Building an SU27 from Bertella plans which use horiz. stab and traditional elevator. I want to build an all moving stab. Is there a standard for calculating where the pivot point should be-fore, aft, or mid-wing?

I would suggest just putting it at the 1/4 chord point unless you have a compelling reason to put it somewhere else.

Ken, I don't know the answer to this myself but I think that your 1/4C reply might be just a bit off-hand.

1/4C of what? Root chord? MAC Chord?

What about area proportions? Like 33% or 10% of area in front of pivot?

Just think for a bit laddie, and remember that the t/p of a SU27 has a fair old sweep to it, it is not a simple plank sticking out there.

Well, if you can manage to get the controls in there, the 1/4 MAC is what I am talking about.

What about area proportions? Like 33% or 10% of area in front of pivot?

Your mistaking this for a traditional elevator. wsmalley is asking about an all-moving stab... big difference.

I recommend the 1/4 MAC, but your going to need a pretty stiff area to put the pivot at (read THICK). By making it an all-moving surface your tail loads are going to go up quite a bit.

First, I'm no expert on this.

But, having followed a lot of the "Jets" stuff over the years, it seems to me that they usually place the pivot point more rearward, say at 40% to 60% chord.

The old Byron kits were that way, with the actuator being in front of the pivot, at about the 20% position.

These numbers are guesses, and I would talk to a more informed jet jockey before adopting my numbers.

Here are some images that might help.

Here's a photo of the stabs on the BVM F-100.

A full size jet would have a very good reason to put the pivot point more rearward... namely the shift in aerodynamic center at supersonic speeds. This would place the pivot point at 40 to 45% MAC for supersonic speeds.

I'm not intimately familiar with the SU27, so I don't know where the pivot point is on the full size aircraft.

Realize that for an all-moving stab, if you place your pivot point away from the 1/4 MAC, the AC of the stab will be moving around when you move the stab.... which may or may not make for some interesting control responses. If nothing else, it will put more stress on whatever torque tube you have the tail pivoting on.

Mike... I'm not sure what to make of the photos you posted, Can't quite see where the horiz stab is pivoting.

You're right, especially about the 2nd photo, Ken. I went back and substituted a better photo, of the BVM F-100 stabs.

Better Picture!

The pivot point looks to be at about the 50 or even 55% root chord, but with the swept tail plane, it "looks" like it may be at the 1/4 MAC... I don't suppose you can pull any numbers out of your hat Mike! (root chord, tip chord, sweep angle)

No, I just copied the image from the BVM site, and there weren't any 3-views or whatever there.

I think the best bet is to get the info from some people who REALLY know, like many of the guys in the "Jets" forum. If I was going to implement all-moving stabs on a design of mine, that's where I'd look for the answers.

wsmalley and Mike, I just found a very good thread here (in the Jet forum):

http://www.rcuniverse.com/showthread...ht=stab+moving

It seems to support my arguments... but I haven't finished reading through it yet D

Ken is correct. I couldn't bring up the tail link he mentioned but the photo of the BVM kit seems to put it at around the 25% mac of the tail.

What you need out of the horizontal tail is both static and dynamic stability about the pivot and small actuator loads.

Static stability is obtained by putting the pivot at the center of the tail load - the neutral point of the tail. That is 25%.

Dynamic stability which is resistance to flutter would need the tail loads biased a little aft of the pivot point (pretty sure I am remembering this correctly, it is morning and I have a headache).

Actuator loads require putting the pivot at the center of the tail load.

It would be hard to go wrong with something 23 to 25% of the tail mac.

50% mac would put a tremendous load on the actuator as would 10%.

I went to the Sullivan, MO Jet Flyin last Saturday. I was very impressed with the state of the art in jet modeling. Beautiful airplanes with beautiful workmanship. Flawless flying and the equipment worked all of the time. I'll have photos on the internet in a few days and will let you folks know where to look.

Hey, did we loose some posts here or what? I was just reading up on this thread when the change happened.

Anyway, I'm with Ben here, with the addition that once you locate the axle for the surface, it should balance by weight at that pivot for dynamic resistance to flutter. Right near the AC of the surface(25% MAC) is where the pivot goes, maybe a couple percent forward.

I have done a flying-stab conversion along these lines to one of the Goldberg Mig-27 foam target drones. Of course, it's not a fast-mover - but std servos are plenty to control a 70 sq in tailplane on a 10 lb slow moving plane like this.

I believe you're right John, we did loose 3 or 4. I don't see mine here any more and theres a couple of others gone too. The one with the MAC sketch lines are missing as well.

If the big guys didn't have their hands full already I'd raise it but I think we'll just let this one ride for now. Perhaps we can go post mining in a week or so when the dust from the big changeover settles.

25% OF MAC IS THE NOMINALLY CORRECT POINT. iN THE CASE OF SIGNIFICANT SWEEPBACK, MOVE THE PIVOT FORWARD A COUPLE OF PERCENT.
REGARDS,
JOHN.