Fishtailing Delta

[Paul Jefferies]

"If flutter is part of the problem my first fix should help."............

Yes, that may do the trick though it would be better if the T.E. of the rudder was square which would create a cleaner "break" for the airflow. Please let us know how it works.......

As for the linkage, flutter can be an incredibly powerful force and very difficult to stop with just good servos and linkages. If it really wants to flutter (IF that is the problem), it will and the only way to stop it is to change the condition that is causing it. If the 6mm tube doesn't stop it then try sticking a small weight at the T.E. and if it still fishtails then perhaps flutter is not the problem........

Paul

[Dave Bran]

Personally I think its unlikely to be either lack of rigidity induced flutter or loose linkages, but I live to be wrong.

Some End Fence only stabilised deltas have a frequently occuring and no real issue low rate wobble around the CG pivot, particularly on high speed wide turns and slower flight when in crosswind conditions.

An example that springs to mind was the end fenced FlyingWings Buzz, but not the more swept Buzz Lightning where the turning moment of the fin action was further back from the CG.

Easiest thing after checking slop would be to tape on some correx or stiff card end fences, can be fitted and removed in seconds with Blenderm.

In case anyone wants to know what approx size, shape etc, look at one of my own hack designs here:-

**LINK**

It is raw video so rough, but jump and freeze at 31 seconds in. If watching it all put the sound on and up as its gliding as well as under power.

Six years ago and still active, it now has another 250 Watts, and the launch "torque" roll is still there in spades if like on the Video you throttle up a little too enthusiastically.

[Dave Bran]

Ron, build one of these, great fun, and it redefines "wobble", you'll never worry about a little wobble again afterwards..............

**LINK**

[onetenor]

Slot in a forward underrfin app 4 long by about 1 high Don't glue in till happy

[ron evans]

Paul, thinking again about flutter there was no noise from the model at the time (the tractor motor is quite quiet), and I know what flutter sounds like after loosing half an all moving tailplane on a slope soarer in a high speed dive. So perhaps it's not flutter.

Dave, I don't think a model like that would be good for my ticker. My model is about 4yrs old and was mostly flown on our old site, which was huge and with no trees, so all wind directions could be accommodated, and I never noticed this affect. Our new site is quite small, runs E / W and is surrounded by trees, so last Sunday with a Southerly, passes up and down the strip were all crosswind, and it was a fellow clubmate who noticed the shimmy first.

Hi Onetenor, what's the thinking behind a forward underfin, most people seem to think I need more vertical area at the the back.

[Paul Jefferies]

Could it be "Dutch Roll" I wonder? Dutch roll is a phenomenon of some swept wing designs and is the reason that fullsized jets have Yaw Dampers. Basically, the aircraft yaws and when it does so, because of the swept wing, the airflow on one side is more across the wing whereas the airflow on the other side flows more along the span of the wing thus creating more lift on one side than the other..... So if it "skids" to the left, it will roll to the right until the whole thing is so out of line that the drag on the fin pulls it back. However in trying to get back to "balanced flight" it has to skid the other way and then it rolls to the left and so on........ in a fullsized aircraft this can build up to an oscillation of 30 degrees or more each way. It is very difficult for a mere human to stop it and hence they invented the yaw damper which is a gyro stabilized device which applies minute amounts of rudder whenever it detects the smallest amount of yaw. If this is what is causing your fishtailing then I think a forward underfin will only make it worse and I think all you can do is increase the size of the fin and/or increase the length of the fuselage by a couple of inches to get the fin further aft....... Perhaps an AFT underfin or big tailskid might do the trick........

Paul

[Paul Jefferies]

FWIW the early Boeing 707 had a bad tendency to Dutch Roll.......

I am not sure if it was due to our CAA or BOAC but before it flew in BOAC they added an underfin to try and reduce the Dutch Roll...... though it still had a Yaw Damper.

A similar underfin should not be too difficult to add to your model.

Paul

[Bob Cotsford]

I seem to recall reading that the P47 needed a dorsal strake adding to increase the fin area when they converted from razorbacks to bubbletop as the reduction in side area caused it to fishtail. Didn't they do the same with the Tempest? I'd just try adding a strake to the front of the fin as a quick test.

[ron evans]

Bob...Paul, more fin area seems to be the popular opinion, just have to try the thickened fin T/E first, if it's windy on the weekend.

I have a 30yr old slope delta which can be hovered nose high above the slope edge, and just a blip of aileron will set it off in a Dutch roll at quite a low frequency, but at high speed there's no sign of a yaw shimmy like the floormate.

Ron

[ron evans]

Flew the floormate today with the thickened TE, and it did seem smoother with just a momentary wiggle turning onto the crosswind leg. However, the wind was much lighter than last week so perhaps I'll not be able to make a fair judgement until I try it on a windy day.

Ron

[ron evans]

A short update on the floormate.

Had four flights on Sunday in between the showers, with TE thickening and without. The strong, gusting wind and turbulence was a bit much for a 20oz foam triangle, but once clear of the worst of the turbulence the model did fishtail less with the thickened rudder fitted.

So an improvement but not a compete cure, and I'll try Increasing fin area next.

Ron

[ron evans]

Hi all, just to wrap up the floormate saga, I can report that adding a temp 50% extra fin area has just about cured the yaw shimmy at speed. I'll now make the fix permanent and perhaps try the thickened TE both on and off.

As a welcomed side affect, the extra fin area has tamed a strong torque roll tendency if power was applied suddenly with the model flying slowly.

Thanks all

Ron

[Former Member]

[This posting has been removed]