|May 11, 2012, 05:51 PM|
Valid Experiment ?
I'm thinking of building a " Wing " for ALES. I kinda like the looks of the Bernard Gross Flying wing from the 1940's and would like to build something along those lines.
And as it stands, I have more questions then answers concerning the dynamics of this type of wing.
So I built this quick and dirty experiment . It's made of MPF foam and has a KFm # 4 airfoil. ( prefer a real airfoil, but this is slightly better then a flat plate ).
There is no "twist " in the wing. Instead I used a fence at the 65% span of each wing. My hopes were to eliminate the dreaded "Death Spiral" And I've never used an inboard fin. Also I wanted to see what those elevons that are larger at the tips would do.
First I built the wing with a straight taper and set the CG at 15%. It took a lot of up elevator to keep it level! A quick circle and semi crash landing and the battery was moved back for a CG of 20% and things were much better! Later 21 to 22 % seemed like the sweet spot for me.
I climbed to about 80 to 100 feet and started applying up elevator until it started mushing down ( high rate of decent) with the nose up but could not get a real "break" in the stall. Making turns in this condition does not make a wing drop. I did this several times, holding full up ( 45 degrees on the elevator) all the way to the ground then climb back up and do it again. No death spiral.
So my question is if "fences" are put on a wing does it need to be twisted?
The KFm airfoil is not the best performer for gliding flight ( kinda like a streamlined rock!) Would a real airfoil perform differently as far as the function of the fence goes?
The Fin size was a TLAR kind of thing, So I cut quite a bit off of them and added triangles to the outboard elevons. The extended elevons did increase its roll rate. It didn't seem to know that it had smaller fins.
Any input or suggestions much appericated!
|May 11, 2012, 05:56 PM|
The wing is "twisted."
When you put in "UP" elevon you are introducing twist. The more "UP" you introduce - the more actual twist effect the wing has at that point.
The wing's behavior sounds as if it has a fairly significant nose heavy CG location.
|May 11, 2012, 06:34 PM|
Since the elevon is nearly the whole wing, you are creating a reflexed airfoil with up elevator input. With a swept wing, having wing twist with elevons at the tips, can improve efficiency. A flying plank doesn't need 10 degrees of wing twist, it just needs a reflexed airfoil, full width elevons and fins. Your plane is kinda right in the middle.
45 degrees of up elevon is too much. Try moving the CG back until the death spiral shows up, then inch up from there.
|May 11, 2012, 07:46 PM|
All right ,I'll buy that the wing is twisted. So I should start moving the CG back.
45 degrees of up is when I'm trying to stall it , not when it's normally flying. Unfortunately I dismantled this ,thinking that I didn't need it anymore! So I'll put it back together and move the CG back.
Thanks for the replies!
|May 11, 2012, 09:36 PM|
OK ,I slapped it back together. Then I moved the battery back 1 inch!! A quick check with Compufoil said the CG was about 29% MAC. I wasn't afraid of destroying the thing, so didn't want to take the time to move the CG back in increments, but move it forward if needed.
At 29% it was difficult to control, and although will drop a wing it recovers quickly. Then I moved the battery forward an 1/4 inch at a time. As the CG moved forward it handled better, then when it got back to about 22% it flies like I want it to. I still can't get it to spiral down in a deep stall.
I think that the fences are helping here.
So another question. If some of the wing is used for stability, how do you figure wing loading? Do you count the total wing area of just part of it?
And is the A/R the same?
|May 12, 2012, 09:18 AM|
Tim - From my experience flying swept wings with fins, a SM of 3% is about the aft limit of good flying qualities. I would probably use 4-5% but SM is just a starting point anyway. You find where you really want it by flying it. The elevon extension probably move the NP back somewhat so your actual calculated SM may be larger than just 3%
Good question about wing loading and aspect ratio. On a flying wing with a positive static margin the outer part of the wing carries a bit of a down load to compensate. That makes the effective wing loading higher than Weight/Area. How much depends on how large the SM is and some other factors. I've never seen a mathematical approach to figuring it out.
As to calculated AR - it would be the same = Span^2/Area. However in some flight conditions the way the lift is distributed on twisted wings, the effective AR can be actually a bit higher than the calculated value. Also, fins at the wing tips can have an influence on effective AR.
Your performance issues may just relate to the relatively low AR, low - or now greater than 1.0 taper ratio and the stepped airfoil. As to the fences - yes they should help with the effectiveness of the outer part of the wing - delaying stall and improving control at high AOA.
|May 12, 2012, 10:58 AM|
Thanks HerkS !
I'm not a big fan of the stepped airfoil. But in the past on non swept wings they do seem to help the L/D over flat plate airfoils.
In my mind the fences are worth the effort to put on. They help to make this ,at high AOA a pretty good parachute!
While this has answered a couple of questions, it has raised many more for me. Guess there are no short cuts!
Well now I'll dismantle it again and give it to the kids down the street, they like playing with my old foamies!
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