Incidence...huh?

[Dwayne]

Just finished reading Al Rabes artical  in Control Line World and it is exelent!! Amazing stuff!! The thing that got  me though was this thing about using posative incidence on the H-stab! Now I've been building and flying c/l models on and off for more than 35 years, I've read just about every artical in Flying Models by Bob Hunt and I have back issues of Model Builder dating back to the 60's and have been a member here and SSW and the old Flightlines forum, and I have never heard of this before! Build it straight! That's what everyone says, over and over, build it straight!!! Now here I find out straight might not be the best way to go! Put some incidence in the H-stab! Why have I never heard of this before?!
My inquiring mind wants to know...lol

[Trostle]

It is possible that Al Rabe was one of the first, if not the first to write about positive incidence in his construction articles in the 70's as well as in his monthly column when he wrote for American Aircraft Modeler around 1973.  (His column appeared in 13 issues.)  Others have written about this from time in Stunt News, including an excellent article several years ago by Bob Whitely about Thngs That Work.  It has not been a closely held secret.

There are some that feel that for a "normally configured" stunt ship that a horizontal tail with zero incidence is preferred.  That becomes something that you almost need to check out for yourself for the way you build, fly and for the particular design you have.  However, there are some who feel that a bit of positive incidence is better than none at all, particularly within the accuracy of most builders, it is better to err on the side of positive incidence than incorrectly building in even a slight amount of negative incidence.

Keith Trostle

[Gordon Tarbell]

How many degrees above level should the leading edge of the horizontal stab  be? 1 to 3 degrees?

[don Burke]

I don't know if this has been mentioned before, but a slight negative in the h-stab might take care of the positive pitching moment caused by the drag of the stab.  Since in most designs the stab is place above the wing centerline.

[john e. holliday]

Reread the article, Al explains why.  Have fun,  DOC Holliday

[Jim Morris]

What I have never understood is when you build in pos incedence you then have neg incidence inverted. Plane isnt suppose to hunt with pos  but when plane is inverted it has neg,but still doesnt hunt.I try to build at 0 deg. Well you know what I mean.

[Bob Reeves]

Doesn't this have something to do with Gyro stuff and compensates more for the engine/prop than the airframe. My understanding is a little down thrust in the engine will more or less do the same thing.

[Al Rabe]

Down thrust in the engine.

Personally, I don't care for the idea of using down thrust in the engine as an alternative to incidence.  If I'm visualizing correctly, pointing the engine down will cause the wing to slightly increase its angle of attack, something like building in positive wing incidence to fly level.  When inverted, the upthrust will cause the wing to decrease its angle of attack, something like building in negative wing incidence.  Down thrust does work and is corrective , both upright and inverted.   I just don't like the thought of the effect these virtual wing incidence variations might have on maneuvering.

The incidence in the stab is there to simply null out the nose down moment which results from gyroscopic precession  inherent in tethered airplanes with props turning clockwise (viewed from the rear) flying in counter clockwise circles.  If the incidence is set correctly, the aerodynamic force from stab incidence should just offset the pitching moment of the prop, both upright and inverted, with neg liable effect on the angle of attack of the wing in level flight or maneuvering.

Al

Granted, a little down thrust isn't going to hurt much but why not just use a bit of stab incidence to neutralize the gyroscopic effects of the prop on a tethered airplane.

Al

[don Burke]

"What I have never understood is when you build in pos incedence you then have neg incidence inverted." 

  The incidence is compentsating for the drag of the horizonatl, upside down the drag of the horizontal is still in the same relation to the wing and CG, no different effect than upright.

[Trostle]

Don,

I think that Al Rabe was one of the first, if not the first to examine, try out and document the reasons for this positive incidence idea for the horizontal tail.  The optimum positive incidence for any stunt ship may vary depending on overall configuration and other factors as well.  It is simply one of those things that work.

Keith Trostle

[Dennis Adamisin]

Al's stuff obviously WORKS, and stab incedence has been part of his regimen for a very long time!  However one more of those perplexing things about incidence: when you build an airplane and it flies "crooked"; does it fly that way because it IS crooked - or does it fly crooked because it is actually STRAIGHT?   

That discussion ought to kill of a keg or two! 

[john e. holliday]

This is something like Byron Meriwether setting the stabs on the racing planes tilted.  The inboard tip is down.  He states the plane will fly level with the stab and the wing will be tilted to the outboard side even tho it is straight with the fuselage.  Never did it myself as I always depended on wing tip weight to keep plane  out on take off.  Have fun,  DOC Holliday

[don Burke]

Stab tilt definitely works as a turn trimming adjustment in Free Flights.  I never intentionally put it in a CL airplane, something to think about.

[Mark Scarborough]

I put int in my profiles to combat the inflight twisties,just a touch, enough to notice and no more. does it help, um sure,, who really knows, I know the stab on the GB Y is level in flight and its the best profile I have flown, is that the reason, probablynot, does it contribute,,, what doesnt?

[Dennis Adamisin]

Al is using incidence - not stab tilt.

[john e. holliday]

Stab tilt definitely works as a turn trimming adjustment in Free Flights.  I never intentionally put it in a CL airplane, something to think about.

Don, don't try it in a stunt/aerobatic plane.  It was was just for straight level flight as fast as you could make it go.  DOC Holliday

[Steve Helmick]

There was an article in SN a few years ago, by Dave Fitzgerald. He experimented with various airfoils and incidence angles on his Stargazer, and liked the sharp stab LE with very slight positive incidence in the stab, like a max of 1/2 degree. On a 3" stab chord, that would be around .025" under the stab LE.

I spoke about this with a local high ranking PAMPA official, and he said that he tried positive incidence angles in his own design, and came to the conclusion that it didn't help with the (different) airfoil he used on the tailplane, which he wasn't about to change.

Personally, I think the whole deal is about "cross controls". It sure works on FF's. A little right thrust, a little left rudder, some washout in the left wing, etc., makes it groove real well. That's my theory, anyway.  Steve