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Mar 18, 2021, 10:20 PM
F3Foamie Pilot
Slopemaster's Avatar
I had similar issues with a 90" coroplast wing I designed a few years ago. Flimsy elevons were one of the problems. I think I'm a pretty good builder, but not a good designer.

https://www.rcgroups.com/forums/show...ast-Slope-Wing
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Mar 19, 2021, 02:36 PM
Registered User
Certainly Davecee is right. It is the same reason that most airliners use the inboard ailerons at cruise speed, as the tip ailerons going high will induce a pitch up of the wing tip, causing roll reversal. This has nothing to do with adverse yaw.
Increase the torsional rigidity of the wing is the only right solution.
Mar 19, 2021, 02:48 PM
the occasional flyer
rothlisburger's Avatar
...Well, and remove any slop in the linkages.

-David
Mar 19, 2021, 03:36 PM
Registered User
Watching the video it appears that the left elevon is deflected up (for a left turn) while the right elevon doesn’t appear to be deflected down. If this is an attempt to reduce adverse yaw, that is the issue. The down deflected elevon creates a stronger rolling moment than the up deflected elevon. I’ve had the same result on aileron ships when too much differential is applied.
Mar 19, 2021, 03:45 PM
Registered User
Davecee nailed it the wing needs Stiffness but I'm not sure that just tape is going to be enough.
My advice is to cut open the bottom of the wing and add a spar ( or two ) . Cheap Fising rod Blanks Fiberglass tubes . Either of tases will add a lot of Strength and not enough weight to be an issue. Get out your Gloves and Apron and buy a big Bottle of Gorilla Glue AKA Polyurethane Glue . It Foams up and is very stick stuff to work with , but its the best thing to use with foam.
Mar 19, 2021, 03:48 PM
David Cairns
dcairns's Avatar
Or you could do it like the B-52 does, and dispense with ailerons and just have spoilers to achieve roll control. But the B-52 does use spoilers for the same reason. The wings are kinda twisty.
Although, if you did this to your plane, I sincerely doubt that would make for a fun to fly plane . But a big thumbs up for thinking BIG in scratch building
Mar 19, 2021, 04:54 PM
Registered User

Rigidity


This creation has all the aerodynamic rigidity of one of my childhood brown paper & bamboo kites ! It was determined to fly right from the get go - a good dollop of lead on the left wingtip would restore lateral balance.
Mar 19, 2021, 07:50 PM
Registered User

Maiden Flight


It wasn't always this bad. This is the third flight on the first day. By then I knew the aileron control was iffy, so I flew gently. Crashed anyway. But spectacularly easy to see, both in the air and in among the bushes after a crash.

After this flight I introduced asymmetric ailerons and doubled the outer control surfaces. As we've all seen, it didn't help...

Maiden with poor aileron control (4 min 14 sec)
Mar 19, 2021, 10:34 PM
aka: A.Roger Wilfong
gnofliwr's Avatar
As has been mention it needs to be stiffer. The tip to root flapping isn't the real problem it is the twisting that is the problem. The observation of the up eleven causing the tip to twist to a higher angle of attack is the big clue.

If you're going to attack the problem with tape, apply linear, fiberglass strapping tape at 45° angles to the spare, along the top and bottom of the wing. Span wise strips may help with the flapping, but will do nothing for the twisting.

The better solution would be to stiffen it internally with a D-box LE, with vertical shear webbing between the spars, and add a secondary I-beam spar 1/4 to 1/3 of the cord in front of the TE. And half ribs running diagonally between the for an aft spars.

The wing on the B-52 did most of its roll with top surface spoiler instead of conventional ailerons. You coul try that - it would eliminate the wing tip warping caused by the ailerons.

Ultimately, you may need to abandon a pure wing and add tip plates or other vertical fin(s) in the center or along the TE.

- Roger
Mar 20, 2021, 03:38 AM
Just Toss It !!!
MATIN's Avatar
I can't say with certainty - because I can't see the planform of the wing properly- but it seems to me that the designer has chosen, for whatever reason (most likely arbitrarily) a wing sweep angle that places this airframe in a gray zone between the two flying wing types of "Chevron" (more sweep angle) and "Plank" (less sweep angle)..... A little more sweep and it'll be a chevron,.... A little less sweep and it will be a plank!

I won't go into the differences between the types here .... but for aerodynamic reasons, chevrons work well with tip fins, whereas planks usually require a central fin for directions stability.

Basically a plank, which has a smaller sweep angle, has a hard time figuring out in which direction it should head and needs a central fin to tell it which way if "Front".
You can see this in the last few seconds when the pilot loses the ability to drive the wing in the direction he desires.
It is not that the wing can't roll effectively albeit slowly (it would definitely do much better with full-span ailerons with a larger chord dimension) ....it is that it cant keep a heading because it doesn't have the aid of a fin.

Can a plank fly with only tip fins ? ....Yes, but not comfrotably.
The later Harris Nelson -the designer of the NCFM Halfpipe - did extensive testing with tip fins of all sizes, shape, geometries and angles, on the Halfpipe prototype .... The prototype didn't like any of them.
At the end a central fin was the solution.
Owners of Halfpipes can tell you that with the central fin knocked out, the model can't really fly, because it doesn't really know which way is "Front".

And as to the stiffness of the wings : I have seen floppier wings fly very competently.
Those who have seen Kevin Bennette's Dumbo fly in Australia know what I'm talking about....That beast is the floppiest wing ever!

So will stiffening the wing help this model fly properly? .....Yes, but not before you fit a central fin and larger-chord full-span ailerons.

Can larger tip fins help?.... Maybe!
As I said, the model is in between two types ....It needs to be pushed towards one or the other!
Last edited by MATIN; Mar 20, 2021 at 03:56 AM.
Mar 20, 2021, 07:51 AM
Robots... robots everywhere
minifly's Avatar
Quote:
Originally Posted by jhbe
It wasn't always this bad. This is the third flight on the first day. By then I knew the aileron control was iffy, so I flew gently. Crashed anyway. But spectacularly easy to see, both in the air and in among the bushes after a crash.

After this flight I introduced asymmetric ailerons and doubled the outer control surfaces. As we've all seen, it didn't help...

https://youtu.be/SPID6cAuQGc
Yep, as said may times you have torsional flex which is negating your control input. Look at 45s in the clip, I assume you're giving right aileron here (I would be), and you can see the right tip pointing way upwards.
Mar 20, 2021, 09:06 AM
“Critical Moments”
Sky Angels's Avatar
Way to go man! I love the trying aspect. You are out doing it! Lots of good feedback.

This is how it all starts.

Good on you.
Mar 20, 2021, 02:51 PM
Registered User
Quote:
Originally Posted by MATIN
I can't say with certainty - because I can't see the planform of the wing properly- but it seems to me that the designer has chosen, for whatever reason (most likely arbitrarily) a wing sweep angle...
You are correct, it was arbitrary. It's meant to be a swept wing. Root chord 1m, tip chord 0.5m and the tip front corner is 1m behind the root front.

How much sweep would I need to be firmly in the swept wing domain?
Mar 20, 2021, 03:21 PM
An itch?. Scratch build.
eflightray's Avatar
A good few years back, (probably 25 years ago), I built and flew a fairly large wing, It flew great off a bungee and line. It was a very stiff built up balsa build around 4 1/4 Lbs.

It certainly needed a bit of speed for any tight-ish turns, but could drift around slowly with gentle turns.

Not having any descent slopes nearby, I never got to try soaring it I ended up adding a power pod to make life a little easier. that much line and bungee was not very dog walker friendly where I flew.

I could easily imagine on a slope they could be a bit of a handful unless the speed was kept up and the control surfaces had no slop.

The model was the The Hump by Frank Moeller, plans are still available via Sarik Hobbies.

Ray.
Mar 20, 2021, 05:23 PM
Just Toss It !!!
MATIN's Avatar
Quote:
Originally Posted by jhbe
You are correct, it was arbitrary. It's meant to be a swept wing. Root chord 1m, tip chord 0.5m and the tip front corner is 1m behind the root front.

How much sweep would I need to be firmly in the swept wing domain?
If that sweep is increased another 15 cm per side, the wing will be squarely in the chevron domain.
You will then need taller tip fins ( more area) and wider full-span ailerons to boot.

If you don't want tip fins, start working on a design for clam shell ailerons.
Once you conquer that mountain, I have a friend in Armidale who will help you come up with the yaw control software for your pure wing!

I probably won't be allowed in this September, but I hope to make it to Victor Harbor next January ...... It'll be nice to see it fly in person.
Last edited by MATIN; Mar 20, 2021 at 05:40 PM.


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