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?
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.
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
Quote from: flying saucer on August 20, 2012, 12:18:01 AM
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.
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
Quote from: Ededge2002 on August 20, 2012, 12:21:48 AM
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
I am not getting involved as I just like to fly circles... lol!
I think you'll find the solution is a more powerful servo.
Robert
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.
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.
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.
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.
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
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.
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.
Agree with Pat/Michael
Start the loop in a dive to build up speed.
Quote from: flying saucer on August 21, 2012, 01:04:27 PM
Agree with Pat/Michael
Start the loop in a dive to build up speed.
That's not what they are saying at all.
But yes, if speed was an issue, diving before the loop, or half loop, would increase the speed of the aircraft.
Whether speed is the issue or angle of attack is the issue, the point is, the model is stalling at the top of the loop because of improper airflow over the wings (I think).
The idea, is to prevent the stall.
A larger diameter loop implies less elevator input, and you may need more speed to complete a bigger loop.
Well, I took the F20 to TEMAC tonight to work out some of the bugs and well let's just say that I won't need to worry about this issue any more with this plane.
Got the plane up on step and flew it in the pattern for a bit. Switched to low rates on the elevator and managed to pull up and into a nice round half loop. So far so good. I'd estimate that the plane gained about 150 ft of altitude, no sign of a stall or snap. Rolled to right side up and headed parallel to the field. Reversed direction and climbed into a half reverse cuban 8. 45 deg climb, roll to inverted at 300+ft, reduce throttle and pull up elevator to begin a descending half loop. As the plane began to descend I knew something was wrong. The plane seemed to skid (not sure how else to describe it, and I lost all control authority on the elevator. Reduced throttle to zero and flipped to hi rates and still could not exit the dive. The plane impacted the beans and was ripped to shreds. Interesting though was that when I found the fuselage it was evident that the plane never impacted the ground as the beans were stuffed in the ducts and they stopped the plane dead in it's tracks, suspended a couple feet above the ground. The deceleration was so sudden that the battery actually ripped through the nose and traveled about 20 more paces before coming to a rest on the ground. The plane is as Ronnie would say "a write off" although all the electronics (save one servo, who's lead got sliced in half) are still viable.
As I sit here I'm trying to sort out my thoughts on this crash.. And here's what I've come up with.
1. Some planes just don't fly well. This one, as much as I liked how it looked, was not a very competent airplane. I could have just flown it in the pattern and enjoyed the speed but the way I figure it, if a jet can't execute those basic moves, then I don't want it. I'm not sure if there was anything else I could have done to save the plane after I executed the descending half loop. I suppose if was higher up I could have but honestly, I started the move at 300ft which in my estimation is sufficiently high enough to pull this off.
2. Well... this plane just sucked.
The good news is that I have all I need to put together a nice 54mm edf. Any suggestions?
Quote from: thehaze on August 21, 2012, 03:54:47 PM
Quote from: flying saucer on August 21, 2012, 01:04:27 PM
Agree with Pat/Michael
Start the loop in a dive to build up speed.
That's not what they are saying at all.
But yes, if speed was an issue, diving before the loop, or half loop, would increase the speed of the aircraft.
Actually Mike if you look on the first page, Michael R. says exactly this.
Anyway it's now a moot point. Although I didn't see the crash, if it's any consolation, I don't think it was a speed issue either, I'am leaning more toward the model itself is simply not capable of tight aerobatic manoeuvres.
Yeah, i found some posts on rc groups where guys reported losing their planes in the same way. The consensus its that there's most likely a design issue with the plane. I've been less than impressed with the manufacturer of this model lately, so I think I'm going to avoid them for a little bit.
I hear that the phase 3 F16 will easily accept the gear from the tiger shark. The airframe is well proven app i might give that one a try. Anyone know of a local shop that Carries this line?
Sorry fir the loss. It did always look great in the air.
I do like the caption at the bottom of the photo that reads "frightening scenes".
Oh, now I see the problem! It's made of styro-foam! ;D
Robert
Quote from: Ededge2002 on August 22, 2012, 09:05:50 AM
Sorry fir the loss. It did always look great in the air.
I do like the caption at the bottom of the photo that reads "frightening scenes".
Thanks
I thought that it was fitting. Made a nice photo.
Yeah, it was a nice looking plane. I guess when a model comes with a display stand in the kit, you should consider the designer had something other than flight characteristics in mind when he drew it up.
Ive heard good things about the phase 3 F16, but i don't think it has rudder control so you may be more limited with aerobatics, also check on the EDF size it takes.
If you want to move up from Foam, look into the HET F16, its about the same size but a lot more performance.