Lifting body discussion

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gtex09

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Apr 13, 2010
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Hi,
Yes I also guess that the better way now is to make measurements on the model. The Re number does affect, but as the scale is already large, measures will be revelant enough..
Regarding my scale 1, the idea keep the same as in the PAV report. But I complexified it a bit, for example to be able to separate the "wings" or the rear part of the cockpit, in order to be able to manage it at "home garage" size. I'm preparing an internal steel pipe frame, which will be integrated in the foam plates, them selves covered both sides on their faces. Actually the idea of B.Wainfain is seducing, but expensive at my level, when you see the price of honeycomb panel....So, as the strength are kept low, I can handle with simple sandwiched foam.
Basically I'll keep 4 modules connected together. In the front cockpit, you'll get the metal frame with engine, landing gear, instrument panel and all command things. As well you'll get the connecting elements (pipes and interfaces) to attach both right and left wing, and the rear block.
Thank you for your interest. Any improvement idea is appreciated.
Regards.
:)
 

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lr27

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Nov 3, 2007
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Although it might make the wing attachment easier structurally, I guess it would be too tedious, even with a power screwdriver, to go with the DC-3 method:
http://farm4.static.flickr.com/3139/2487999141_2d9b883128.jpg?v=0
However, perhaps it's possible to do something similar with fewer fasteners. Or perhaps even flanges with a bump, and then a c-section piece to slide over. Assuming the stress is low enough. Might be easier to reach from inside, depending on the structure.

If steel tube (not pipe, that's too heavy) is carrying all the load, why not just use fabric covering and save weight?

If you wanted a relatively cheaper panel that would bear at least some loads, what about a sandwich with aircraft ply and foam? You could make it fairly weather resistant on the outside by using 1 oz. (34g/m^2) fiberglass cloth. Applied with care, the glass wouldn't add much to the weight. Once you start to look at it, though, the carbon isn't THAT much more. Less than double, I think.
 

Workhorse

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Mar 6, 2010
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Southern Spain
Hi Gtex. Do you think if a negative pitch at the wingtips may cause some structure stress?. Which is the C.G range % you have considered?, it is a total area % or MAC%?. Did you try a maximum aft C.G for stability and performance?.
 

gtex09

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Apr 13, 2010
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Ola Workhorse, que tal ?
This model (scale 2/5) has not yet been tested at its limits on aft or fore CG position....but that is inside the program.
The previous version, same type of model, but smaller (scale 1/4), wre showing a great range of CG position.
Actually the all surface is lifting, then the 6 m whole cord allows a absolute great range.
Regarding the tips, and if you develop the sections, you will see that all the sections are twisting from the center to the tip, giving an interesting flow circulation. This contributes to longitudinale stability, and is verified on tests (see GTx9-05 flights).
Cheers.
 

gtex09

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Apr 13, 2010
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France (Lille)
The tip angle is 10%, negative. But the lift at the tip is roughly zero. There is no additional stress. And aanyway the loading is very low....
I'm always considering the stability in % of MAC. Good results are given in our case with stability margin around 7%. We never went under 5%.....
Cheers GT
 

Workhorse

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Hola Gtex

Could you tell what are the numbers in the 1:1?, area, wing loading, h.p., wingspan. If you have a 25% MAC C.G. what would be the % area before and aft the C.G. whith this setting?.

Merci
 

lr27

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25 percent MAC is not correct for delta wings. They are different. According to Hoerner, the AC moves back as the aspect ratio goes down. Not only that, but apparently it depends on whether the l.e. is sharp or rounded. I kid you not.

BTW, I seem to recall that the Facetmobile had a 15 inch c.g. range.
 

gtex09

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Hi,
OK, to give your some more details, please look at the attached screen copies of simulation.
Sorry, it's in german, I'm still using the trial free version of FlzVortex.
You have :
Plan area = 25m²
Mass = 450kg ...so wing loading =18kg/m²
Wingspan 6m
Engine BMW 90hp, suitable for ultralight regulation

Cheers
 

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gtex09

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Messages
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Location
France (Lille)
Dear all,
Attached pls find 2 simulations, at high speed and low speed.
Note that, at low speed, the simulator doesn't integrate Polhamus vortex effect to increase lift at high angle of attack.
But as a first approximation, it is acceptable, since we observe the same good behaviour on the 2/5th scale model, when flying at very low speed....
Rgds
;)
 

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gtex09

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Apr 13, 2010
Messages
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Dear all,
Last weekend we let the wing fly again.
At this stage, we have three main conclusions :

1. The view from cockpit is OK, for take-off and for landing. Good.
http://www.youtube.com/user/gtex09#p/u/8/G4xJmscT36k

2. At high speed, for example when diving, we observe a lack of control on elevons. This has been also observed just after a diving, even in horizontal flight, but always at high speed. Some modifications will have to be performed.

http://www.youtube.com/user/gtex09#p/u/1/j2xurG6Ni1U
http://www.youtube.com/user/gtex09#p/u/0/Yvyg2HaAzec

3.
At low speed now : very good, very manoeuvrable and responsive. Good.
http://www.youtube.com/user/gtex09#p/u/16/1CuLzW1pMZs

Regards.
gtex09


 

rotax618

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Evans Head Australia
G'day Gtex09,
I found the same problem with reduced pitch control at speed, I believe that control surface blanking is a problem because of the aspect ratio and wing thickness - I believe that Hoffman encountered the same problem in the Arup aircraft and was eventually forced to place a tailplane/elevator on the top of the fin on the S4 model.
tailless aircraft in the USA

My next experiment was to have 2 fins with the pitch control surface on the top, between the fins, and as you have done, to extend the ailerons past the wingtips.

Cheers Tom C
 

gtex09

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France (Lille)
Hello Tom,
I hope every thing is fine for you.
It's interesting...but according to the tests, I guess this could be induced by the sharp edge very close to the extremity of the airfoil.
Among the factors, I believe that the length of the chord , and the shape of the airfoil, especially at the root, gives me a laminar /turbulent transition start point really far from trailing edge, then the turbulent flow should be big enough to perturbate the control on elevons.
As far as a prototype was born to be modified, I will operate and cut this sharp edge on the top skin, on both wings and also on the rear "fuselage". I attach a sketch to let you understand what I mean, and I will try it asap.
If it doesnot work, I'll then reconsider the thickness of the all wing and reduce it till something like 11%.
Of course I'll keep you informed and will post the videos.
Please give me your comments.:)
Cheers,
Gtex09
 

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rotax618

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G'day Gtex09,
I'm not so sure your mods will eliminate the problem, if you look closely at the photos of the Arup S2, (whose wing was both curved and relatively thin) you can see the very large elevator surfaces that were required. Hoffman fixed the problem by placing a much smaller pitch control surface above the wing on the S4.
Other very low aspect ratio aircraft have used a "Junkers" type elevator below the trailing edge to overcome the "dead air" problem.

Cheers Tom C
 

Norman

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G'day Gtex09,
I found the same problem with reduced pitch control at speed, I believe that control surface blanking is a problem because of the aspect ratio and wing thickness - I believe that Hoffman encountered the same problem in the Arup aircraft and was eventually forced to place a tailplane/elevator on the top of the fin on the S4 model.
tailless aircraft in the USA
I believe that the pitch down of the Arups was due to the airfoil pitching moment, not control surface blanketing or local separation.

Although separation on the lower surface is often a problem for reflexed airfoils I don't think that would have caused the diving tendency that I've heard the tailless Arups had. Flying wings that rely solely on the airfoil pitching moment for pitch trim have a very definite, built in, speed limit. Since AoA is controlled by adjusting the amount of reflex at some speed you will be flying with the elevators neutral with regard to the nominal airfoil profile. Attempting to trim for a higher speed will remove the reflex and create a negative pitching moment. Since by definition flying wings don't have an external pitch control surface there's no way to maintain level flight with a negative Cm.

The external horizontal stabilizer of the S4 allowed the plane to be trimmed for high speed without the wing's pitching moment going negative.
 

lr27

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Nov 3, 2007
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That doesn't make a whole lot of sense. You could make the same argument for a conventional airplane, but what it really means is that the c.g. is too far back. With models, we sometimes do a dive test to evaluate the c.g., and what we see is behavior like that you're describing.

A tail adding negative moment will sum up to the same thing as changing the reflex. Unless it breaks off, of course. ;-p
 

rotax618

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G'day, The Arups had an enviable safety record, none having ever crashed. Charles Lindbergh flew one of the Arups and said that it was a pleasure to fly. The performance envelope of the Arups, considering the 37HP engine was and still is remarkable.

Cheers Tom C
 

bradyaero

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Mar 3, 2010
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Canada
Wow that Arup is absolutely fantastic! What a great plane for its size!

 
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