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Nov 28, 2008, 04:07 AM
I plan to take over world
sayno2glo's Avatar
I dream of a multibumm that has adjustable CG. In the turn it would get nose up deflection from the elevator, after that or at the same time the cg would go back enough to destabilize the plane and then the ailerons would drop (in gyro? controlled fashion) to keep the turning flight path and creating high lift cambered profile. I'll let someone to test it for me...
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Nov 28, 2008, 05:21 PM
Caution:Makes sharp left turns
Troy's Avatar
How about outboard pitcherons? Keep elevators in the middle and you can have both pitch and roll with pitcherons. Would need some big-torque servos.
Nov 28, 2008, 06:53 PM
52 legs done, 53 next.
jjmouris's Avatar
Troy, pitcheron fuction does not increase the lift potential of the airfoil.

That is the problem here, we need high lift in the turns without creating a pitch down moment.

Joe
Nov 29, 2008, 04:23 AM
Registered User
WimH's Avatar
Quote:
Originally Posted by jjmouris

Lets say i wanted to use the same concept for an F5B model

Joe
Have you ever seen this one turn? Does > 40 legs (even does 40 with F5F power)

Nov 29, 2008, 06:52 AM
smokin electrons...
skyzking's Avatar
Wow, thats nice. How do they launch that thing with the prop at the back?
Nov 29, 2008, 07:08 AM
52 legs done, 53 next.
jjmouris's Avatar
Wim i have seen it but not first hand.

It still has a design where the outer half of the wing is not deploying any tricks to develop more lift. So in effect only half the wing is optimised during the turn if that makes sence.

Joe
Nov 29, 2008, 07:42 AM
Needs brain lubrication
biber's Avatar
Thread OP
Generally I don't see a lack of turning crispicy in the Multibumm and Mubu II, anyway.
Compared to other Limiteds in competitions, the Multibumm did very well.
If you'd wring more lift out of the wing as it is, you'd have to increase the AR to keep an optimum balance of induced drag to friction drag.
i don't see lift as an independent issue.
If you make it lifty you'd better push up the span and AR, otherwise the induced drag jumps in your face.
What's the benefit of a low turn radius, when you exit the turn with all airspeed bled off?
AR does make it harder to built and high lift devices do so aswell.
That makes it a building challenge.
Not impossible to come out ahead, but not assured, either.

My goals are a clean, light and simple airframe, rather.
An in flight adjustable CG (few mm will suffice) would be the gadget to start with, though I believe it's not necessary to have it.
Pitcheron is not easy to get aerodynamically clean and weight distribution and flutter issues have to be dealt with, aswell.

BTW, the low wingloading by the rules of F5F (as i understand it) favour a tailless design far more than other competition formulas.
By the requirement of a certain minimum wing area and the necessity to minimise wetted surface area, anything that's not a wing must be minimised (as the wing is the only thing that can't be minimised).
That's the card we are playing.

What else can we do?

Keep induced drag low in turns by span, winglets and optimum lift distribution.
Make sure the airfoil can deliver enough lift for that
(no need for enormous lift, aswell as airfoil Cd does not count as much as in straights, Cd_i is king).
Make use of all the lift potential of the already predetermined wing surface,
you don't want to have any part of the wing significantly less loaded than the reasonable maximum efficient lift.
No wetted surface ought to twiddle its thumbs in turns.

For the straights keep it clean, avoid excessive laminar separation bubbles and turbulent separation.
Be as laminar as possible while complying with the above.
Make sure the Cd_i is really zero at zero net lift (not every model does comply with this).
Make a well tracking plane, no corrective inputs should be necessary to correct a model developing its own ideas of a track to go.

The Cece in Wims picture has a very nice and clean fuselage pod and beautiful prop, that's what I like most in it.

biber
Nov 29, 2008, 10:00 AM
Hi biber,

Might be a dumb question for the discussion at hand, but what's the advantage of having 4 control surfaces (2 elevators + 2 ailerons) versus 2 surfaces (elevons) on a model like this? Elevons would require only 2 servos, ergo less weight, yes?

Jose
Nov 29, 2008, 12:14 PM
52 legs done, 53 next.
jjmouris's Avatar
Quote:
Make use of all the lift potential of the already predetermined wing surface,
you don't want to have any part of the wing significantly less loaded than the reasonable maximum efficient lift.
No wetted surface ought to twiddle its thumbs in turns.
That is what i am going on about. Trying to get the outer wing halves to do as much work as the inner halves.

I guess moving the C of G back will allow you then to also deploy the ailerons as flaps since the pitch up moment is guaranteed. Whould that be the idea?

Joe
Nov 29, 2008, 02:12 PM
Registered User
WimH's Avatar
Quote:
Originally Posted by skyzking
How do they launch that thing with the prop at the back?
Motor off... Goes wrong sometimes...
Nov 29, 2008, 08:01 PM
One Idiot is plenty...
Dbox's Avatar
Wim,this wing of Yours killing me for a long time.
It 's fasssssssssssst even when You holding it
Yuri.
Nov 30, 2008, 12:07 AM
Caution:Makes sharp left turns
Troy's Avatar
Quote:
Originally Posted by jjmouris
Troy, pitcheron fuction does not increase the lift potential of the airfoil.

That is the problem here, we need high lift in the turns without creating a pitch down moment.

Joe
But you are trying to change AOA as efficiently as possible without adding a ton of drag. If adding things like leading edge flaps create lift, it also changes the pitching moment countering the "power" of the elevators. My experiments with my Beach Bumm found that the power of the elevators to change AOA was super critical to the planform. I will eventually rebuild another to Biber's original plans (I fudged some layout specs) but the CG change seems to be a good one for adding more bounce in the turns.
Nov 30, 2008, 12:33 AM
One Idiot is plenty...
Dbox's Avatar

Biting the bullet


Carbon or glass Glass or carbon
Yuri.
Nov 30, 2008, 05:24 AM
Registered User
WimH's Avatar
Quote:
Originally Posted by Dbox
Wim,this wing of Yours killing me for a long time.
It 's fasssssssssssst even when You holding it
Yuri.
It's not mine... Joint effort by JF Ney and B.Bossmann. They fly it in F5F, and sometimes F5B since the new lipo rules arrived. They have been developing it for several years. Only saw it fly once on an Eurotour contest in Belgium and was amazed...

http://franz-josef-ney-online.de/f5f_modelle.htm
Nov 30, 2008, 07:09 AM
52 legs done, 53 next.
jjmouris's Avatar
Troy, changing the angle between inner wing and outer wing during the turn will not help either. In one direction it would help to create a higher pitch up moment but at the same time decrease the amount of lift produced by the outer wing half. Move things the other way and you end up with needing more elevator to counter the pitch down moment. No, not the solution i am looking for.

Having LE droop over the whole wing will increase the lift potential of the whole wing much like dropping the flaps and ailerons on a conventional wing. However LE devices tend to move the centre of presure foward if i am not mistaking.

The elevator/flaps are a nice feature to create the needed pitch up and some more lift. Guess we will have to live with the fact that there is a difference between the inner and outer wing during this stage of flight. Not that bad considering a conventional plane would have an elevator at the back with up deflection creating a lot of drag.

Yuri, what have you done!


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