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Old Apr 12, 2006, 07:22 PM
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Multibumm

In my last entry I mentioned a F5D Limited Wing.
That thing will be named 'Multibumm'.
And it's going to look like pictured below.
The other pics show what it looks like about now.

The special gimmick in this design is its distribution and management of control surfaces.
It won't use a common elevon mix like most wings do.
The outer control surfaces are supposed to act as ailerons only and the more little inboard ones as elevator only.

That means e.g., to pull up the inboard controls are lowered some degrees.
On most common swept wing designs up elevator means that 50% or more of the airfoil's TE
is seriously reflexed during turns and pullups wich does not really match the increased lift situation.
The rest of the TE remains unchanged.
That is different with the Multibumm.
It will have 75% of the TE unchanged, 25% slightly cambered in the same situation.
No reflex to pull up.
That is possible due to the high sweep of 30 degrees combined with the quite high aspect ratio of 9.3 (without the winglets).

Hm, have to continue this tomorow, it's now late over here in Germany.

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Last edited by biber; Apr 13, 2006 at 04:26 AM. Reason: fixed broken pics
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Old Apr 13, 2006, 12:45 AM
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This looks pretty neat.

I'm somewhat a newb when it comes to aerodynamics... can you explain why the added reflex on a normal wing when using the elevators is bad?
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Old Apr 13, 2006, 05:06 AM
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Thanks for the compliment.

Commonly (not specifically for tailless wings) the main reason for
reflex on airfoils is to minimise the airfoil's drag for a low lift situation.
But that comes with a increased drag for high lift situations,
for wich again more camber would be an appropriate measure to minimise drag.
For a model with tail that means simply lower the TE if you want to go slow
and raise the TE to speed it up, to get the optimum out of the wing at any time.
And that is what all the F3Bs, F3Js, F3Fs and so on are doing all the time.
On a tailless wing the elevons are moving just the opposite direction to manouver,
wich is anything but ideal in terms of performace of the wing.
That is not the case with the Multibumm.
That is the reason, why I'm confident, that this wing
will be exceptional in its performance on the racing track.

I will certainly pic up that topic again in a future entry in the blog,
explaining the aerodynamics of this particular design.

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Old Apr 13, 2006, 10:04 PM
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I think I understand you. Hopefully my next question will test my understanding...

Where is the CG of the plane?

Would it be possible to put so much sweep into the wings such that your elevators actually wind up being in front of the CG point? Is that what you're sort of shooting for here?
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Old Apr 14, 2006, 06:29 AM
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For the aimed static margin of 5% the CG of the Plane is 9mm aft of the TE at the wing centre.
The elevators are just about on the CG,
but the wing portion in front of them is also affected by their deflection.
Actually a downward deflection of the elevator does not only increase the elevator's top vs bottom pressure difference,
but also does so on the parts of the wing direct upstreams, in front of the elevator.
As that wing portions are located in front of the CG, the plane pitches up when the elevator goes down.
We did tests on that principle, to make shure, that it works.
One thing was to have a buddy, who works at the TU Delft,
stuff the geometric data of the model into a paneling program
to verify the calculations we performed using the Ranis code
(wich itself is based on lifting line theory).
The other test was to build a simpel 2/3 rc model out of depron,
with lipos and Feigao, and get it airborn.
That finally convinced us to be on the right track.

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Old Apr 14, 2006, 10:03 PM
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Awesome.


I have some depron laying around that I was going to use for something else, but I think this looks more interesting...
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Last edited by GoatZilla; Apr 14, 2006 at 11:58 PM.
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Old Apr 15, 2006, 06:27 AM
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Put some more passion in yours than we did, if you are going to build one, ours was a real quick and dirty build.
After we found, that it flew quite well and was actually big fun to play with, we thougt it was a pitty that we built it that hasty.
Paul is still thinking about a complete rebuild with a sanded balsa wing.
But first we have to get the 'real' one airborn and win some races .

For (some) more details on the little one, see also:
http://www.rcgroups.com/forums/showthread.php?t=456839

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