cactusflyer

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OK folks, Here is a drawing of an all flying stab on my next project. Is there anybody out there that can help me determine the proper pivot point? .......Calling Bob Parks!!

Thanks in advance,

John

Ben Lanterman

About where you have it would be OK. Subsonically the ac of the stab is 25% of the mac. If your mac is correct then something a few percent ahead of the 25% point would allow the stab to be stable and not load up the servo too much.

Aft of the 25% point would make the loads unstable and make it difficult to achieve a setting - the stab would go from one tolerance extreme to another.

Too far forward of the 25% point would really be stable and the servo loads would be extremely high - not good.

The exact number of percent ahead of 25 with model servos in mind would be 2 or 3 percent as a starting place. The fuselage will have some effect, etc. I would look at an airplane successfully flying and not tearing up servos and copy his location.

cactusflyer

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Hi Ben!

I was thinking/hoping that you would chime in on this one! Questions like these make me miss the old MAC R/C Club think tank! I think that the pivot point is at around 20% mac. I thought this would work but I got askeered when I saw a picture of a Yellow Aircraft F-16 stab over in the Jets section with the pivot WAY up front. What about static mass balancing?

Tailwinds,

John

Bax

My Feedback: (11)

Hmmm. It all depends upon the speed of the model. Several gliders I have put the pivot point at about 30%40% of mean aerodynamic chord of the horixontal surface. No particutlar problems with stability or divergent behavior of the surface.

The DynaFlite Bird of Time has the pivot at about 35% or so.

Today's servos are plenty strong. Look at the deflections on 3D-type models! Some of them have so little fixed surface that they may as well be all-flying tails, and they're hinged at about the 0%-10% point (when you consider any aerodynamic balance area).

Tall Paul

Any distance aft of the 25% mac point is asking for a surface hard-over lock at worst, and lots of flutter in between.
A couple percent ahead of 25% adds a degree of slipstream fairing to the surface, without placing an undue load on the servo gears.
As many of these gears are made of congealed margarine, they'll strip at the slightest opportunity.

cactusflyer

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Sooooooooooo 20% is cool.....Right?

John

BMatthews

Yep. 20% is very conservative and definetly safe.

Also don't confuse aerodynamic balancing (the topic here) with mass balancing ( done with weights ). They are two different items. But having said that it's good to add some weight as far forward on the surface as possible to move the CG of the surface forward of the pivot point for very high speed flying. With the pivot ahead of the MAC and the CG on or slightly ahead of the pivot I don't think flutter would ever happen shy of the transonic region.

Mike James

Just to reinforce the above opinions...

I did some research last year on this, before designing my "Eliminator 2" sport jet. ( http://www.nextcraft.com/j47_sportjet01.html )

I agree that 25% MAC, or a tad ahead, is the most often used position, and this proves that it works. Bob Violett, Yellow Aircraft, and a bunch of others do it that way, on planes that have proven qualities, so we know it works.

Hey, show us some photos of the rest of the plane !

Ben Lanterman

Bax are you thinking of the hinge point of the elevator as opposed to the pivot point of an all flying stabilizer. They are different things.

cactusflyer

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Mike I would show you some pictures, but then I would have to delete your posts!!

John

Rotaryphile

I have used flying stabs, and about 20% MAC is about right - any farther back tends to get you into trouble with oscillation now and then - sorta a low frequency flutter, and any farther back than 25% can produce a model that flatly refuses to trim in pitch.