Ok, what effect does moving the stab up or down have? I have heard lots of things but lots of them conflict. I have a pattern plane that I'm going to have to raise the wing on about an inch, so I was wondering if i should move the stab too. it's very low as it is.

I am talking about vertical position relative to the thrust line, not incidence.

Thanks,
-Mike

By raising the wing you have raised the center of drag of the whole plane a bit. This will increase the tendency to climb as the throttle is advanced and may require a bit more down thrust than you originally had inorder to compensate. Any other pitch effects are likely to be caused by inadvertantly changing the angle of the wing slightly when you move it. The downwash behind the wing has a large vertical dimension and the difference in down wash angle over the stab will be very, very small and unlikely to have a noticable affect.

Sorry Ollie, you're a little off on this one. I don't really follow pattern design that closely but a few years ago there was a series of columns in one of the magazines. One whole installment was devoted to the horizontal stab. I read it just out of curiosity and while I don't remember the meaty details I do remember that the positioning of the horizontal stab makes a big difference to knife edge flight and somethng else that I don't remember at this point. Perhaps it was the vertical tracking but I'm not sure here. There was one pattern model that did quite well that used an anhedral stab. The author (I think it was Dean Pappas) told of how he did it that way because it was too high and he didn't have time before a big contest to do it properley so he just hacked the stab in two and joined it with some anhedral. It worked and he and the model went on to be US champion or place well at the worlds or whatever.

Either way MHester needs to find this sort of article(s) and follow it.

BTW, a WHOLE INCH for the wing???? From that same series of articles I remember that even 1/4 inch makes a big difference.

Good luck with this and be sure to dig up those articles if no one here has the info.

Actually on modern pattern models stab height has little to no influence on KE flight. Changing the angle of the rudder hingeline is proven to be more effective at fixing KE pitch mixing problems.

Mike I would try to keep the stab above the wing if possible, I think you will run into mixing issues if the wing is above the stab. You could place the stab on the thrust line and the wing just slightly below it, and run with a 0-0 incidence setup, you should not have a problem with that.

hahum, there is a VERY famous pattern design with an anhedral stab, it's Hanno Prettner Curare. The "couple" won a certain number of world champs in the 70s, if my memeory serve me well. Hanno won everything for about 15 years......

Bernard

Well, I DID say that I didn't follow pattern very closely LOL

But there was something about stab height and some aspect of pattern flying....

Here is my take on knife edge flight. In knife edge flight the fuselage and rudder are producing the necessary lift equal and opposite to weight. The wings are producing no lift (operating at the zero lift angle of attack). The stab is no longer operating in the wash of the wing because the wash is zero. Therefore, the stab is operating at a different angle of attack than it would be in level flight. That difference will result in the need for elevator offset to keep the plane on a constant heading. Also, the couple formed by the center of drag and thrust line will be different than it was before the wing was raised. That difference will result in the need for additional down elevator or additional down thrust to keep the plane on a constant knife edge heading. In this analysis I don't see any forces associated with the stab position except those that are associated with the need for elevator angular offset.

What have I overlooked?

It has been a long standing thing that stab position does effect KE flight, however recently designers are moving away from this and focusing on rudder areas above and below the thrustline as it apparently proves to be more effective in reducing pitching in KE.

Ollie

I dont think you are missing anything, I agree with what you are saying, but it has been my experience with most of my pattern planes, that they pitch to the belly in KE flight when setup to fly a perfectly vertical downline. It is usually minor and I just mix it out.

Extra downthrust is not a problem, the original design needed a tad more anyway.

Why am I doing this? Am I nuts? Well, maybe. Truth is, the pattern plane that I want doesn't exist. So I'm tweaking a good design to make it a little better.

The original design had a straight trailing edge and a HUGE amount of wing and stab area. This made for a very stable plane, but it's snap and spin characteristics suffered. Elevator was also extremely sensitive.

I designed a new wing with a double taper, and raised it a bit. This plane had the wing set at 2 inches below the thrust line, and the stab one inch below. if I move the wing up an inch without moving the stab, they are both then one inch below.

The original design was very neutral in knife edge. Tapering the trailing edge reduces the dihedral effect somewhat, so I elected to raise the wing to bring it closer to center. It won't go any higher. I also dropped some of the area on the stab, it's now about 24% of the wing. (Was about 31%)

I admit it, I know just enough to get me into deep trouble. But all i am looking for is a good starting point and I'll cut and adjust from there. I feel confident that I'm on to something with this thing, just not sure about some of the particulars.

-Mike

The simplest solution to an over sensitive elevator is to reduce the elevator throw.

Reducing the horizontal tail area by 20 percent will move the neutral point of the the whole plane forward. This will require that you move the CG forward to achieve the same stability that you had before. The smaller horizontal tail will also result in a reduction in pitch oscillation damping so that the new configuration will not be quite as groovy in pitch. The pitch damping can be improved by about 24% over the original design by lengthening the tail moment arm by 20% even with the smaller horizontal tail. The location of the neutral point will be restored by the longer tail moment arm to compensate for the smaller tail and the original CG can be used.

The roll rate will not be affected by the changes in the tail length or area. The wing's stall characteristics and CG location have a lot to do with how the plane spins. A more tapered wing planform and and lighter wing tips will improve the roll rate as will larger ailerons. A more tapered and swept back wing planform will increase the tendency to tip stall and make spin entry easier both upright and inverted. The sweep back comparison should be done along the wing's 25% chord line for a valid comparison.

This question has bugged me for a while. Many equally stable pattern planes have the stab in differing vertical positions, and I've wondered why.

This summer, I kit-bashed an "old reliable" Ultra Sport .40, with the idea of reducing the pitch coupling, especially in knife edge.

I moved the stab down about 1 inch, to place it on the thrust line, rebuilt a lighter vertical fin/rudder from sheeted sticks, and changed the rudder hinge line from angled to upright. (90 degrees to the waterline) I also reduced the frontal area of the plane, by removing the turtledeck and rebuilding it with a lighter, lower one.

This greatly reduced the coupling. I had assumed that the stab re-positioning was the cure, but from reading the above posts, it may have been ALL the changes in combination.

If you want to see the article and photos (just one page) it's at http://www.nextcraft.com/us40bash.html

Mike
Just found this thread.

Whatever you decide, try to avoid the wing and stab on the same vertical plane. This will cause the stab to hunt. Think about the airfoil making dirty wind at the rear of the wing, This turbulance will flow a somewhat straight path toward the tail, therefore turbulated air hitting the stab. Hence the tail always hunting.

I did this back in the late 90's on pattern design of mine. Made for a great highspeed s plane. Locked in!! Slow it down, to make it pretty, and the tail lost its lock in. . Later moved the stab 1" above the previous location and the plane became a great performer in any attitude or speed.
I think it was all associated to the stab getting somewhat undisturbed air accross it.

Hi Gerald,

I think you're right, that the effect of airspeed on the downwash can wreck the "lock in". I had a discussion with Ivan Kristensen about a year ago on this subject, and he mentioned a similar trait. So...

On my upcoming (.90 to 1.20 size) pattern plane, ( http://www.nextcraft.com/j45.html ) I've done what you suggested, and have the stab 1 inch higher than the wing. (image attached)

Great design you have going there. Nice lines. We also found (with center mass mounted wings) that sweeping the Hingeline of the rudder back about 5-7 % made a great difference in knifeedge and point rolls as well.

Thanks GW.

(It's not radical, but it's mine.)

Since this one has plug-in wings, that should be an easy thing to test. I'll definitely keep that in mind if there are coupling issues.

Thank you for the input.