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Old Apr 26, 2016, 07:28 AM
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Help!
new plane diving into the ground?

I have designed and built many planes that fly well however this one has me stumped. This is my 1st time using DTF. I am really surprised how strong this foam is as this plane has lawn darted it self 5 times really hard and is still in tact.

DTF construction. High wing RET with a fair amount of TLAR dihedral.

Wing is 40 X6 Armin. 1 inch top skin over lap in case I wanted ailerons.

Fuse is 20 inch long and 4 inches tall under the wing. Shaped sort of like a cub.

H stab is 20% of wing area. V stab is 1/3 of H stab area. Elevator and rudder both have 2 inch chord. LE of H stab is about 8 inches from the TE of the wing.

The plane will take off and appear normal and then suddenly pitch abruptly nose down and fly into the ground vertically just like you dropped it holding it up by the tail. The elevator does nothing once it pitches over.

Now if I do manage to keep the nose up with lots of elevator the plane will wallow around just like a flying wing does with not vertical fin.

It seems like the V tail may be too small however this does not explain the sudden nose down pitch over. I thought it might be a bad servo so I changed the elevator servo and it did not help.

The CG is dead on the 25% mark of the wing chord. I also moved it more forward with no change in behavior. The weird thing is the plane seems normal until the pitch over. The plane never gets far. I think it has only made it about 50 feet from takeoff before the problem begins.

I usually build 4 channel planes with little to no dihedral and they all do well so i am a bit stumped as to this behavior. Even very tail heavy planes are wild but never do what this one is doing.
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Old Apr 26, 2016, 07:50 AM
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Have you done a range check recently ?, especially is you have had a few vertical landings.

There is a formula somewhere on the forum for sizing a 'V' tail. it does sound like it may be too small. But still do a range check, just in case.

Is this your first 'V' tail ?. How about a picture(s) of the model.


Ray.
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Old Apr 26, 2016, 07:53 AM
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By V tail, I mean Vertical stab. It is a conventional tail. The radio was removed from a flying plane with no problems.

I will get pics later today.
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Old Apr 26, 2016, 08:00 AM
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Tail volume is .02 for the Vertical and .48 for the Horizontal so it should be ok.
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Old Apr 26, 2016, 11:11 AM
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Have you tried any glide testing? I would try a power off, all control surfaces set at neutral throw from shoulder height into the wind over tall grass and watch what happens. If she glides straight and stable in a gentle arc to the ground then I would think something wonky is happening in your radio system As opposed to the basic airframe/trim. If it does anything else, then that behavior may provide a clue. I would also try a similar throw with power set at barely able to keep it in the air, kind of an extended glide. And see what happens.

Ray's range check suggestion is right on, and along that line, what happens to the controls in the plane when you shut off the TX? (Loss of signal). By the way, what radio are you using?
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Old Apr 26, 2016, 11:57 AM
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I will try these tests and see.

Hitec aurora 9 and mini rx.
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Old Apr 26, 2016, 12:06 PM
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Quote:
Originally Posted by grant31781 View Post
Now if I do manage to keep the nose up with lots of elevator the plane will wallow around just like a flying wing does with not vertical fin.
Is it stalling? What is the AUW and how many Watts?
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Old Apr 26, 2016, 12:10 PM
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Weight is less than a pound. No it is not a stall. It looks like you put in full down elevator the way it dives. The elevator is non responsive when it happens.

My TX low battery beeper came on however I do not think that should cause a problem.
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Old Apr 26, 2016, 12:59 PM
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What are your failsafe settings?
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Old Apr 26, 2016, 03:40 PM
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Look for flex in the flying surfaces.
A plane may be perfectly rigged when static but go wildly out of trim when aerodynamic loads cause distortion of the structure.
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Old Apr 26, 2016, 03:42 PM
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If it per chance isn't the radio, which it sounds like it is. Consider the wing flexing as the speed and load picks up.

edit: Whiskers said it. I'm slow hands.
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Old Apr 26, 2016, 04:03 PM
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Great minds and all that...
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Old Apr 26, 2016, 07:36 PM
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One of the things you will need to do is determine if the issue is with the plane or the radio. (or worst case, both but hopefully not) My glide test suggestions were aimed at determining if there was anything obvious in the plane. Same for Whiskers asking about surface flex. The test of what happens when you lose signal is aimed at finding out if the radio is at issue. Along that second line of reasoning, where is the RX antenna pointing and where would the TX antenna be pointing when the plane does it's nose dive? How many of us have had RX antenna laying straight aft in fuse, Plane going away from us and somehow point TX antenna straight at the plane??? Mea Culpa!
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Old Apr 26, 2016, 08:47 PM
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Well I feel stupid! It was the radio setting. For some reason the alieron stick was also controlling the elevator. Once I disabled the stick, she flew fine. Then I set too little throw in the rudder and got her stuck high in a tree. It took over an hour to get it down. Oak trees really hold on.


Preflight I only was checking the rudder and elevator. Flying I am so used to 4 channel I used the alieron stick without realizing it.
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Old Apr 26, 2016, 09:44 PM
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Sweet! Been there (approximately) done that. Glad ya found it.
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