence why he is dropping like a stone. You could try to both fly at the exact same angle of attack rather than same speed. You should get the same mileage then. Cheers

[@MrTurbohampster]

Correct 👍👍 Thanks for saving me the effort of writing the same thing

[@SM_Fato]

@@MrTurbohampster 😂

[@D00kerT]

Sort of better to say that the AOA is the same regardless of weight, and hence the "glide angle" is the same. But you're right, that is why RC was gaining, they both should have flown AOA, not speed.

[@MrTurbohampster]

@@D00kerT Not sure what you mean AOA is the same regardless of weight. To be clear an aircrafts L/D ratio is a fixed value, it doesn't vary with weight. The AOA at which the best L/D ratio is achieved is also fixed. To achieve the same AOA at a higher weight you need to fly faster. Hence a heavier aircraft will fly faster and descend quicker than a light aircraft when flying at best glide AOA. But both will cover the exact same distance

[@D00kerT]

@@MrTurbohampster I dont think we're disagreeing on anything. AOA is the same for stalls, L/D ratios, etc. I was saying that Cap and RC were flying the same speed, and that they should have been flying AOA and that might have had something to do with the fact RC was outpacing Cap.

[@piotrrkacperr4262]

Weight doesn't change L/D max, heavier aircraft just needs to go faster to have optimum AoA - 7deg for F-16. Actual formula for F-16 is 200kt +5kt per 1000lbs of fuel / stores, so 200 kt for you and 235 for guy with 7100 internal gas.

[@William3000x]

6:34 That "little fan thing" underneath the aircraft is called RAT (Ram Air Turbine). It's one of the last levels of saftey systems redundancy that every aircraft needs to have. It provides the minimum electric power needed to keep basic flight instruments working on a minimum power consumption level. This electric power is what keeps your Fly-by-wire system working, so you can control the aircraft, also keeps your basic navigation systems working so you know where are you going and what/how the plane is doing.

[@tahaejazkhan6037]

When you guys started off, the plane with the more fuel (more mass) had more Kinetic Energy (Ek = 1/2 mass x velocity to the power 2), while the velocity was constant, thus farther distance travelled due to more Ek. Man, I love Physics, shout out to my Physics teachers Sir Rehan, Sir Fayyaz and Sir Shams for making me love the subject!

[@michaelkelly7328]

L/D max is a function of alpha, so as others have commented a heavier airplane will have a faster glide speed than a lighter one. What's missing here is the energy state of the two aircraft. Total energy is Kinetic Energy (KE) plus Potential Energy (PE), both include mass terms (KE=.5mV^2, PE=mgh). So, the heavier aircraft has both more Kinetic and Potential Energy. Usually, best glide speeds are listed for gross weight and for most general aviation aircraft the variation between a light vs a gross weight aircraft glide speed is small enough to disregard for the skill level of the average GA pilot. All of that said, the heavier aircraft is actually closer to L/D max so it makes complete sense that it glides further because it has more energy and is flying closer to the optimum glide speed. That V^2 term in KE makes a difference! FWIW I am a private pilot and have done this in training, also professionally I was a USAF flight test engineer who took Test Pilot School short courses flying gliders characterizing things like this. Finally, real world I actually had an indicated engine failure on an aircraft I was flight testing in Iraq and actually had to dead stick an aircraft over 20 miles back to base over northern Iraq back in the day, fortunately it was just a bad sensor.

[@grimreapers]

Thanks MK

[@Nails077]

As a lot of people have already pointed out, the lighter you are the easier it is for the air to slow you down. The heavier the vehicle the longer the brake distance. It't not entirely linear as at some point the weight is too much for the available lift.

[@srky19]

as mentioned in comment by Anvil, some sailplanes carry water balast which gives them greater L/D ratio at a higher airspeed. In tests you should focus on holding indicated airspeed from manual, all the way down and dont pump the stick before touchdown. Edit: if you want to test how much terrain you can cover while gliding than you should hold the airspeed and not chase the angle because ideal glidepath is something you GET when flying at correct airspeed for given weight (not including effects of wind here). If weight is changed -maximum glide ratio obtainable will change. It will also be at a different speed.

[@jamesmckinney660]

I spoke to an retired fighter pilot about this test. He basically said all things being even except weight, the heavier plane has more mass behind it when you started the sim. It will be forced futher in the air "before" glide takes over. It went futher because the weight pushed it further before begining to glide with your plane with no fuel.

[@grimreapers]

thx

[@geogeorge2039]

Just like the gliders have water in the wing tanks

[@stephenzenerak7846]

It's the momentum of extra weight that works against the forces that slow the plane down, that's why heavy trains are hard to stop.

[@mercatorarts7663]

Subsonic best glide speed for a given configuration and centre of gravity position varies proportional to the square root of weight. External stores increase drag, making the glide steeper and best glide speed lower.

[@AnvilAirsoftTV]

Not that I know anything but don’t actual gliders launch with water ballast?

[@Gert-DK]

Now you know how to get to homeplate low on fuel. Does anybody have the glidespeed on the Hornet? It would become handy in a low fuel situation.

[@adhdrenalin4392]

Weight theoretically doesn't impact glide ratio your Vbest glide will be a bit higher but you will have the same ratio. This is also the reason gliderpilots fill their wings with water on good days to achieve higher average speed with no real drawbacks other than a slighly smaller climb rate in thermals. (Wrote my comment without reading the other comments, then noticed I wasn't the first to comment this)

[@2384SKIPPER]

The extra weight had to be accelererated at some point so there is extra energy in the form of inertia in the heavier plane.

[@jorge8596]

Also more potential energy

[@frqgrenade]

Maybe glide ratio is higher because at 10k feet ground speed is higher than cas? If you test this at lower altitude you might get results closer to the flight manual.

[@georud54]

More weight also means more potential energy. The extra weight would therefore effectively help the plane push through the air as it descends, allowing greater speed to counteract the extra lift required. Maybe :-)

[@mercatorarts7663]

Subsonic best glide speed for a given configuration and C.G. occurs a fixed angle of attack.

[@Nerb1]

Eek... the whole idea of an optimum glide speed is to maximise the glide distance. Speeding up a bit shortens your distance Cap! I dont think the weight makes any difference?

[@golffoxtrot5721]

with more weight you can fly the same ratio on a higher speed

[@101ECVGADGET]

Please consider that best glide speed (BGS) is weight dependant. Glide ratio does remain the same in the same aircraft, but with heavier weights it will fly faster and will arrive to the ground sooner than the very same aircraft carring a lighter weight, but BOTH WILL TRAVEL THE SAME GROUND DISTANCE FROM THE SAME ALTITUDE. This is the reason why competition gliders carry a heavy water ballast, to allow a faster flight still flying the same distance per altitude. It would be interresting conducting a practical experiment that would permit to draw the glide polar of certain aircraft. In the same sense, as A320 has a representation of the max LIFT/DRAG ratio in the form of GREEN DOT SPEED, some aircraft in DCS can also show this data, said F-18 and probably F-16, and for the case probably all "digital" fighters of the same period. Manuals often describe BGS for MaxTOW of the aircraft. To find the new speed for other wheights, simply aply this equation: NEW_BGS = BGS_MTOW * SQR [ACTUAL_WEIGHT/MTOW] Thx for the video

[@grimreapers]

Thanks Sir

[@reubensteel4632]

I would assume the slight difference in distance is due to slight differences in maintained airspeed and as the airspeed should have been higher for the heavier aircraft ? given the following assumptions. "Variations in aircraft weight do not affect the glide angle provided that the correct airspeed is flown. Since it is the lift over drag (L/D) ratio that determines the gliding range, weight will not affect it. The glide ratio is based only on the relationship of the aerodynamic forces acting on the aircraft. The only effect weight has is to vary the time the aircraft will glide for. The heavier the aircraft is, the higher the airspeed must be to obtain the same glide ratio. If two aircraft have the same L/D ratio but different weights and start a glide from the same altitude, the heavier aircraft gliding at a higher airspeed will arrive at the same touchdown point in a shorter time. Both aircraft will cover the same distance but the lighter one will take a longer time to do so." Reference: www.skybrary.aero/index.php/Glide_Performance

[@MF-og1ct]

Something that has more mass has more kinetic energy. This is why mass does not change glide ratio.

[@RobertClarkSherman]

Inertia is also something you have to consider. More fuel = more mass = more inertia (?). Just a thought.

[@grimreapers]

Interesting...

[@danvercillo]

Cap and RC that was an oddly calming display. I though you were going to play Yanni live at the Acropolis for a second. LOL (Try playing "Yanni - One Man's Dream" while watching OMFG LOLZ )

[@grimreapers]

Yanni! Yes was rather calm.

[@JakusJacobsen]

Lots of people are saying that weight doesn't affect glide ratio and that gliders carry ballast and so on. I am no expert but I'm with CAP. A heavier aircraft must have an effect. There is after all a reason that gliders are made of advanced super light materials. If weight doesn't matter can I make a glider out of concrete? I'm puzzled why glide ratios in manuals would be for anything but zero fuel? If you need to glide then either you have no fuel or the engine is dead so dumping the fuel would seem to be the thing to do anyway. My Father was a senier commercial instructor and ex military fast jet pilot so I am sure he would have had the answers I need. Its always the little things that you miss the most.

[@sam0899gamer]

Cap, most aircraft have an optimum speed where the drag is minimised. That determined by lookibg at a wings polar diagram. This sort of diagram shows the relation between alpha and lift. At that optimum angle of atack you can obtain maximum glide ratio. Weight does NOT change the distance, it only changes the descend speed and airspeed.

[@grimreapers]

This is so bloody confusing lol

[@CNCTEMATIC]

Ever made a glider, e.g. from balsa wood? They fly a lot further if you weight the nose...

[@grimreapers]

Interesting

[@connorAv8]

Weight only changes the time to ground out you will fly the same distance, this is what I’ve learnt from atpl’s

[@hugiebessie4513]

Cap, gliding characteristics depend on the wing layout and AOA largely. The sleeker the wings, and the smaller sweptback angle, the higher the lift over drag ratio L/D. Let's call L/D like rd. Then, drag is lift divided by rd (D=L/rd). Since lift must equal weight, higher weight means more lift, thus more drag inevitably. But a better (greater) rd, reduces drag.The reason gliders can reach longer, is that they have long sleek wings, and low weight. To gain the optimum in any plane, the pilot needs to keep it's AOA and speed right. To do this, he adjusts the horizontal tail. If he needs to increase his AOA, he must pull back, making the tail push down. This he must compensate with more lift and thus even more drag. The water tanks in gliders are probably in the nose and the tail, making it possible to adjust AOA without having the tailplanes push down. Hope this helps, keep up the great work!

[@grimreapers]

Thanks Sir

[@jordannewsom4578]

Only thing you weren’t 100% right on was the water (ballast) tanks location in the gliders, they are in fact in the wings just like fuel tanks would be. And if I remember correctly most gliders actually have a LOT more ballast weight than one might think, pretty sure the competition gliders carry like almost 200lbs in each wing(: (please PLEASE don’t quote me on that I’m not sure if I’m 100% correct and if someone knows the actual weight/gallon capacity of gliders please let me know!!!)

[@shrekrecker]

love watching the avionics mathmatical videos 👍

[@geogeorge2039]

the F-16's glide ratio is actually closer to 6000:1000 . meaning1Nautical mile/1000 feets

[@grimreapers]

thx

[@MrPnhartley]

I think the test of the L-39 need tweaking to give rational results. You may have to try different airspeeds for the different weights...desired result is lowest ‘sustainable’ descent rate for the condition. Now I introduce you to Ground Effect Compressibility where near the ground the compression of the airflow between the ground and the bottom of the wing gives you way more lift and dramatically increases the glide ratio. Effect causes the ‘float down the runway’ effect when landing. The Russian Ekranoplan (Caspian Sea monster) flew full time in this area. And...discuss. Lol.

[@ottoottensen4836]

MrPnhartley Except lowest descent rate will not result in furthest glide. Speed for L/Dmax will always be higher than that.

[@srky19]

I don't think desired result was:"lowest ‘sustainable’ descent rate for the condition. " , rather the speed at which most ground would be covered, i.e. -at which speed should I fly, with this weight, to fly furthest that I can? Otto beat me to it.

[@Mich_Angel]

When trying to glide a aircraft the amount of lift at what speed a wing can produced is good to know. different wing(aircraft) have different lift based on it's area, angle, curve, form, weight and speed. So when using identical aircraft same start speed to see glide using a... ..Very very simple way to explain might help. More mass = more momentum more speed = more lift.. But yes this have a threshold where to much weight and drag will take over the lift factor of what the wing can produce in X speed. And then the effect would be opposite and you drop like a stone. So how much fuel you need to have left to get the best L/D glide distance vary a lot on how much lift the wing can produce at what speeds. Aerodynamics can be very frustrating and some time it seem to break all laws of physics but it doesn't, it's all about variables and numbers and they are pretty straight forward. CHEERS! Keep up the awesomeness ;)

[@grimreapers]

Thx Mich

[@Mich_Angel]

@@grimreapers You're welcome. ;)

[@yjwrangler7819]

Check out the story of the Gimli Glider, Cap

[@JakusJacobsen]

Lots of comments are saying that weight doesn't matter and that gliders carry ballast and so on but I'm with CAP, weight must make a difference. I'm no expert but there is a reason that gliders are made of advanced super light materials after all. Can I make a glider from concrete? The ballast on a glider surely could only be a small percentage of total weight, Or a small increase to wing loading meaning they trade speed for climb ability. VS a jet with full tanks weighing 50% more, or even more so for a fully fueled A320. What about aircraft with internal bomb bays? If they lose power is it of no benefit to get rid of the bombs? That doesn't seem likely. I'm also puzzled why aircraft manuals would give glide specs for anything other than empty? If you need to glide then either there is no fuel or the engine is dead so getting rid of the fuel would seem to be the thing to do anyway. As a side note I know the normal procedure for flame out is to immediately turn your airspeed to altitude and a fully fueled plane won't be able to get as high. My Father was a senior commercial instructor and ex fast jet pilot in the RAAF so I'm sure he would have had some good answers for me. Its always the little things you miss the most about people.

[@AgneDei]

As mentioned in other comments gliders take water ballast for competitions to increase their speed initially, and later on they can drop it if they predict that they won't make it to the finish. The thing here is that with more weight you need more speed to generate lift. More speed means more drag, and that means that you can't possibly get the same glide ratio as with a lighter plane and lower speed. The tricky part is that it's not that easy to predict the optimum solution to the more speed vs more drag question, as the drag reduction must be must be considered over time and distance, as you do reduce your speed. Should be quite easy to find that speed in a plane/simulator though looking at the altimeter, speed gauge and correcting the glide angle. That's basically what gliders training is mainly about...

[@MrTurbohampster]

This is basic aerodynamic theory. Your dad would have told you that weight does not affect glide ratio. www.skybrary.aero/index.php/Glide_Performance

[@grimreapers]

Thanks JJ

[@AgneDei]

@@MrTurbohampster Don't spread untrue information. To understand it simply put 10 tonnes of ballast into a glider and see how it goes.

[@MrTurbohampster]

@@AgneDei false information LOL. What are you qualifications? I have about 1000hrs in gliders. Full ATPL and 20yrs commercial experience, currently in the left seat of a B777 Do you know the lift formula? If you did you would know why glide ratio is unaffected by weight Glide ratio, which equals D/LD/L, does not change with weight. Hence, starting from a given height hh, the longest gliding distance will be d=h⋅max(L/D)=h⋅max(CL/CD)d=h⋅max(L/D)=h⋅max(CL/CD), i.e., independent of weight. However, best glide speed and, consequently, the gliding time, do change with weight, and significantly. If CL,maxCL,max is the lift coefficient when CL/CDCL/CD is maximal, the gliding speed at best slope, V¯V¯, is obtained from W=(1/2)⋅rho⋅V²⋅Sref⋅CL,max Don't take my word for it though www.skybrary.aero/index.php/Glide_Performance www.askacfi.com/1759/effect-of-weight-on-glide-ratio.htm

[@cmibm6022]

Compare that to the glide ratio of a sail plane: 60:1

[@skinnyflea2628]

What aircraft in dcs has the best glide ratio?

[@grimreapers]

Interesting. Has to be a warbird right? Best wing loading by far.

[@D00kerT]

"La-Guar-dia"

[@liamhines3194]

he can push through the air better because he is weighs more