horizontal stabilizer...which way is up

[steve hartle]

Acknowledging the fact that an effective hs is neccesary for the safe and stable operation of most, if not all gyros, i pose this question. Given that size matters, that symetric is better than flat, that in propwash is better than not, that some negative incidence is neccesary, would it also be advantageous to use an inverted airfoil design ? I believe Zenith uses such a stabilizer on its STOL 701 model aircraft. Maybe in our quest to save weight, minimize size, and improve effiency someone better versed than I could explain if this design would be appropriate, of significant benefit, and if so what airfoil profile might work best.

[Aussie_Paul]

Originally Posted by steve hartle

Acknowledging the fact that an effective hs is neccesary for the safe and stable operation of most, if not all gyros, i pose this question. Given that size matters, that symetric is better than flat, that in propwash is better than not, that some negative incidence is neccesary, would it also be advantageous to use an inverted airfoil design ? I believe Zenith uses such a stabilizer on its STOL 701 model aircraft. Maybe in our quest to save weight, minimize size, and improve effiency someone better versed than I could explain if this design would be appropriate, of significant benefit, and if so what airfoil profile might work best.

An effective small h/stab. A piece of extruded rotor blade with the heavy spar removed works well for this 2.5" HTL machine with a pod. The faster the machine goes as the nose is pushed down the more the h/stab works tp keep it from goi9ng too far.

Aussie Paul.

[Passin' Thru]

Originally Posted by steve hartle

Acknowledging the fact that an effective hs is neccesary for the safe and stable operation of most, if not all gyros, i pose this question. Given that size matters, that symetric is better than flat, that in propwash is better than not, that some negative incidence is neccesary, would it also be advantageous to use an inverted airfoil design ? I believe Zenith uses such a stabilizer on its STOL 701 model aircraft. Maybe in our quest to save weight, minimize size, and improve effiency someone better versed than I could explain if this design would be appropriate, of significant benefit, and if so what airfoil profile might work best.

.
Steve, that is precisely what I did on my previous dominator. When I first started examining the overall layout, I had to make a few adjustments to accomodate a 68" prop on the Rotax 618-E box. (I really wanted to go 70", but settled for 68.) this neccesitated raising the engine about 4" and an attendant increase in mast height. Next, due to a hip and leg problem I needed to lower the seat, so I reduced the amount of keel drop by about 4" with another increase in mast length. (Gee, I'm really going away from home, aren't I?)
At this point it was only on paper, so I consulted someone who I consider an expert in these matters. After describing what I was trying to accomplish, he suggested the inverted asymmetrical horizontal stab. ( I won't mention his name, he may not want to be party to the crime! ) The results were very satisfying. It worked great. The machine is completely stable in every test. Hands off and trimed, I could drink a soda and smoke a cigar with my feet on the panel. Just like a Cessna. I really hate I sold it.
.

[Fl90]

I hate to get off topic, but I have a head full of questions looking at that airfoil.

It looks to have a heavily aft loaded mean line, a laminar attempt with the major thickness location and heavy reflex. I would think the separation at about 3/4 chord would be high at most rotor speeds and the drag on the airfoil would make for an inefficient rotor. I'm just wondering how the airfoil performs as a set of blades, and what disc loading does the manufacturer recommend?

If you would rather PM me, so as to keep this thread on track, that would be fine.

Sorry for the hijack, Phil.

[steve hartle]

Just viewed R&D at BD ,confirmed alot of beliefs I had but as usual far more complicated than I realized so although Iam convinced this would be beneficial I would like to hear some of the more versed opinions . Starting to realize more going on than meets the eye. Speak slowly , use small words.

[Doug Riley]

An asymmetrical airfoil and a symmetrical one with negative incidence are virtually the same thing. The camber contributes the equivalent of 2-4 degrees of incidence, depending on the severity of the camber.

A cambered airfoil is slightly more efficient at generating X pounds of lift than a symmetrical one making the same lift (by using more incidence). The difference is very slight for a wing as small (and slow-moving) as the usual gyro H-stab.

Example: A NACA 0012 airfoil (a typical HS foil) has a drag coefficient of around .007 at a lift coefficient of 0.4. The same airfoil, but with camber, becomes a 2412, and the drag coefficient for the same lift goes down to .006. That's a savings of 14% of the drag. But the drag of a gyro-HS-sized wing at 60 mph at the 0.4 lift level is less than a pound to begin with. Who cares about saving 14% of 1 lb. of drag?

Do the simplest thing.

Chum's rotor-blade HS is too low to catch any propwash and is quite small to boot. You'd need to do a little arithmetic to see if it's powerful enough to prevent PPO and/or drag-over caused by the pod. I'd be surprised if it were.

[steve hartle]

Thank you Mr Riley , clear and to the point as usual.Thought there might be more to be gained . Zenith seemed to make big deal about benefits ( beware the hipe I guess). The point you keep making about hs in the propwash is well taken. FW with T tail above propwash don't experience the same pitch movement under throttle changes as do tails in propwash. Don't know statis of Starbee T tail but seems to me to be the optimum for those searching ( definitely my personal pick). Good job at BD , we all need to do more practical work and less speculating. Wright brothers wouldn't have got off the ground if they had sat back in Ohio and talked about it. For us looking for the Wright stuff the gyrobee is a good inspiration as the Wright Flyer was but it needs to evolve . Just because something is simple, cheap to build and is flyable dosen't mean we should stop there or else we'd all be buzzing around in Wright Flyers. Before the hate mail starts flowing I mean no disrespect and applaud Mr Taggart for inspiring us.

[Aussie_Paul]

Originally Posted by Doug Riley

Do the simplest thing.

Chum's rotor-blade HS is too low to catch any propwash and is quite small to boot. You'd need to do a little arithmetic to see if it's powerful enough to prevent PPO and/or drag-over caused by the pod. I'd be surprised if it were.

Doug, without the stabs there is a slight tendency to nose down with a rapid application of power, so it is very slightly HTL. With the stab it behaves nicely. I would not say that it could not be improved on BUT it does work well. This was the first single seater built to the Hybrid tested geometry. The fact that Chum is short so he had to keep the seat lower. Otherwise it would have turned out very close to CLT.

Using the upside down rotor blade idea was just an out of interest idea. Once you are close to CLT the h/stab is nowhere near as critical as it is on a RAF. I seem to compare everything to a RAF BUT that is where I started from, and use that as my datum I guess.

Aussie Paul.

[Doug Riley]

Paul, you really need actually to find the CG-thrustline relationship with a double hang or equivalent static test. On gyros, HTL has a nasty habit of sometimes not revealing itself readily in flight tests. The gimbal head and other mechanical factors can mask it -- but that does not mean that it isn't there, waiting to get you.

Body pods are the same. Their effects may only pop out when you shut the engine off and do a fast deadstick dive. In normal flight, they can behave just fine.

Again, some ground analyisis and testing will often reveal the problem more quickly and safely than flying about and trying to find the "coffin corner" of the envelope.

If a gyro fails such a ground test, flight testing should not proceed. The fact that a given craft appears to "fly good" doesn't mean that HTL or other airframe instability can be ignored. Flight tests do NOT trump the math -- not at this basic level.

[Doug Riley]

Steve, fortunately for us, the Gyrobee is a much better gyroplane than the Wright Flyer was an airplane. The Flyer is a canard craft that is NOT set up like a modern Rutan-style canard. The Flyer doesn't use the canard as a lifting wing. As a result, it's pitch-unstable for the same reason that a HTL gyro is: the CG is aft of the lifting surfaces' combined aerodynamic center. This is an unstable setup on any aircraft, whether fling-wing or fixed-wing.

Various 'Bees have tested out at perfect CLT, slightly LTL and an inch or two HTL. All three situations are within the "tolerance" that's easily accommodated by a 5-6 square foot HS in the propwash.

The principal shortcoming of Ralph's old prototype Gyrobee is the use of a Brock HS forward of the rudder and low on the keel. Most 'Bee builders have used a more effective HS than this, though.

[Aussie_Paul]

Originally Posted by Doug Riley

Hi Doug. Paul, you really need actually to find the CG-thrustline relationship with a double hang or equivalent static test. I agree On gyros, HTL has a nasty habit of sometimes not revealing itself readily in flight tests. The gimbal head and other mechanical factors can mask it -- but that does not mean that it isn't there, waiting to get you. Absolutely

Body pods are the same. Their effects may only pop out when you shut the engine off and do a fast deadstick dive. In normal flight, they can behave just fine. Absolutely

Again, some ground analyisis and testing will often reveal the problem more quickly and safely than flying about and trying to find the "coffin corner" of the envelope. Absolutely

If a gyro fails such a ground test, flight testing should not proceed. The fact that a given craft appears to "fly good" doesn't mean that HTL or other airframe instability can be ignored. Flight tests do NOT trump the math -- not at this basic level.Absolutely

Points taken Doug. Many thanks.

Aussie Paul.

[birdy]

If Chum's machine is so it is very slightly HTL, then Mate, i must be sitn on my head.
Not only is he sintn low, so is the fuel. And the bulk of a heavy engine is nearly ON the TL.
Add to that the fact that theres a larg pod, with a 'down forcing' windsheild, both way below the TL [ adding more nose down forces] and dragy mains gear even further below the TL.
If this thing flies with even the slightest resemblance of stability, with that pissy little stab to counter all that weight/drap offset, then ill stand r##tn.
Yes, it dose have a long mast with heavy rotors, but after wot i saw at Lammeroo with the double hang testing there, Chum's machine could not possably be remotely close to so it is very slightly HTL.

[Aussie_Paul]

Originally Posted by birdy

If Chum's machine is so it is very slightly HTL, then Mate, i must be sitn on my head.
Not only is he sintn low, so is the fuel. And the bulk of a heavy engine is nearly ON the TL.
Add to that the fact that theres a larg pod, with a 'down forcing' windsheild, both way below the TL [ adding more nose down forces] and dragy mains gear even further below the TL.
If this thing flies with even the slightest resemblance of stability, with that pissy little stab to counter all that weight/drap offset, then ill stand r##tn.
Yes, it dose have a long mast with heavy rotors, but after wot i saw at Lammeroo with the double hang testing there, Chum's machine could not possably be remotely close to so it is very slightly HTL.

Well once again you are wrong Birdy.

Aussie Paul.

[SamL]

The other important factor about sizing HS's is to size them large enough to place the machines centre of presure behind the CofG.
Otherwords allowing you machine to weathercock in pitch correctly.

Sam..............

[Doug Riley]

Certainly, Sam.

Where some machines get into trouble is in regard to the C.P. of the frame at large airframe pitch angles. The C.P. can be behind the CG when the frame meets the air in a level stance, but the C.P. may forward dramatically at a nose-down angle.

This can happen with open-topped pods. When the nose gets low relative to the flight path, the open "bathtub" acts like a scoop, creating tremendous drag way forward and possible pulling the nose over. The rotor may not have enough power to keep the nose up, even at full aft stick, especially if RRPM declines because of the nose-low stance.

Igor Bensen encountered this effect in a famous incident involving the Gyroboat.

It's important when using a pod to make certain the H-stab is large enough to prevent this sort of "drag-over." A simple silhouette (paper cutout) test is a start, but isn't conclusive in this case. A 3-dimensional model tested in a breeze is better.