Help needed with Durafly Tundra

[Moiraobrien]

I am a recent returnee to fixed wing flying and am now comfortable with general flying away and towards me. I cut my teeth on Eflite Sport Cub S and Champ S+ (which I stupidly lost downwind when I forgot that there was a panic button!). I moved up to the Durafly Tundra.

My first flight was OK though I had problems with one of the aileron servos so had a bad takeoff. I reattached the nose and replaced the servo but now have this problem. When I try to take off without flaps (with flaps seems to be no problem) the plane skews violently to the left and crashes - ailerons don't seem to have much effect to stop this. Having rebuilt the nose again the geometry of the fuse was not 100% but it was not enough to cause this repeated effect, causing another broken nose. Note that I checked the control surfaces and CofG every time I rebuilt.

As I have now crashed in the same way on every attempt at no flap takeoff I am now wondering if this is a problem with the design. After each rebuild I took off successfully a number of times with half or full flaps.

Any ideas?

 

[Wingman04]

The motor torque may be causing the roll. Have you tried advancing the throttle slowly so that the torque isn't so sudden? The motor from the factory is supposed to point a little to the right.

Do a range test too. Maybe, after your first several crashes, something when wrong in the Rx and during takeoff, the plane loses link and goes into Failsafe. Just a thought.

 

[sprzout]

Ok, dumb question - when you say it skews violently to the left, is it ROLLING (like an aileron turn) or is it yawing (like a flat spin) to the left? And are you rolling into the throttle, or just going full bore, from zero to 100 percent throttle?

I ask because there's an effect across the rudder where you'll get turbulence from the prop that will blow against one side of the rudder, making it want to push to the left or the right. If you look at the designs for the FT Bushwacker, it's actually got the engine cocked to one side in anticipation of that thrust.

Again, I'm completely guessing that's what's going on, I could be WAAAAY off, but if you're saying it's kicking to one side, it COULD be that thrust. Having the flaps down could give it enough of a disturbance so that it's not throwing it hard across the rudder...

 

[nhk750]

I had a problem with my Tundras aileron servo connections. They were terrible and would cause the ailerons to stop working so I tore them out and directly wired the ailerons with servo extensions. I think I posted the fix somewhere on this forum.

 

[Hai-Lee]

Sadly I have seen such behavior before.

When you did the first repair you may, (according to the planes behavior), have removed the majority of the right thrust that the fuselage and motor mount had built in. Additionally if you broke the original propeller the replacement might not be exactly the same in Dia or Pitch.

If when you reattached the nose you inadvertently altered the centre line of the fuselage giving it a mild "Banana" shape towards the left the thrust line is lost and the vertical fin is now canted towards the right causing the plane to want to turn left.

You mention that the problem does not occur when the flaps are deployed. I suspect that with the flaps deployed your wings are generating far more lift when you take off than they do without flaps and so the increased speed of the airflow, (caused by the diagonal flow of wind from the tips to the flaps, - a kind of venturi effect), is allowing better aileron roll control and the low speed airflow across the vertical fin/rudder is of less effect due to the lower take off speed.

All is not lost! Make your ground runs without flaps longer and faster so that you maintain control at rotation and at lift off. Also plan to realign the fuselage to get the various lines back where they should be!

Have fun!

 

[Bricks]

Have you checked the programing in your radio, check how the ailerons work with flaps up and flaps switched on to down, and do you have the flaps programed to work with the ailerons or are you keeping them separate at all times?

 

[Moiraobrien]

Thanks for all your thoughtful responses. Being relatively new to powered fixed wing flying, I have a lot to learn.

@Wingman04 First off, the right tilt of the motor is if anything more pronounced after rebuild (there was a bit of compression of the fuselage after the first crash) - so that is unlikely to be the cause of roll to the left. There is no problem with range - once safely taken off, I can fly well into the usable (good visibility) distance.

@sprzout It rolls to the left, drops the wing and dives into the ground. I may have been going to hart throttle too fast - though the last crash was from longish grass where I HAD to roll the throttle just to stop it nosing into the ground. There is a slight twist in the fuselage with the tailplane showing a very slight tilt to the left relative to the wing, so with your theory that should produce right movement. I do think you may have something in your theory of turbulence over the vertical stab.

@nhk750 the issues I had with servos was the servos just failing completely. However, your mod sounds sensible - if the wings shifted (the screws are prone to being pulled out if you catch a wing on landing or in a crash) direct connection would avoid disconnection.

@Hai-Lee I am using original carbon props, so they should be OK. See above re angle of thrust. I will take your advice on board and try longer take-off runs, though I suspect I will always be using flaps on take off from now on .

@Bricks I have mixed flaps with elevator only. I have not noticed anything untoward with aileron movements.

All in all, I think the most likely explanation is turbulence over the vertical stab and not enough take off speed with no flaps. All this give me things to experiment with on my next flight.

Thanks everyone.