Crash Analysis - Great Planes Decathlon 40

[@ememeable]

thank you for the objective methodology in problem solving

[@RCJim]

Thanks for the encouraging comment. I'm well on the way to having the plane fixed. Hopefully we will have all of the issues sorted out! Getting ready to post an update video on the repair.

[@HornetHobbyRC]

Def looks like an accelerated stall, that turn was pretty sharp. I did something similar with a Hangar 9 Spitfire… hate to see a balsa model go down.

[@RCJim]

Yes, a model like that does go "crunch"

[@gogogeedus]

I don't fly my spitfire, it's only had about ten of fifteen flights because it's built from plans and you never get the time back it took to build. these things are not worth money they are worth hours of your life.

[@rafaeltorres2886]

Carnage 😢 Great analysis 👍 I think your over thinking this a little. You're not flying fast enough to make such small amount of aerodynamic difference with that little peace you intend to add, just add more to the throws to your control surfaces and add some expo . You should be golden. 😊

[@RCJim]

Thanks for the tips. Hopefully it will be great with increased control throws.

[@budmartin8297]

I'm not an expert, but I have crashed my share of Decathlons. You haven't addressed the elephant in the room...Decathlon models are super prone to tip stalls if you turn too deeply while having the throttle way too low. What I saw in the video was a plane that wasn't responding well to aileron or rudder input and tip stalled on that last turn which caused it to fall out of the sky. One concern I was curious about was did you move that heavy servo outbound on the wing when you went for a shorter aileron? If so, that is gonna contribute to sluggish aileron response as the angular momentum is increased. If it was designed that way, then never mind on that count. Ask your field buddies why they think Decathlons fly bad. Looks like from the beginning, you never really had much throttle after initially cutting back. I felt your pain on that maiden and am sorry for your loss. Best of luck on the rebuild and make gentle turns under power when setting up for landing. I understand that most of the flight was not usual flying and know you are a competent pilot. If things like this never happen, then maiden flights would not be a little tense.

[@RCJim]

Thanks for the input. One of our really good pattern flyers at the club also thinks that it stalled near the end. Around that time I was in a state of mild panic, and wasn't thinking about the throttle. With most of my planes the descent to the field keeps enough airspeed going, so I can cut the throttle until I begin to level out near the end of the field. If I had better control earlier on, then I wouldn't have gotten either the plane or my mind in that sort of state!

[@RCJim]

Re. the aileron servos, they are HiTec HS225BB servos. Smaller and lighter than standard servos but very quick and powerful. They were not the problem.

[@MO-PA28-140]

The size of the ailerons is not the issue, full scale airplanes use small ailerons as does your model. The upward deflection is also different than the downward to minimize adverse yaw. However, they also have designed in washout so the root of the wing stalls before the tips, as @budmartin8297 states. Something to look into. Sorry to see this happen. A gorgeous bird. Looking forward to seeing it fly again.

[@Xplanker]

I hope i didn't offend you with my original comment. It wasn't my intention. Great to see you working this out

[@RCJim]

No worries. Thanks for your concern. When I know that if I knew then what I know now, the crash probably wouldn’t have happened, it helps me to take in all of the inputs and observations with a positive willing to learn mindset.

[@ememeable]

thank you

[@RCJim]

You're welcome!

[@lechiffre2617]

so did you recover and analyze the cockpit voice recorder (orange box) ?

[@RCJim]

:-) Of course, I do have my great commentary on the video!

[@marcobuonavita2757]

Dispiace vedere un crash di un modello , ma confido nella tua caparbietà di ripararlo e di portarlo in volo. È la dote piu importante per un aereo modellista, buon lavoro

[@RCJim]

grazie per il tuo incoraggiamento e supporto

[@gogogeedus]

The time between this model flying well and crashing was a millisecond, when you were at a 45 degree angle of bank you had a split second to unload the elevator and rudder and add some power, ideally you would anticipate a situation that could arise and avoid it, happy skies next time.

[@RCJim]

I certainly prefer not crashing planes, but I wouldn’t be in the hobby today if I didn’t willingly take the risk. My boys and I crashed our Great Planes Trainer 40 at least a dozen times when we were teaching ourselves how to fly 25 years ago, and it’s still flying today!

[@jeffp6324]

The plane seemed to not be stalling to me but spiraling. Only thinking from a full size perspective - the secondary effect of yaw (rudder input) is roll and the roll seemed to be increasing? Using the rudder to level the wings would be too slow and using elevator while banked like that would make it worse. All too much happening so close to the ground with the plane minus some of its control. If this happened higher up maybe you would have recovered but needing to get it safely back on the ground did not give you that option to be higher up. It is such a good looking (and the effort you put into it) model to not fly again. Reminds me to fix my daughter's Beaver foamie which she stalled in trying to extend the glide after the motor lost power (my fault) by pulling back on the elevator. Looking forward to the fixing series to get this beautiful bird back where she belongs in the sky.

[@RCJim]

Thanks for your good observations. I’ve gotten started on the rebuild and I think it is going to be a good outcome. One more set of lessons learned!

[@tomrammel]

Jim, Another possibility could have been the covering coming loose. this can cause a ton of drag on one wing and at least contributed to the crash.

[@RCJim]

Possibly, but one piece came off early on, and the second was also just a thin triangular trim stripe.

[@weshillsberry6275]

Its called gap sealing do it with your covering not hard plastic keep up the good work not alot of people build anymore sometimes the analysis and rebuild is the best part lol

[@RCJim]

Thanks for sharing the technical term. What I am doing is one step beyond simple gap sealing. On one of my planes I've run the covering right across the join to the ailerons, but it follows the contour. With that there is still a V shaped notch that can disrupt airflow. Where the aileron is thin, that's not a big deal, but with these big fat ailerons on this model I was concerned about that. The stiffer plastic should stay flat, even when air pressure is applied. Being attached only on the wing side allows it to stay in place without inhibiting movement of the aileron. If I did the same thing with a strip of covering, I would be concerned that 1) it might flap in the breeze, like a flag, and 2) that the side that has positive pressure might cause it to indent, creating the V between the surfaces and thus creating turbulence. Maybe I need to hold it out of a car window while someone is driving and observe what happens at various angles of attack. Of course that still wouldn't tell me whether or not the turbulence is a problem with the effectiveness of the aileron.

[@briancoss8482]

I built a great planes Decathlon, and crashed on landing, I got to slow and it stalled and spun in to the left. Dey jaa veu, you have to fly them fast , and use power on landing

[@RCJim]

Yes, and having a lot of foamies, that’s something I need to remember every time I’m fling one of the heavier ply and balsa models.

[@Workerbee-zy5nx]

Model airplanes need washout to make them more stable. I have a old model plane book from the 70s that describes this subject.

[@RCJim]

That makes sense but the design of this model did not incorporate washout, and my son had already built the wing structure when I took over the project.

[@Workerbee-zy5nx]

@@RCJim ideas? Vintage book says washout prevents tip stalling.

[@RCJim]

@@Workerbee-zy5nx Yes, but to build in washout requires the wings to be twisted. It’s too late to do that on this model, and I expect that washout would be counter productive when the plane is doing aerobatics.

[@Workerbee-zy5nx]

@@RCJim Ahhhhhh, got you.

[@joelzwerk4162]

Sorry about the crash and thanks for the updates. I have another opinion on the washout. I built an RCM Trainer last winter and put about 1/4" washout at the tips. It's very visible, almost looks banana shaped. It is by far the smoothest flying plane I have and will do any aerobatic maneuver that the airframe will allow. There's more to washout than what meets the eye. The plane looks almost comical but the laughing ends when they take the transmitter. Best of luck on the repairs.

[@johndd8186]

My take-away is do not fly a Decathlon!

[@RCJim]

Many have told me that is the case - they certainly have their challenges. But I don't give up that easy! (I might change my mind if I do it again to the rebuilt plane...) :-)

[@jeffrapp7278]

If you fly, you crash, we get to walk away every time😂

[@RCJim]

That’s a good reason for flying rc!