Why do some planes snap and others don't?

[DeaninMilwaukee]

Had a bad crash today with my jk aerotech P-51 caused by snapping hard at the entry of a loop. I pulled up hard, and it just rolled right over and plunged in. :(

I have had and self designed quite a few planes now, and have rarely seen this sort of thing before and only when I dialed in really extreme elevator throw.

The thing thats confusing me on this particular case is that the throws on the p-51 were only 1/4" up and down. At lower speeds I was actually wishing for more elevator authority, but at the high speed I was going before the crash, it was obviously too much.

I know I'm not nose heavy at all, ( glide is flat and impressive ), so what could be the cause of this? Could it be due to a sorta heavy wingloading? ( 41 oz, 250 in/sq).
Its really quite baffling. :confused:


Dean in Miwaukee

[vintage1]

In your case, I think wing loading says it all.

There ws an extensive thread on Usenet about this, and I learnt a lot from listening...and it made sense.

Lets say that your wing stalls at some combinations of speed and angle of attack. Take that wing at a given speed through a given loop on a light model, and it has to generate the amount of lift necessary to lift the model weight, times whatever 'G' force the loop needs - say 2g.

Now double the model wieight, and to keep the speed, loop size and hence 'g' force the same, it has to generate MORE lift (even upside down!) to get the loop the same size.

The only way to do that is to increase the angle of attack, taking you dangerously near a stall - which typically occurs towards the top of the loop where speed is lowest.

With a high wing loading, you simply have to fly the loops bigger, and faster, to get the same 'g' Or faster and the same size but pulling more 'g' to keep the airspeed up..but even here, you are into dangerous territory, because to generate the extra 'g' you need extra angle of attack...and so once again, the stall is possible.

The final answer is that high wing loading models can't do 3d sized loops :)

Lighten the model, or accept larger loops and fly it carefully and smoothly all the way round. Washout will help to tame the stall onset for normal loops (but will make outside loops worse). However, if you have outboard ailerons, dialling in a little 'up' on both may stop it being so violent on the stall.

[DeaninMilwaukee]

Thanks for the clear explanation.

There really is no way to remove significant weight unfortunatly.

The plane does however have full span ailerons, so I'm wondering now about your comment about giving some reflex to them.

What exactly does this do?

Its kind of interesting on this plane. The elevator power is'nt so hot, but the ailerons are unbelivably effective. I get about 2 rolls a second with them at full speed. I've never seen another warbird that rolled even 1/2 that fast. I wound up using -50% expo just to tame its response.


Dean in Milwaukee

[radfordc]

Originally posted by DeaninMilwaukee
The plane does however have full span ailerons, so I'm wondering now about your comment about giving some reflex to them.

What exactly does this do?

Dean in Milwaukee

Raising the ailerons has the effect of reducing the angle of attack of the wing. With full span ailerons the washout effect isn't as great as with conventional ailerons.

In your case, adding a stall strip to the leading edge of the wing at the root may also help. A 1/16" square strip of balsa about 3" long glued along the center line of the leading edge at the root end of the wing. This will cause the root to start stalling before the tip.

[vintage1]

Originally posted by DeaninMilwaukee
Thanks for the clear explanation.

There really is no way to remove significant weight unfortunatly.



Of course there is! drill holes in it, throw away the motor and put in something lighter, use NiMh etc etc...:-)

Out of interest, what *is* the spec.? maybe you just need better power-to-weight so that it does enormous WWII style power loops!

The plane does however have full span ailerons, so I'm wondering now about your comment about giving some reflex to them.

What exactly does this do?

Its kind of interesting on this plane. The elevator power is'nt so hot, but the ailerons are unbelivably effective. I get about 2 rolls a second with them at full speed. I've never seen another warbird that rolled even 1/2 that fast. I wound up using -50% expo just to tame its response.


I wouldn't bother if its full span. Just concentrate on NOT pulling up hard. Some really heavy planes will allegedly snap roll instantly at speed in level flight if you pull full 'up'.

I wouldn't have thought it was *that* heavy tho...that's not a bad weight for the plane size...I suppose something didn't break, did it? Like the elevator hinge? I can imagine one elevator sticking full 'up' doing that...

[DeaninMilwaukee]

Weight: What I should have said was that theres no cost effective way to remove weight.

Oh I have plenty of power! :cool: The powertrains 7 rc1700's on a atomic force geared 2.5:1 with a 9x7 folder. Motocalcs says 26 oz thrust, 30 amps, 65 mph ps speed.

The plane is fairly easy to launch, goes fast and climbs very nicely. I know I could get some gains by gearing 3.8:1 and using a 11x10 apc-e, but it's power is pretty decent as is.

I am realizing that my only solution is to just not pull so much elevator. Nothings broken BTW, all is still solid. Did'nt even strip the servo.

[Neil Stainton]

Snapping isn't just caused by an unexpected stall, it is caused by the plane TIP stalling. If starting from scratch the easiest way to avoid tip stalling it to avoid too much wing taper and/or add washout (no good for aerobats though).

For a given planform a very effective way to lessen tip stalling is to make the wing leading edge more rounded - ie make the roots sharper than the tips. If it is a foamie just remove the LE tape, sand the LE at the tips to make tthe front very rounded, and reapply tape. I'll bet it works.

Neil.

[GooberVA]

Wouldn't adding a little up on the ailerons effectively increase the wing loading even more since it effectively shortens the chord?

[Tim Green]

I believe that a plane snaps easier, the further back you have the CG.

[Neil Stainton]

So Dean in Miwaukee, have you tried my suggestion of blunting the LE near the tips? It will work.

Neil.

[DeaninMilwaukee]

Actually no. I still have'nt even finished putting it back together yet.

I'm not sure if I will try that or not though. The plane does fly pretty well otherwise, and will do great big pretty loops too. You just have to ease in the elevator instead of jamming it.

I think the key is for me to realize that it just is'nt a fun fly plane, and requires some finesse to wring it out.

It is worth noting here however, that its quite possible to tip stall it when trying to banked over and yanking too. A pylon type turn is out of the question.

I may try out the turbulators though, as they are easy to add and remove agin if they don't work. Not a permanent change.

Dean in Milwaukee

[Neil Stainton]

Strange. You ask for help to improve the flying charicteristics, are given an easy fix that only requires you to sand 1/16" from the wing leading edge, and has no downside, and now you decide not to bother, or to try an alternative method which is much less likely to work.

I guess people don't understand why blunting the LE delays the stall, and aren't willing to try it.

Neil.

[DeaninMilwaukee]

"Strange. You ask for help to improve the flying charicteristics"


Actually Neil, I asked for help understanding why it happens.

I did however try your idea and fnially got an opportunity to try it yesterday.

Result: Improved, but still does it.
This time I got it way up high first, and then yanked back hard. Now it tries to climb before the snap occurs, where before it just snapped. It is easier now though to make tighter " bank and yank" turns. Before you really had to watch it, now its fairly predictable.

So the result is not 100% cured, but definately better.


Dean in Milwaukee

[Neil Stainton]

> So the result is not 100% cured, but definately better.

Well done! I am glad you tried it. Blunting the wing leading edge is an amazingly effective way of impoving the wing stall behaviour. I'm sorry it didn't cure the problem entirely, but I hope you think it was worth the effort.

Neil.

[DeaninMilwaukee]

I do think it was worth it. The reason I did'nt want to try it right away is that I had done such a nice covering job and I did'nt want to cut it up. It turned out ok though.

This is a fun plane BTW, lots of power and REALLY big loops,
( 100 ft?), plus it rolls extremely fast, ( blink, missed it!), especially when you consider that its a warbird.

I am tempted to build another one with a REALLY rounded LE, just to see what'll happen. Give me a chance to build one that does'nt look crashed and repaired too. :rolleyes: It would be nice to build it lighter, but that would take some serious bucks.


Dean in Milwaukee