HO-229

Is uncommanded pitch oscillations (hunting) a characteristic of a wing divergence?

da Rock

Define what divergence is and you might get an answer quicker.

readyturn

My Feedback: (9)

The charactistic of longitudinal instability is genrally due to the cg being to far aft. You need to check where it is and move it forward 2 or 3 % MAC.

Cheers

HO-229

My observations of the aircraft are as follows:
* the aircraft shows pitch oscillations (uncommanded).
• The CG seems to be to far forward, as the commanded pitch up inputs seem to be dampened excessively.
• The pitch down inputs seems very responsive.

Observations of the canard inputs are as follows:
o Large canard inputs can be seen, pitch up poor to say it best (slow to excecute/dampened)
o (Canard stalling maybe?), but I believe the pitch down would be poor also…
o Is the aircrafts balance optimized, I would say no as I/we have not found the sweet spot yet.

Regards

da Rock

Would you be so kind as to share with us what aircraft you're observing?

You really should take the time to more fully describe and explain what you're talking about and what you'd like to know.

HO-229

here you go
This aircraft has 12+ flights on it & the I/we have been moving the CG aft in an effort to make it more interesting to fly (U-line). Proof of cocept prior to RC build
The attached pic is of the spar that I have added to get rid of divergence if it in fact is the issue. The aircraft experianced a mechanical failure yesterday so it was a good time to add the spar

Lou Crane

I wish the photo had shown the model before the 'incident' ...

It is possible that a 'soft' material for the wings could be bending (aeroelasticity) from lift loads. With a forward sweep, the tips would more likely flex to a higher angle of attack if structure isn't sturdy enough to prevent it. That could be part of your observations...

Your drawing shows a few reference points: NP and CG I recognize for sure. So, I presume you've done the calculations or construction lines to find Mean Aero Chord length and its quarter chord location relative to the masses and other surfaces?

If you have guidance for a simple canard layout, you could rough out the design with an equivalent rectangular mainplane and canard foreplane, then locate your forward swept planform to be aerodynamically the same or very similar. (If you've done this, before asking help, apologies...)

Another idea might be to cut up a cardboard hand glider to check for a stable CG. The back cardboard on letter or legal size pads isn't to far off... Or cut such a test piece of sheet balsa. ( You'd need some vertical surface for yaw stability. ...possibly also some dihedral.)

But to the initial question, surprisingly, there shouldn't be a divergence due to forward sweep. Particularly in pitch motion and control response. You'd expect there'd be a negative dihedral effect in yaw - when the model yaws to one side, the span length presented to the air on that side increases, so why doesn't that make the yaw get worse? The opposite effect - with a swept back planform - IS said to have some dihedral-like stabilizing effect. Apparently, both influences are slight enough that when the masses and areas are good, either sweep causes no problem.

HO-229

I think the powerful dihedral effect in yaw is caused by Asymmetric vortex/spanwise flow... Unlike sweep back planforms the spanwise flow & the canard vortex is moving inboard.
(Binding flow)

Allot of reseach has been done on vortex flow. A very good example is the F-18., The F-18 uses a huge LERX to excite vortex flow & Bind spanwise flow across the main wing & tail
The great thing about having a LERX is that the vortex gets even more excited at higher AOAs (better control) When I was on active duty we did the 100% Lerx testing on the AV8B her at Pax & it was a noticable improvemnt in control at high AOA

I like to think that if you control the vortex you can achieve better contol but remember nothing is free in aero

iron eagel

Ho-229

Don't discount what Lou said "It is possible that a 'soft' material for the wings could be bending (aeroelasticity) from lift loads. With a forward sweep, the tips would more likely flex if structure isn't sturdy enough to prevent it."
I do know that this is a major problem with forward swept wings. and the rate of the divergence problem you are seeing, might not be vortex, but rather the mechanical resonance of your wing. I would think that you problem pitch up rather than pitch down would confirm that it may be related to loading of the wing structure.
That your pitch response is so different at +/- AOA is the one thing you said that made me think that the flex of your wing may be a issue.
Good luck with your design.

HO-229

Iron eagle,
I am with you brother...

I made two changes to the airframe & that is a big no-no when trying to find root cause problems, oh well it's only foam & an arrow shaft

The canard was just a single sheet of blue foam (not the best stall characteristics) now its 3 sheets sanded to an airfoil shape
Added a triangle spar to the bottom of the wing .015 thk (I did not want to have to do this as it adds allot of weight)
Ideally... I should have a compression spar on top of the wing but I am feeling lazy today

I have two other wings that will get a better designed spar (less weight) next go around

Anyway, this is what it looks like when its all together for anyone wanting a peek. I think it would be a great RC flyer as is but I will optimize it prior to building an RC version
The cord is 18% for anyone wanting to know... I only did this for divergence reasons. When I build this in RC I will most likely scale down the cord thickness a bit.

BMatthews

Pitch divergence won't come from the wing being swept forward other than in terms of how it affects the tail volume ratio. So I'm sort of inclined to suspect that flexing is more likely the issue. Strongly forward swept wings having been shown to have a tendency to naturally flex the tips to a higher angle of attack when loaded. The new spar layout looks like it should help control any flexing but for it to work as intended it really does need to have a matching set on the other side or else the foam will still flex almost as easily. Weight be darned. If you want to learn if the flex was the cause you'll need to match the spar layout on the other side. Let us know how the flight testing with the new setup goes.

HO-229

your a tough man Bmathews...

Ok... I will put the compression spar on it

Regards,

iron eagel

I like your design, hope all works well with it, if you see an improvement then you know you are on the right track.

da Rock

BTW, what're you doing for yaw control?

BMatthews

It's a controlline testbed model. So the lines are the fin....

HO, it's all about the tough love, man...

HO-229

It’s all back together, but my co-worker & U-line flier friend did not bring in his power plant & control assembly today. Normally we will fly the aircraft at lunch.

It was a fine day to fly so I am a little sad that we were not able to fly it.

U-line flight as simple as it is is quite interesting & unique.
I am amazed at what can be done with a U-line aircraft.
I am not a U-line flier & I only know the basics of U-line flight

SFW Aircraft data
Vertical stab area is slightly more than 16%
I have not weighed the airframe but it is a pig now with the spar, I am ok with the weight as long as the pitch oscillations go away
The power plant is a AXI 2217 & it zips it along at a good clip (fast)
The motor/prop assembly has 0 degrees X, Y, & Y axis thrust line angle & does not appear to need any at this time

Other than the pitch oscillations & the CG that is not in the correct place it fly’s well.

A question for all who are in the know ….
When an aircraft is in a hard maneuver be it pitch up or a stall & the audible tone of the prop chopping through the air changes what is this phenomenon called?
Is the prop cavitating on some level or is it Doppler shift?

Also, I want to thank Ed Mooran & everyone else who offered guidance on this project

If you don’t know Ed has made some wonderful design tools that he is more than happy to share

Regards
Pax River RC Flight test

da Rock

Lines usually do not provide adequate yaw stability with control line models.

And with any swept wing, yawing can contribute to "hunting" roll which if constrained by the lines may show up as hunting in pitch.

After having flown years and years of CL precision aerobatics, combat, and racing, hunting can often be a result of poor location of the leadouts. We started placing movable leadout locations in our stunters very many years ago. It was found that having the leadouts poorly placed had a number of different bad effects on our ability to fly the model level as well as fly it through insides or outsides.

The very fast models used in combat and racing and speed often could get by without yaw control, but the slower planes were helped quite a bit by rudders. And the result wasn't just for more pull on the handle. But we did pay close attention to the line sweep on combat wings (no rudders on most of them) and on rat racers (usually no rudders there either). Both those needed to accurately follow our control input and needed to fly level with some degree of dependability.

You ought to experiment with two things. Line rake and side area.
No predictions nor guarantees.
But both could affect your level flight pitch stability.
And neither will cost you much in time, effort, or money.

HO-229

THE END WE FOUND THE CG LIMIT

iron eagel

Ouch!
Sorry for the loss of you test unit, are you going to try again?

So it made a good lawn dart?

HO-229

Iron Eagle,

Yeah.... It's not as bad as it looks.

We found the aft CG limit just after release. The aircraft did not have enough speed to keep the lines tight as the left/inboard wing stalled & the plane came in.

I am a little mistified with this planform. control/performance Predictions & cg are not what I expected them to be. This aircraft is not well suited for Contol line flight

The plane is "very" fast & is well suited for RC as the power off performance is very good & predictable.

I'm not sure but I think the canard needs to be thicker & the leading edge well rounded.
When I Changing the canard to a thicker cord it helped control performance

I have a couple other things I am going to do to it when I convert it to RC (Vector thrust & elivons)

The vector thrust rig I have designed is really neet & weighs almost nothing

I think the below statement is one of my issues

"QUOTE"
Additionally, longitudinal stability is compromised by the fact that the main wing must be close to the c of g since net lift must act opposite to weight. Conversely directional stability was easier to achieve because the fin was placed well behind the c of g and was not required to produce any force under normal flight conditions.



It is now more obvious to me now why this aircraft behaves as it does.

FIX
With a trapezoidal half span wing & an outer sweep forward wing & the addition of a LERX the longitudinal stability should be promising/more predicable or I could just add a gyro in the longitudinal axis

The plane still hunts & any wind makes it drop or climb

iron eagel

Interesting, I am also working on a design that uses vectored thrust (oddball design in the scratch build and design forum). I am also using aerodynamic controls in case of power out type of conditions. My vectored thrust setup is still in the process of being designed right now I am going with 45 degree throws, I hope I am not going to far with the throws. I guess I will find out when I get the test model built. I have gotten as far as making templates for cutting the foam, and am now putting together my own wire cutter for the foam.

Your design shows a lot of promise, I still think you stability issues was more due to structure rather than design. I think the flex of the structure along with an aft COG was causing a lot of your problems. Keep in mind with any aerodynamic design the further aft the CG the less stable, and you had been moving it aft for some time. Also with CL keep in mind that your lead outs are going to effect performance so do not make a lot of changes before you try the RC version, because it will fly a lot different from the start.

Perhaps you should do a larger foam version for RC before scrapping you original design totally. Your thoughts regarding the thicker profile and rounded edges of the canard are valid, I would add that you might also want to set up 2 degrees positive incidence on you canard as compared to your main wing. Doing so will insure that the canard always stalls before your main wing and may help in your design. Da Rock, BMatthews, Ed Morman, and Cwesh could be of help.

HO-229

Ironeagle,
After a little research I have come to the conclusion that an elevator on the main wing of this platform should be explored. From what I have read in a few other threads “the RC jet folks” sometimes use the canard as a speed break & most if not all pitch control is done utilizing the main wing & vectoring.

iron eagel

That should work also although I like the no stall type of setup you can do with a canard. Thrust vectoring can also liven things up a bit also either way good luck with your design.

HO-229

IRONEAGLE,
Take a look at the following thread
"is it possible to make a rc plane like this???" authored by "speedbrake" #103
It seems this planform has this issue, other theads on the subject same-same

Enjoy

iron eagel

These forward swept wings designs have some issues, one thing I have though about is using a gyro for pitch stability. One along the lines of those used for heli yaw control should suffice. Keep in mind with forward sweep just a small change in AOA is going to cause a large change in your altitude. As far as the elevator response tie the canard to your rear wing elevons like the control line stunt guys do if you want to enhance control response. Although if I recall correctly you had thought about using vectored thrust as part of your control system.