HStab in Prop wash

[birdy]

Now that I seem to have the attention of a few experts from around the world, I have a question for you.
To what purpose is an H stab that is 100% immersed in the propblast or wash.
To my understanding, the stab is to stablise the airframe pitching tendencies against variations in oncoming airflow speeds and angles. If the stab is hidden from the oncoming airflow by the propwash, how can it react to the changes in the airflow.{especially the ones on tall tails, they are centered to the prop and very close.}

The only affects these stabs can have that I can see is to counter some of the torque roll from the prop and, if there is some negative incidence, would apply down force on the aft of the machine. And wouldn’t the same down force effect be gained by having the thrust line angled up with no stab???

Stabs like the ones on the Magnie’s and such would have the desired effect because they have the nearest thing to undisturbed airflow to react to.


Please, keep your criticisms brief, I confuse easily. ???

[Doug Riley]

Birdy: It's a common belief that the propwash "shields the HS from the wind." This is one of those common-sense ideas that turn out simply to be wrong. In fact, the propwash moves about in response the wind coming into the prop disk. The angle of "output" depends on the angle of the "input."

I didn't believe this until I tried it myself.

To see this effect in action, get two electric fans. Tie some pieces of yarn to the grille of one of them, so they stand out downwind when you turn it on. The slipstream is strongest near the tips of the fan blades, so that's where the yarn will catch the best wind.

Take the other fan in hand. Turn them both on. Point the wash coming out of the hand-held fan into the intake side of the other fan. Now move the hand-held one about. Point it at the intake side from various angles -- from below, from either side, from above. Vary the angle any way you like. You will see the yarn tufts move in response to your movements of that upwind fan. The effect is impossible to miss, and really quite surprising.

You can easily detect the effect in flight on a rough day. Among other gyros, I fly a Dominator tandem. The H-stab on this bird is about as "immersed" as you can get. Flying in substantial thermal turbulence, the nose of the craft points down into updrafts like a dowser's stick near water. This seems very strange until you realize that it preserves your airspeed -- with no input from you. It also smooths out the bumps (as much as you can in a 400-pound aircraft, anyway). All you do is hold the stick still. You don't have to "float" the stick or try to "feather" the gusts with stick inputs. You simply ignore them and the gyro does the corrections for you. A Gyrobee, with partly-immersed HS, reacts exactly the same way.

I flew without a HS for years. The difference is remarkable and very gratifying to me. I wouldn't ever go back to the old way.

[Udi]

If I may add one detail to Doug's excellent reply - having the stab inside the prop wash actually magnifies its power. The same size stab will be more effective when mounted in the prop wash than out. We should make sure, though, that the stab is powerful enough even when the engine quits. This is my only gripe with some immersed stabs.

Udi-

[Doug Riley]

Udi's point is well taken. The prop slipstream doesn't shield the HS from "the wind," but large objects upstream of it can. Wide cabins, blank walls making up the backs of fuel tanks and similar items can result in disorganized flow to the HS. Especially if the HS is stabilizing the craft against instability caused by parts of the airframe, it's important that the HS receive good flow even with the engine off.

[mceagle]

Doug,
Your previous post makes a damn good argument for having the tail feathers outside the prop wash.
Do you know if any experiments have been done to compare the efficiency of a stab in the propeller altered slipstream operating at 50% deflection efficiency relative to the incoming air but with higher airspeed, compared to one operating in the "real" airflow with 100% deflection efficiency and with less air speed.
I appreciate the fact that when straight and level, the prop slipstream would also be straight, but gyros do move around in rough air so the slipstream would not always be in line with the flight path.

PS - why are you a Llama?

[splashdown]

Udi, how does the stab in the prop wash increase power?

I could understand if the stab was deflecting the prop wash at some angle, but apart from that I fail to see how it can increase the stab performance?

Can you explain it to me?

[birdy]

Doug,I'm not goi'n to argue about the two fans thing,but you reckon the stab in the propblast is just as effective as one in free air??
Would'nt an immersed stab need to be bigger to have the same effect,given not only the desturbed air but also the short moment arm???Or dose the increased vilosity counter these.


BTW,I asked on the oz forum if anyone with the T tail setup[short tail boom]if they ever herd of anyone smack'n their blades on the ground while landing???Noone has answered yet.I once watched a video someone took of one of my spot lands and I reckon if I never had the longer tail boom, the blades would have hit.[only about 4" clearance]
Have you ever had it happen over there??

[Udi]

Lloyd,

I am sure Doug can explain this phenomenon much better than me, but I will give it a shot. The lift of any airfoil is a square function of the airspeed, for any given angle of attack. For example, an airplane flying at 200 mph and a given angle of attack will have 4 times the lift is has at 100 mph, with the same angle of attack.

Back to a gyro stab - the relative wind is entering the prop disc at a given angle say, 4 degrees. The prop will accelerate and also somewhat straighten the angle of the air stream hitting the stab. Let's say the incoming airflow is at 60 mph, and the prop accelerates it to 120 mph. The prop wash will hit the stab at 120 mph, and 2 degrees.

Will the stab have more lift with a relative wind at 60 mph and 4 degrees, or 120 mph and 2 degrees? Since the lift is a linear function of the angle of attack and square function of the airspeed, the latter case will have double the lift.

Doug, please correct me if I screwed it up.

Udi-

[CLS447]

Hey guys ,this is an interesting topic! The new Butterfly has the AC style rudder & stab. The stab is mounted up high on the rudder.

If I moved the H-stab on myAC 447 up , I wonder what the results would be? Doug, did you ever try this?

I have an extra set of the AC flat plate stabs. How about I install them on my tall tail at prop center in addition to my keel mounted stab on my new machine? The best of ,or part of, both worlds !!!

Has anyone out there in gyroland ever tinkered with AOI on the flat plate stabs or should they just be parallel with the rear keel?

[Doug Riley]

Udi, that's right. The strength of the force created by an airfoil is a function of the SQUARE of airspeed. Increase the airspeed of your airfoil from 50 to 100 mph and your lift, drag and pitching moment will all increase fourfold.

OTOH, the forces change only linearly with changes in angle of attack. Halve the AOA and the lift, drag and moment are cut in half. Double AOA and the forces double.

It's true that the changes in AOA that an immersed airfoil "sees" during a given updraft/downdraft are less than those that an un-immersed one sees. However, the "airspeed squared" rule more than makes up for this. You end up with a more effective HS if it's immersed (and the the throttle is up, of course). (I can post some numerical examples if people really want to see this in action. Math isn't pretty to everybody, though.)

In fact, gyros are capable of such slow flight that an UN-immersed HS is too weak to do much at the low end of the speed range. It almost HAS to be at least partly immersed to have much effect at low speeds. The professionals who designed the McCulloch J-2 and the Air and Space 18A knew what they were doing. They put their HS's squarely in the prop blast. Same for boat-hulled amphibian planes.

If the thrustline is through the CG, there is no need for much negative HS incidence. A little does improve pitch stability. If the thrustline is above the CG by a small amount, some negative incidence is likely necessary to avoid instability. On a stock 1-place Air Command rudder, I don't feel the structure is strong enough to start loading up the HS with lots of incidence. On the Gyrobee, I put about 3 degrees neg incidence on a six-square-foot HS. This more than makes up for the 1-2 inches of thrust offset and produces pitch behavior very similar to a Dominator. On the 'Bee, the HS is about nine inches above the tail tube -- partly in and partly outside the prop blast.

[Aussie_Paul]

Doug, I am seeing some value now, after a lot of reading, of the partly in and partly out of the prop wash stab position.

The required effectivness of a stab at slow flight is governed, I believe, by the amount that the thrust line is above the VCoG.

A dangerous scenario for a stab outside the prop wash would be low aispeed and full power, accentuated by the amount that the thrust line is above the VCoG.

Would that be a fair comment?

Aussie Paul.

[splashdown]

Thanks Udi and Doug.

Another question then. If the incoming airstream is at an angle to the stab (updraft or downdraft) and the acceleration of the airstream by the prop straightens this angle out a bit, then doesnt that reduce the effectiveness? Isnt the airstream now at a lesser angle and less effective on the stab? This seems to follow what Doug says. from my perspective it would seem the effectiveness of the immersed stab would average out when the throttle is high, but then become a liability when the throttle is low. Wouldnt a larger stab outside the wash be a better solution across a range of throttle settings?

Also, I understand your comments on how the stab 'lift" increases based on the amount of airflow (high throttle), but wouldnt it be more effective to just angle the prop thrust to give you the same result?

[Doug Riley]

Paul, what you say is generally true. However, in the case of a small, slow pusher gyro, the use of propwash augmentation MAY be necessary for reasons other than high thrustline. The structure may just not be big or strong enough to allow the size HS that you need if you're going to operate outside the wash. IOW, the wash lets you get away with a smaller HS and still get the stability you want.

Granted, the wash effect won't be there at low throttle settings, but that problem may (again, with a light, slow design) not be all that serious.

Lloyd, assuming that you don't have thrustline above CG, a larger HS outside the slipstream would be OK. When you start doing the numbers, however, you see how MUCH larger you're talking about. At 40 mph, for example, a simple HS produces about 0.3 lb./sq. ft.of lift per degree of angle of attack. That's 4.8 OUNCES! Not much bang for the buck. Combine that with the very short lever arms that a pusher gyro lets us use and you can see that the un-immersed HS is a real paper tiger in our speed range.

Check out the lever arms that tractor airplanes have for their HS's: typically 3 or more times as long as our pusher gyros' tail arms. OTOH, pusher planes typically use an immersed HS.

[birdy]

Doug,I like reading your posts,your thinking is logical,and your explanations are understandable,[even for a thick bastered like me].
Keep it up mate.

[CLS447]

If I moved the H-stab on myAC 447 up , I wonder what the results would be? Doug, did you ever try this?

I have an extra set of the AC flat plate stabs. How about I install them on my tall tail at prop center in addition to my keel mounted stab on my new machine? The best of ,or part of, both worlds !!!

Has anyone out there in gyroland ever tinkered with AOI on the flat plate stabs or should they just be parallel with the rear keel?