The lowly stab....
Ive been cruising through pictures of many popular pylon racing stabilizer planforms (cruciform that is, excluding vtails) basically looking for any trends or commonality for my design. There seems to be a lot of variation & artistic licence in terms of the outline shape. Some are curved, some are straight, some have swervy tips, some are plain vanilla rectangles with radiused corners. I would expect structural/weight issues to influence a portion of the design decision, but there doesnt even seem to be agreement between models of the same racing class.

Assuming the basic stab area & airfoil is engineered to do its job of balancing the wing moment during cruise & initiating/maintaing the turn during pull, Im interested to hear comments on what parameters would logically go into a 'good' racing stab design.

- is there any stall related issues so that an eliptical type planform might be preferred?
- is there an optimal aspect ratio analagous to wing design?
- what would make a good stab airfoil & why?
- does the affect of wing wash or prop wash influence planform shape?
- is there any relationship to wing shape influencing stab shape or are they completely independant?
- typical pylon racers never (intentionally) fly inverted yet most racing stabs are symmetrical. They only have a job to do in 1 direction - between neutral (crusie) & turn (lift). Would an 'inverted' lifting stab offer any benefit (like a car rear wing?)

Here is a snappy looking F5D (electric) pylon racer I fly for reference.

This ought to be interesting...

-David (fellow Avionik F5D jockey)

Ok, I'll throw my hat in to get things started.

First, these are very good questions. Second, I don't know all the answers.

I think a lot of the planform shape design comes down to style and manufacturing techniques. If you have CNC you can get as fancy as you want (Avionik) but if you are using hand made plugs for your molds, then simple straight-tapers will be what you see (like the one I'm building right now).

I haven't done any math to prove it, but I think the elliptical shape also increases stiffness a bit, thus minimizing the structural material (weight) required.

The wing pitching moments and the required max Cl for the stab are pretty small (and I've never seen one stall) so the symmetrical sections seem to do ok. I guess there's a tradeoff between drag sources... the profile alone vs trim drag, etc.

I think the Avionik D99 stab is a little too small (forcing a forward CG) but that's just my opinion from flying them.

Other ideas??
-David

The purpose of the horizontal is to restore the plane to a trimmed condition after a disturbance.
You want it to have as little drag as possible when the plane is trimmed.
Small horizontals have less drag than large....
And the lower the drag, the more power to be used to go faster.
To get the minimal stability needs, the small stabs go further aft; longer fuselage, more "wetted" area.
As the "disturbance" is almost always large, the 180 turn, there's no particular reason to worry about the drag increase then, it must be large, so size the horizontal for the level flight area.
Shape.. as mentioned the elliptical gives the optimum lift distribution for structural purposes.
A bit tougher to build though.
As with everything else in aircraft, any one part must be integrated/compromised with the whole package.
And in racing, it's mostly motor, then pilot, then airframe.

If you want a controllable airplane, the most aft flying surface surface CANNOT stall at all. One of the production cessna had a problem of a stalling stab in certain loading conditions and was fixed by using a slot on the stab.

Usually while designing a aft tail airplane you want to make sure the Cl max of the airfoil is less than that of the main wing. and the incidence is used for trimming condition as Tall Paul said in his earlier post.

The rutan canards can't use more than 0.8 - 0.85 Cl max because of the main wing being the aft most surface.

Some people think the 3 surface airplane is the best. a canard, a wing and a stab.

my 2 cents

e=mc2

ptxman,

There is really no benefit (that I know of) to an elliptical planform per se. An elliptical lift distribution gives the minimum induced drag, everything else being equal. An elliptical planform is one way of getting an elliptical lift distribution. The important thing to note is that induced drag is only important for a lifting surface (wing, stabilizer, ...) which is operating at high coefficient of lift. The first thing to do to limit induced drag in the stabilizer is to avoid operating it at unnecessarily high CL. To do this, you need to get CG at or just a little in front of the Neutral Point, which is to say that the plane is set up neutral or slightly stable. This will mean that the stabilizer generates almost no lift in level flight, and doesn't have to generate any extra lift overcoming excessive stability during the hard turns. This has another benefit. If the tail doesn't generate large amounts of lift, the stress on the empennage is reduced, and it can be built lighter. Even so, the stabilizer will still need to generate some lift, so there will be some induced drag. An elliptical lift distribution is therefore ideal. It may be worthwhile to use an elliptical planform, but a tapered planform, with a taper ratio of about 0.45 or so, is a pretty good approximation, and is probably easier to build light.

I'm not sure what all the tradeoffs are concerning aspect ratio. Certainly, higher aspect ratio would reduce induced drag. It is possible to reduce the chord so much that the Reynold's number gets too low, though. There may be some issues surrounding stall with high AR stabilizers as well, but I'm not the guy to tell you about that.

A good airfoil would have low CD at zero and low CL. It might make sense to have a slight inverted camber, as you mention, but certainly not much. I'm not sure about this tradeoff either. There are some sites on the web that list favorable stabilizer airfoils, and I don't know offhand that any of the URLs are. A Google search may turn them up. If I find any, I'll let you know. You definitely want a real airfoil shape, rather than a blunt flat plate.

Good luck,

banktoturn

banktoturn> There is really no benefit (that I know of) to an elliptical planform per se. An elliptical lift distribution gives the minimum induced drag, everything else being equal. An elliptical planform is one way of getting an elliptical lift distribution. The important thing to note is that induced drag is only important for a lifting surface (wing, stabilizer, ...) which is operating at high coefficient of lift.

This brings up another grey area for me, maybe more related to wing design. Does this eliptical planform / lift distribution relationship have any (unmentioned?) underlying airfoil assumptions? In other words, if you had 2 radically different root & tip airfoils, would an eliptical planform still be better (meaning minimized induced drag) than a trapezoid type planform of similar aspect ratio? Or is there some 1940's Spitfire folklore built in there & the advantage is only confined to constant airfoil family accross the wing panel? (Gee, I dont even know, did they blend airfoils on those eliptical warbirds back then?)


banktoturn> A good airfoil would have low CD at zero and low CL. It might make sense to have a slight inverted camber, as you mention, but certainly not much. I'm not sure about this tradeoff either. There are some sites on the web that list favorable stabilizer airfoils, and I don't know offhand that any of the URLs are. A Google search may turn them up. If I find any, I'll let you know. You definitely want a real airfoil shape, rather than a blunt flat plate.

Nope, the blunt plate was never an option! The Avionik D99 (F5D pylon rcer above) references a naca0006. The Ariane crowd (link = http://www.delago.de/ariane/EProfil.htm ) favours an HD800 & HD801 for similar application. From what I can see on the polars seem pretty comparable.

- Assuming the max thickness is defined, what about the high point placement? With the elevator proportion like shown in the D99 & maybe 10 deg max deflection, is there any pros/cons of the resulting polars by having the high point occur at 25% vs 30% vs 40% etc?

ptxman,

The effect of the elliptical lift distribution (minimum induced drag) is independent of how you accomplish it. If everything else is held constant along the span (same airfoil, no twist, etc.), then an elliptical planform gives an elliptical lift distribution. If a non-elliptical planform is used, then twist, or varying camber (blending different airfoils), etc. can be used to get the elliptical variation in lift from the root to the tip. The elliptical planform itself has no value, only the elliptical lift distribution. In any case, a trapezoidal planform, with the appropriate taper ratio, gives a lift distribution close enough to elliptical that it is hard to justify the trouble to build an elliptical one.

I'm not sure what all the issues are regarding the location of the high point. One major issue is laminar flow. A high point which is relatively far back (40% to 60%, say) is a primary characteristic of a laminar flow airfoil. This reduces drag by delaying the transition of the flow from laminar to turbulent, because laminar flow results in lower skin friction than turbulent flow. Generally, the flow can be kept laminar up to the high point (if the surface is very smooth & not wavy), but not much past it. I'm assuming that a high point fairly far back is better for this reason. It is not uncommon to use a turbulator somewhere near the high point, to cause the laminar flow to become turbulent. This helps prevent separation, which is more likely for laminar flow than turbulent. One thing to keep in mind: it might be hard to get the benefit of a laminar flow airfoil on the stabilizer, with propwash and other 'dirty' air hitting it. I don't know offhand how likely it is.

banktoturn

This is a very good question. A lot of the very low Re sections (ala Drela, etc) are thin with a very forward high point. I've been flying a section like this on my latest S400 racer for about a year and I swear the aileron response is more sluggish than on the same planform with an MH30. Maybe we need to play with xfoil some more.

-David

I have a very old copy of the David Faser sailplane analysis tool that used the early Selig airfoil information along with some other designing formulas to help predict some of a sailplane's performance. One of the parameters that is in the printouts is a listing of the stabilizer Cl as a function of the speed. When the CG was set back near the neutral point you could see the Cl for the stab go from slightly positive to slightly negative and you could move the 0 point around with a few minor changes to the design. The numbers in these cases was always very small. This is also why for a properley "close to neutral" trimmed model there is no real advantage to using an upright or inverted lifting stab. As I recall the Cl is positive at low speeds and negative at high speed.

There was also some info I read somewhere that said that at low Reynolds numbers the higher aspect ratio planforms stall earlier. The outcome was advice to always have the aspect ratio of the stabilizer be about .5 to .8 of the wing. That ensures the stab chord is sufficienct to prevent an early stall compared to the wing.


With the lift coefficients being so small in a well trimmed design the actual planform becomes more one of style. Only one item changes this. I've seen information about the turbulence of control surfaces that extend right to the tips. When deflected they iduce vortices that create drag. Having a small fixed portion of the main surface outboard of the movable surface acts like an end plate to reduce this vortex formation. There is obviously some turbulence created by the angled break but it's apparently a smaller issue compared to the votices. This is why a lot of high speed designs have the tips that are full chord. The outer fixed portion needs to be about 3 times the chord of the control surface for this to properley dampen the vortices. This is shown on the diagram of the Avionik surfaces.