Deano642

I have a Goldberg Bucker Jungmann Bipe...Flies awesome EXCEPT it keeps snap rolling out of the top of loops. I have the elevator throw set to the published LOW rate and it still does it.. ANY IDEAS how to fix this???

Thanks

wellss

CG is too far aft if it snaps on low rate. You can reduce the elevator throw a bit more and/or move the CG a little forward.

mesae

If you like the way it flies otherwise, just use less elevator at the top of the loop (before you adjust the CG). This is a somewhat common rookie mistake in full-scale Decathlons, Great Lakes Biplanes, Stearmans, and other relatively low-powered aerobatic airplanes. In fact, every now and then a rookie in training will pop into a spin from the top of a loop if he is really ham-fisted. Good fun!

Since gravity is working to tighten the loop, AND airspeed is relatively low, much less elevator deflection is required around the top of a loop than around the bottom. When I used to compete in Decathlons, I pulled 4-1/2 to 5 g and 140+ mph initially, but floated across the top at about +0.5 g and 50 - 60 mph. That's one-half g positive. That's a big difference. I suspect your model has a bit more power relative to a full-scale Decathlon and doesn't lose as much speed in proportion but the principle is the same.

I like those Bucker Jungmann/Jungmeister bipes. Enjoy.

JPMacG

My Feedback: (2)

Give it full power as you enter the loop and keep the power on full until you're past the top. Make the loop bigger if necessary.

As mesae says, stalling out of the top of a loop is a common full scale mistake. It gave me lots of practice in recovering from stalls and spins. That was when I had a younger stomach.

Deano642

Thanks for the help everyone!! I think I will try moving the CG fwd a little to see if that makes a difference. I only mentioned that it snaps out of loops, but it will do it from any attitude with full nose up elevator and slow enough airspeed. Now I know you are probably thinking..."well no dah!" At a slow enough airspeed the wing will stall and depending on a lot of different variables, you may or may not snap roll. The problem here is that this occurs at a relatively low elevator throw (published LOW rate) and happens all over the flight envelope...making the model less fun to fly because its somewhat unpredictable.

All good suggestions so far...I will try the CG and less elevator throw.....Any more suggestions???

Flypaper 2

I bit of up aileron, maybe 1/8 in. may help. Acts like washout. I would back off the el. throw till you get it ironed out too. A panic pullout at low altitude will put you upside down it the turf real quick. Been there

T.W.

Hi Deano642. You may want to adjust things in order to move the balance point further forward before toning-down your elevator. Tail-heavy needs less elevator, but if you're nose-heavy you're probably going to need the elevator setting you have at the moment. Try one thing at a time is my motto when adjusting the balance

Good Luck, Tony

rmh

That model --the Goldberg --is a scaled down version of the Buckers I did for the TOC years back - really a very nice model
Like all of the aerobatic stuff -especially the old bipes -- the performance was always based on power - always
They easily ran out of power quickly -yours is apparantly doing the same thing -and- as speed drops - ANY trim setting responds differently -
All the improvements in aerobatics are based on power improvements
you will never see one where engine sized is reduced -except- for basic club aerobatic craft
My guess is that you simply can't maintain speed thru the loop size chosen - and so - as noted by others - you stall out
The idea is NOT more speed - simply more available thrust from any speed

First aerobatics were loops -where max speed was first established - to just get thru a loop.
newer and current high performance craft - can do a loop form any attitude and gain or maintain speed (which can actually be an almost constant G loop.)
The Bucker never could do this.
you will have to learn to fly it thru the loops and low speed maneuvers - correcting inputs as needed.
as a side note --the original model Buckers flown by Dave Patrick of Goldberg Models, that the Goldberg was scaled from -- were quite large - I built the fuselages in two pieces -they were too large to move in the workroom - the 4 cyl engine power available for was really not as much as needed either - which made maneuvering more difficult than the smaller ones which had higher power to weight setups .
power to weight is everything in unlimited aerobatics .

Globemaster3c17

Slow speed truely has nothing to do with stalls. The one, and only requirement to stall is exceeding the critical angle of attack, which can occur in any condition, at any speed, and at any attitude. Moving your CG forward will reduce the elevator's effective authority to some extent, and that'd probably be a good way to solve your snap problem. BTW, use your left thumb more. The difference between a simple stall and a snap is whether or not the plane is coordinated.

mesae

This is of course true and indisputable and I in no way disagree with you. I would like to add that at the top of a loop, where the airspeed is low and the power and slipstream gradient are near maximum, and therefore the elevator effectiveness is high, it is very easy to exceed the critical angle of attack and stall, if one doesn't ease off the elevator pressure as one climbs toward the top of the loop. It is even possible to float across the top of a loop (not necessarily a competition loop) at less than the 1 g stall speed at less than one g. Naturally in this condition, simply trying to level off will cause an immediate negative stall.

One of the excercises I gave my aerobatic students was a series of 45 degree up and down lines (sawtooth) with 90 degrees of alternating positive and negative pitch change. If done properly, on the downward-pitching portion, the airspeed indicator would indicate well below the normal stall speed, yet the wing wasn't stalled, since it was generating well under one g of lift and the stalling AOA wasn't exceeded. The plane followed a nearly parabolic path with the accelerometer indicating near zero g until the downline was re-established. With my very advanced students I would have them start this maneuver inverted with a push. Great fun!

This brings to mind the reason I don't like the term stall-turn. In a properly executed hammerhead turn, the airspeed is (or should be) below "stall" speed (the 1 g stall speed), but the wings are generating zero lift, (part of the inside wing might even be flying backward), yet the wings are not stalled. If the airplane does stall, it means the pilot really botched it up, an inverted spin will likely begin and the maneuver will be given a zero.

rmh

Hmmm -a sharpshooting reply that ignores half the statement - grrrrr
Dear Globemeister --
I thot I noted that as speed drops , trim settings RESPONSES react differently .
moving cg around has no bearing on this .
This a common error on the part of those are new at doing loops .

I feel I can say with some conviction that the trim must change as the loop progresses
Slow speed does cause stalls
once the speed is mismatched to the trim setting.
(Libby insisted I comment )-

mesae

Globe's statement is true. The problem (the stall) the OP is having is caused by exceeding the critical angle of attack. The rolling portion of the problem is caused by exceeding the critical AOA with the "ball" not centered (the resultant lift vector not perpendicular to the lateral axis). This can be solved in several ways; use less elevator (don't pull so hard), dial in some more expo, reduce the elevator throw, or move the CG forward, and always use just the right amount of rudder (there is far more to this than most modelers realize).

I like to set low rate so that full elevator gives the best precision snap rolls - and no more than elevator that. High rate is only for beyond-stall AOA and low speed stuff (otherwise known by the wholly inadequate description, "3D")). Snaps are my only criterion for setting low rate throw. Beyond that I set expo to get approximately linear movement at the control surface per stick deflection, and then I learn the airplane. My Giles will snap all over the place at any speed if I pull too hard, and I like that. So I don't pull too hard.

I would like to make the point that there is nothing wrong with the airplane for showing this characteristic. Most biplanes are aerobatic and snaps are part of aerobatics. It's up to the pilot to make sure snaps happen only when he/she wants them to. I prefer a snappier airplane because that usually means intentional snaps are clean and easy to enter and exit.

rmh

Im too touchy anymore - his first two sentences were true - sure
just obvious
My point about trim setting -I thought -covered all of that
Personally -I never set the controls for any maneuvers - just fly em
my models are usually set with 40-45 degrees throw at all times - I just add inputs as reqd.
the expos are 70% which takes the twichyness out.
I want my models to snap -anytime I hit the inputs for a snap
You can't swap around from smooth flying to departure maneuvers -unless you use a similar setup

But- there are a lot of modelers and full scale guys who simply are not into departure maneuvers such as the various tumbles that go into flat spins etc..
I'll bow out -

mesae

Nothing wrong with your setup. Just personal preference. My method does present the "difficulty" of switching modes, but with flight modes, it's only one switch - two modes (of three available). Not too bad. If I had to use individual rate switches, it would be that many times worse.