Some advice on flying an EDF with some quirky flight characteristics.

[thehaze]

I've been putting some time into flying my small great planes  EDF Tiger Shark. For the most part the plane is fast and very responsive on a 4S setup. I have been having some issues with one of it's characteristics and I'm struggling to figure out what the solution is so I'm putting this out there for some outside opinions.

The airplane has a tendency to snap (or more appropriately stall) out of looping maneuvers such as loops and Immelmann turnarounds. I don't think it's a speed issue as the plane has more than enough thrust to complete the move (plus it's entering the loop about 80+ mph). The plane just seems to fall out of the maneuver, usually to one side, and I need to correct the plane's orientation and dive to regain control authority. It can perform a split s but then again it seems to skid through the bottom of the half loop and takes a lot longer than it should to pull out of the dive.  My guess is it's an elevator issue, either too much, or not enough. But I'm not sure. Never had a plane that acted this way.

Any thoughts?

[flying saucer]

Doing aerobatics on a smallish jet with a higher than average wing loading is possible, but definitely takes more over-all speed(and power) than a sport plane. On my great planes F16 (which I believe uses the same edf unit), I need over half throttle just to stay airbourne, and nearly full power to do a simple turn. If the plane starts becoming unresponsive or loses altitude, you are in the beginning stages of a stall.

PS thanks for the report, my next model will likely be an all plywood/balsa 70mm F20.

[Ededge2002]

Is the elevator pushrod pull or push for up? Is there possibility it's flexing?  How solid is the joiner between the two elevator halves? Is the servo up to the task? Is the elevator directly behind the wing so it could be an airflow situation? Cg?
Just some ideas. Good luck

[thehaze]

Doing aerobatics on a smallish jet with a higher than average wing loading is possible, but definitely takes more over-all speed(and power) than a sport plane. On my great planes F16 (which I believe uses the same edf unit), I need over half throttle just to stay airbourne, and nearly full power to do a simple turn. If the plane starts becoming unresponsive or loses altitude, you are in the beginning stages of a stall.

PS thanks for the report, my next model will likely be an all plywood/balsa 70mm F20.

As I said before it's not a power or speed issue. Although yes, with an EDF you need to maintain a little more speed to have control since there's no prop blast coming from the front of the plane to wash over the control surfaces. The behaviour noted from the plane is similar to a stall but rather is occurring with ample power and airspeed.

I'm pretty sure the problem is aerodynamic. I'm just not sure what variables to tweak to make this work right.

[piker]

I was thinking the same thing, Ed, at first.  Maybe elevator blanking (by the main wing), although this usually happens at high angle of attack.  Sounds like Grags case is at high speed, so not high AOA.

Now I'm leaning toward sloppy linkages or weak servo as you suggest.  I had an aerobatic /pattern plane many years ago that would lose it's line in a loop then snap out.  The solution was a more powerful elevator servo.  Solid as a rock after that.

Robert

[thehaze]

Is the elevator pushrod pull or push for up? Is there possibility it's flexing?  How solid is the joiner between the two elevator halves? Is the servo up to the task? Is the elevator directly behind the wing so it could be an airflow situation? Cg?
Just some ideas. Good luck

I thought about those as well. I don't see anything wrong. The elevator linkage is above the stabilizer so it's a pull to go up scenario. The push rod is short and is glassed so there is no flexing. There is a joiner and it looks pretty solid, but who knows up in the air. Maybe I'll add a CF strip to it and see if anything changes.

Thanks,

Mike

[Gregor77]

I am not getting involved as I just like to fly circles... lol!

[piker]

I think you'll find the solution is a more powerful servo.

Robert

[thehaze]

Hey, it's all just flying in circles. Just on a different plane.

Rob, i was thinking that replacing the servo might be the answer. I just refused to believe that Great Planes would have put the right one in the model when it left the factory as the model was bought as a receiver ready kit, servos and power system already installed. But then again, G P has done some pretty bone headed things in the past.

[Ededge2002]

You also have to take into account that you are pushing the plane and being a demanding pilot. Although GP did provide servos they only realy needed it to fly. For a servo recommendation the hitec HS65 is a great servo at only $22 from your local shop.

[thehaze]

True,

But gp billed it as an 85mph plane. And what kind of jet pilot would i be if i didn't do a split s and immelmann?

New servo it is.

[Michael]

A small plane like that can easily stall in a tight loop as speed slows down quickly.

Try a bigger (diameter) loop.

Dive slightly into the loop for additional speed and keep full throttle until after the top.

[pmackenzie]

Contrary to popular opinion, slow speed does not cause stalls - excess angle of attack does.
You will exceed the stall angle at low speed because you need more angle of attack to maintain altitude.
So for one G level flight there is a corresponding stall speed, hence the confusion.

If you are snapping out of loops and/or tight turns reduce the amount of elevator throw. This will prevent the angle of attack from getting too high.

Pat MacKenzie

[thehaze]

Lots of varied opinions out there. But I'm convinced it's not an air speed issue. Since I understand Pat's explanation the least, that's the one that I'm going to try first.

I'm going to program in a new flight mode into my radio that reduces the elevator throw slightly. After the plane is up on step I'll switch to that and attempt an Immelmann turnaround and see what happens.  I'll note any improvements and adjust my rates accordingly.

If that doesn't work, and the plane isn't a giant crater in the bean field. I'll think about a new servo.

I'll report back..

Thanks for the input everyone.

[Michael]

Pat is right.

So am I (sort of), except I explained it wrong.

The solution, less elevator throw, will result in a larger diameter loop, preventing a stall.