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Jul 29, 2004, 01:04 PM
Tee Sqaured
Thomas Manson's Avatar
Thread OP

Freak crash, what the heck happined??


Hey guys.

Im sorry to say, but my pride and Joy F4U corsair plowed herself into the ground last night at a very high rate of speed.

I was flying my Corsair in a scale like manner. I was practaceing some scale war-bird manouvers with a much more experienced pilot that myself, Warren Jones, coaching me. While doing a loop, i had bizzare incident happin at the bottem of the loop, see the diagram below.

After impact, the plane was in reasonably good shape, but i may not repair it now. It send parts of the plane for quite a distance, i found the battery,motor, and ESC a good 50 feet from the point of impact. To abd i didn't bring a Camera, it left quite a crator where it hit.

Anyone know what happined??


Thomas
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Jul 29, 2004, 01:17 PM
JAS
JAS
Registered User
JAS's Avatar

Possibly?


Looks like it was a downwind loop. If there was not adequate power around the backside of the loop, the airplane would mush (stall) around the back-bottom of the loop. This (mush) is also increased by a "heavy" wing loading. Between the downwind, low speed (assuming) and "high" wing loading (also assuming) it sounds like the unfortunate scenario you experienced. Just my take on the matter.

Jas
Jul 29, 2004, 01:28 PM
Registered User
FLYWINGFAN's Avatar
Hey Thomas,
Bad times, hate it when stuff like that happens ! From what you describe, it sounds like your batt may have slid back(tail heavy planes at higher speed can do that, you don't often see it because most tail heavy planes don't stay in the air long enough to get enough speed to do that)or the elvevator started to flutter/loose control horn.

-Sam
Jul 29, 2004, 01:29 PM
Scott Stoops
sukhoi26mx's Avatar
Thomas,
Was there any rotation to it? Did it stall and snap? It sounds like a traditional high speed stall to me. FWIW, I would think it unusual to have to use full up elevator in a high speed portion of a loop (at least I rarely have to-- usually only to avoid hitting terra firma).

Scott
Jul 29, 2004, 01:53 PM
Nimble with Gimbals
I would definitely agree with FLYWINGFAN. The last time I saw a tail heavy Corsair it flew exactly as you've described it. A snap would typically simply put the airplane on its back, but it does sound as if you were right on the edge of the flight envelope.

Looks like it's time for CSI: Crash Scene Investigation
Jul 29, 2004, 01:58 PM
Departing at 'O' dark 30
Cass's Avatar
might have snapped or bent the push rod, too much force on the elevator at that speed, maybe blew out the hinges on one side? Check your elevator and aileron servo and see if it still works. I have fried a pico and a Naro servo in the ailerons. I believe too much torque caused them to fail, both failed in flight.

When this happend i lost complete control both times. I crashed badly the first time with my Stang, second time the corsair but not nearly as bad as the first time.


Dan
Last edited by Cass; Jul 29, 2004 at 02:02 PM.
Jul 29, 2004, 01:58 PM
Registered User
FLYWINGFAN's Avatar
Yeah, Scott voiced my other thought. With my SA CAP 232 I can exactly emulate the first part of your crash description. Comeing out of a loop, at the bottom I pull full up elevator, plane begins to mush and if I don't release the elevator one of two things will happen:
1) If I am close to the ground and do not quickly release the elevator and re-apply a lesser amount I will mush right into the ground.
2)If I am higher up i can begin to go into the oscillation mode becaue the wing isn't getting air over it(too high an AOA) even at high speeds and it is "high speed stalling".

The verdict is pretty much, with heavier planes(your F4U is running large BL & large Lipo correct?) if you go into a mush because you pulled to much up elevator release the elevator to get the air moving correctly over the wing and tail and then input LESS elevator.(the letting go of elevator is the scariest part Esp. if you are closing in on terra firma) Hope that helps you in the future!

-Sam
Jul 29, 2004, 02:24 PM
Tee Sqaured
Thomas Manson's Avatar
Thread OP
Yes, My corsair flys very fast with it's 8 turn mega and an 8x6 prop. power comes from a 3S 2000 mah lipo.

The battery could not have shifted, so that is out of the qestion. the hinge and linkage did not break (although it's possible it could have bowed)

After it mushed, i knew i had to relase the up elevator, and as i relased it to try and gain speed, it started to oscolate.

Both servo's still work. nether are stripped.

It did not snap out or tip-stall. and what was even weirder is the nose had a very extreme AOA every time i snaped up and down. like it was being dragged by the nose.

Both of my flying buddies, and myself, agreed that it was likely a high speed stall, but the osscolation realy stumped us, is it possible the wing blanked out the airflow to the H-stab and cuased a loss of pitch stabilety?


Thomas
Jul 29, 2004, 03:31 PM
Cleared Hot!
Merlin_FAC's Avatar
I don't subscribe to the (incorrect) theory that doing a loop (or turn) upwind or downwind matters to the plane, since it absolutely doesn't care... the plane sees no wind either way, that's purely a phenomenon related to your perception of the aircraft as an observer planted in one place on the ground. However, judging by your diagram, I would suspect that there may have been some turbulence and possibly sinking air near where the ground sloped down... it would have been sort of the opposite of slope soaring, if you will. If you got into a stalled situation, and then had turbulent, sinking air around you, that very well could explain what you described. You may have also had something where the tail got blanked out by the wing. It's hard to say without trying to reproduce the problem again, up high.

Air can have a lot of force, you'd be surprised at what can happen in situations that we normally don't see (but are acheivable in certain conditions.) If you were going fast, and got an oscillation set up, it's very possible that you could have gotten a pushrod to start vibrating, or even a whole wing, and that the oscillations were self-feeding. It could have been another part of the plane entirely even.
Jul 29, 2004, 03:31 PM
Departing at 'O' dark 30
Cass's Avatar
Hmm.

The weird thing bout my servos is that they didnt strip. they just stopped working,and the motors pulled all kind of juice from the esc. The esc gets hot when plug them in!

As for what happened to you... i havent a clue, dont do it again .

Dan
Jul 29, 2004, 03:58 PM
CO of The Flying Circus
The Blue Max's Avatar
If it didn't snap over in a stall it could not have been a CG problem and Thomas already said that it was impossible for the battery to come loose and move. I think that it sounds like it was a problem with the fact that the loop was attempted down wind. no matter how much power you have available if it was flying too slow (remember that during down wind maneauvers the wind lowers the aircrafts effective airspeed) during the pullout it would cause it to "mush" in just like Thomas explained. Not only does the wind decrease effective airspeed during down wind maneauvers but it can also hit the top or bottom of the wing, fuse, and tail surfaces causing the plane to do some very strange things! If it was as slow as Thomas said it was coming over the top of the loop, when he applied more up elevator after it started to "mush", the wind could have hit the top of the plane and literally pushed it down into the ground! In this type of incedent, the only lift that would have been available to pull out would have been the thrust of the prop and even with a brushless and 3s Li-Poly's it wasn't enough!

Thats my opinion of what it sounds like happened.

TBM
Jul 29, 2004, 04:17 PM
CO of The Flying Circus
The Blue Max's Avatar
And as a side note, Merlins assumption that the aircraft does not care if it is traveling upwind or downwind is incorrect! Wind does effect the flight characteristics of airplanes, and especially so with very slow flying aircraft like an RC parkflyer!

An example of this is if you are flying a parkflyer that will fly at 10 mph and you are flying it at 10 mph groundspeed into a wind that is blowing at 10 mph then you have an effective airspeed of 20 mph. But if you fly that same parkflyer downwind at ten miles per hour groundspeed, then you have no effective airspeed, Zero, zilch, none! And the plane will drop like a rock! You would have to fly it at twenty mph groundspeed to maintain the same 10 mph airspeed and you would have to fly it at 30 mph groundspeed to fly it at the same 20 mph airspeed! So to try and say that the plane does not care about the wind is nonsense, and if you adhere to that belief you will have nothing but problems trying to fly your slow/parkflyer in the wind! Anyone who has flown RC models or full size aircraft long enough will tell you that you have to increase your groundspeed when flying downwind to keep your airspeed at a high enough level for the plane to fly, and the slower the plane flies (airspeed) the less control authority your control surfaces will have!

TBM
Last edited by The Blue Max; Jul 29, 2004 at 04:19 PM.
Jul 29, 2004, 04:46 PM
Registered User
I think Merlin_FAC is just saying that if you think of the plane and its speed relative to the air instead of your point of view from the ground, then you shouldn't have to worry about changing how you fly when you're going upwind vs. downwind.

Of course, if you don't try to fly the plane reletive to the air/wind, you'll have problems...

The groundspeed should be higher at the bottom of a downwind loop and lower at the top of the loop, right?
And that should occur naturally, without any change in the pilot's flying technique.
Jul 29, 2004, 04:47 PM
Tee Sqaured
Thomas Manson's Avatar
Thread OP
Blue max, good ideas tot he cause. I was somewhat confused though as my Corsair was moveing at a good clip when it hit. It only lost speed as it began to mush, but rapidly accleeratid before the oscolations begun. It hit at about 25-30 Mp/H, easy (it was moveing along at a good speed!)


Thomas
Jul 29, 2004, 05:20 PM
CO of The Flying Circus
The Blue Max's Avatar
Sorry if I misunderstood Merlins statement about wind and I apologize if I offended anyone since that was definitely not my intention!

Also I must have missed the part about oscillations occuring before it impacted terra firma! I still think that it could have been a downwind problem though. I was flying my Zero at the local club field the other night to show off for the slimer pilots who can't fly after 5PM because of noise and noticed some really strange tail oscillations while on the downwind leg. It only occured on the downwind leg though and it happened on every circuit of the field, so it had something to do with turbulence on the tail surfaces while flying downwind! Merlin may be correct too in that it could have had something to do with the tail surfaces being blanketed by the wing and not having enough air flowing over them to make them effective.

Sorry to hear about your Corsairs demise and hope she is airborne again some day soon!
Good luck, and watch your airspeed on those downwind maneauvers!

TBM


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