Tail plane in the prop wash?

[Fliterisk1]

Please excuse a question from a non-pilot. I fully intend to pilot a gyro, even before anything else. But other commitments prevent me from taking such action at this time. Though I’ve no flight experience, I have studied the gyro and have an engineering (mechanical) background. I am interested in the gyroplane, with questions. That is why I joined this forum.

I am new to the Forum, so I would think this subject has been discussed. But public descriptions of the tailplane apparently show this question as unresolved. May I pressed it again?

My question is why put the tail plane (aka “horizontal stabilizer“) in the prop wash? Some manufacturers do, some don’t. I’ve seen a recommendation on the web saying that should be done, without explaining why; though the aircraft in the website did not. It’s tail plane was beneath and out of the way. That is where I think it should be, and for reasons which are not difficult to understand. Others, I've read, think otherwise.

Jean Fourcade’s article “Longitudinal Stability of Gyroplanes” was rather well-known some time ago, though I couldn’t find it on the web recently. In it he states, “It is important to note that the lift is proportional to the square of the air speed while it is only proportional to the angle of attack. It is therefore preferable to place the stabilizer in the slipstream of the propeller to use the benefit of more air speed. It is especially important in a gyro as these machines are not flying fast.”

I disagree, of course. I would agree if he wrote that with respect to the rudder. And I would agree if the stab was variable in pitch, acting similar to the rudder. But it is not. The tailplane needs free wind just like the arrow’s feathers stabilizing it in flight. Putting a little propeller in front of those feathers would do nothing to stabilize it. That prop wash can only interfere with its stabilizing effect by interfering with the wind, by blowing it into a parallel flow with the tail plane. The tailplane needs wind independent of the gyro in order to work properly. (Can you feel my frustration?)

There may be exceptions. I can think of one. How about an airfoil which works off the prop wash in order to counter the forces which may destabilize the aircraft; specifically, those due to the HTL prop thrust itself. I have not seen an example of this, though it would seem a natural step to stabilize a particular design.

It may be possible to combine the effects of tail feathers with a prop thrust vectoring system. But the examples I’ve seen show a tail plane which is essentially parallel to the prop wash. This combination will do nothing to stabilize the gyro. (Though the tail planes I’ve seen seem large enough to reach the relative wind.) And such stabilizers centered behind the prop are also blocked from the relative wind by the engine and the pilot. Expecting the prop wash to make up for this blocked location, or even improving on a free-air tail plane is just plain silly!

The gyroplane can be flown without a tailplane, while some are strongly suggesting it be present. So I'm wondering if, in some cases, the tailplane isn't mostly for show.

My other question has been already inferred, and it is why isn’t there a second stabilizer behind the prop to vector thrust so to counter the HTL design? It isn’t difficult.

Otherwise, it is my opinion that the gyroplane would become a more acceptable aircraft when its shape (its apparent *Design*) takes on a generic appearance, (built on fact, not opinion); and does not vary so much between all the manufacturers. (There are still questions about a tail plane?!) Differences between designs should be easily understandable by the layperson on sight. A generic design infers predictability because it is predictable (even if it is predictably dangerous); because that is what the manufacturer wants the gyroplane to be—a known quantity. If a generic design bores you then maybe flight isn’t what you really want. Maybe you want notoriety (even if its only you boasting to yourself), and some of that notoriety comes with the inherent danger of flight. You see, I don’t think I can trust some of you.

(Off my soap box, with some worry about what I just wrote.)
--
Fliterisk1

[rehler]

Bryan,

Jean Fourcade is correct: "lift is proportional to the square of the air speed while it is only proportional to the angle of attack. It is therefore preferable to place the stabilizer in the slipstream of the propeller to use the benefit of more air speed. It is especially important in a gyro as these machines are not flying fast.”

What this means is that when the air is flowing faster it acts stronger on the rudder or the HS, so when the wind comes from an angle (like from the bottom) it diverts the air stream from the prop upward. This faster flowing air actiing upward moves the HS up more efficientle than if the HS was out in the unaffected air.

You said: "I would agree if the stab was variable in pitch, acting similar to the rudder." There is no difference if it is a fixed HS or a movable HS or rudder. Both are more effective if in the prop wash.

You said: "Putting a little propeller in front of those feathers would do nothing to stabilize it." This is not correct. It would be more stable with the propeller.

I believe the problem you are having is thinking that the prop makes the air shoot straight out and is not affected by the wind. This is not true. The prop blast IS affected by the wind. The wind redirects the prop blast just like it does the air outside the prop blast.

It may be difficult to believe but real life testing will clearly show what actually happens. Take two fan (one as the prop and one as the wind) and try it out.

[Udi]

Bryan - I enjoyed reading your post. I can see your engineering background through your critical thinking. Some of your observations are intuitive, but intuition can be deceiving. Let me assure you that everything about the design of gyroplane tail feathers is a known quantity. Chuck and Doug may pop in and give a more elegant explanation, but I will give it a shot.

There are three main reasons to placing the h. stab in the center of the prop wash:

1. As ken has said, the prop blast is acting like "power steering". It adds power to the stab. See additional explanation below.

2. As you have mentioned, the stab can be mounted with a negative angle of incidence and use the prop blast to counter other nose-down moments, such as a HTL. This is in fact being done in many gyros, most notably the Gyro bee and the Magni (the Magni stab is located on the keel, but it is huge, and it gets enough of the prop blast to counter the HTL). Some gyros have an engine thrust line offset too large to counter effectively with a stab, but a small HTL can easily be mitigated with the stab.

3. A stab that is located in the center of the prop blast acts to counter some of the propeller torque. This is due to the spiral slipstream that hits the stab from both sides, which creates a rolling torque, countering the prop rolling torque. A tall rudder is doing the same thing.

To convince you of the validity of reason #1, here are some numbers: Say you are flying at 50 mph. Also, lets assume the air is exiting the prop at 100 mph. As long as the air is entering the prop at zero degrees, it would also exit the prop at zero degrees. Now, if you hit an updraft and the air is entering the prop at an angle of 5 degrees - due to the acceleration, the air would exit the prop at half the angle it entered - 2.5 degrees. If you don't believe this, do the experiment that Ken suggested above, or do more research on this subject.

So, the air that is leaving the prop has a velocity of 100 mph (relative to the tail) and an up-angle of 2.5 degrees. Since the lift curve is pretty much linear at small angles, half the angle would produce half the lift. However, doubling the velocity would quadruple the lift. So the NET effect of the stab being inside the prop wash is doubling the lift compared to a stab that does not get any prop wash.

It is true that the engine and the pod/pilot affect the airflow going into the prop, but take another look at the proportions. Today, most props are 5 ft in diameter or larger. The width of the engine, pilot and pod is less than 2 ft. The prop is mostly seeing free airflow. Even with the wider-bodied side-by-side gyros, the airflow going into the prop may not be completely laminar, but it maintains the airflow direction to the most part.

Look at some of the gyroplanes designed by “real” aerospace companies - the GBA Hawk 4 and the Carter Copter. Both have the stab located in the center of the prop wash, for the same reasons. This is hardly unknown quantity.

Udi

[gyromike]

Originally Posted by Fliterisk1

Please excuse a question from a non-pilot....
(snip, snip)
...You see, I don’t think I can trust some of you.

(Off my soap box, with some worry about what I just wrote.)
--
Fliterisk1

Something's a little fishy here.

Fliterisk1,
your name wouldn't also be Michael Rhodes, would it?

[PW_Plack]

Insider abbreviation alert for newcomers:

Bryan, when Udi says "HTL," he means "high thrustline," as in a center of thrust above the vertical center of mass, causing the prop thrust to produce a nose-down torque moment unless countered by a horizontal stab.

[Doug Riley]

The question of whether the direction of the propwash changes with changes in the direction of the inflow to the prop has been discussed ad nauseum on this and the prior forum. It does, and Fourcade's analysis is correct.

Most professionally designed small pusher aircraft (gyro and FW) have the HS placed at or near the center of the propwash. This placement helps to compensate for the necessarily short lever arm of the tail boom that we see in pushers. Have a look at the Air & Space 18A gyroplane and the McCulloch J-2 gyroplane (both FAA type-certified). Then inspect the Seabee, Lake Amphibian, Cessna Skymaster and similar FW pusher craft, all certified. If their immersed H-stabs didn't work, the aircraft could not and would not have been FAA type-certified.

You can experiment with two small electric fans. Tie some yarn tufts to the grille of one fan and turn it on. Take the other fan, turn it on and point its wash into the intake side of the tufted fan. Move the non-tufted fan around and watch the corresponding movement of the tufts.

Or go take a flight on a rough day with an instructor in a Dominator gyro, a gyro having a fully-immersed HS. The aircraft points its nose into updrafts and downdrafts like a dowser's stick near water. By the same token, the H-stab loses a considerable amount of its power, and the aircraft develops a spongy feel in pitch, when power is reduced to idle for the landing approach -- even though airspeed is not reduced.

[Fliterisk1]

Okay, I think I see my error. Everyone who disagreed is correct, I think. Udi provided the explanation (below my reply). And his statements make Fourcade's intentions clearer. Thank you sir! (And apologies for top-posting my reply, which I rarely do.)

It's a mass flow problem. Say your example was changed so that airflow exited the prop at 3 times the speed it entered. Then the change in angle of exiting air would be 2/3 the angle at entry. (I did not know that detail.) Since lift is ~proportional to AOA then lift's effect would be reduced to 1/3', while it is also proportional to velocity squared so it would be increased by 3^2. The result equals 9 times 1/3 = 3. This is how many times faster the prop wash is than the relative wind.

So the effectiveness of the stab IS increased by the presence of the propwash. And its amount is proportional to ratio to velocities of prop wash divided by its forward speed. This factor is used by multiplying it to the original force of the relative wind on the HS if it were unaffected the prop wash.

I think I got this straight. But one question. Shouldn't the prop's thrust respond against the aircraft? And push the aircraft in the direction of the airflow? I've not heard this property of the propeller. This is not a torque, such as the p factor, but a change in thrustline direction.



Fliterisk1
--
Udi said: To convince you of the validity of reason #1, here are some numbers: Say you are flying at 50 mph. Also, lets assume the air is exiting the prop at 100 mph. As long as the air is entering the prop at zero degrees, it would also exit the prop at zero degrees. Now, if you hit an updraft and the air is entering the prop at an angle of 5 degrees - due to the acceleration, the air would exit the prop at half the angle it entered - 2.5 degrees. If you don't believe this, do the experiment that Ken suggested above, or do more research on this subject.

So, the air that is leaving the prop has a velocity of 100 mph (relative to the tail) and an up-angle of 2.5 degrees. Since the lift curve is pretty much linear at small angles, half the angle would produce half the lift. However, doubling the velocity would quadruple the lift. So the NET effect of the stab being inside the prop wash is doubling the lift compared to a stab that does not get any prop wash.

[rehler]

Bryan, you asked: "Shouldn't the prop's thrust respond against the aircraft? And push the aircraft in the direction of the airflow?"

This sounds right to me - but ...

It would not "turn" the aircraft into the airflow (the tail feathers do this)

I believe it would push the entire aircraft in the opposite direction of the out flowing prop thrust air flow (not the direction of the incoming air flow which is not the same, as the air flow changes direction when it goes through the prop).

[Udi]

Originally Posted by Fliterisk1

...But one question. Shouldn't the prop's thrust respond against the aircraft? And push the aircraft in the direction of the airflow? I've not heard this property of the propeller. This is not a torque, such as the p factor, but a change in thrustline direction...

A propeller thrust line, Bryan, is perpendicular to the plane of the tips, even when the inlet and outlet airflow are not perpendicular. That is because the direction of the thrust is a result of the acceleration of the air - not the direction of the airflow. The air is always accelerated at 90 degs to the prop plane even when it enters the prop at an angle. The air maintains it's original sideways momentum, but the prop does not add any momentum to the sideways vector.

This is very evident in the large propeller that we call rotor... The air is entering the gyro's rotor disc at a very large angle (about 80 degrees), but the rotor thrust line remains perpendicular to the disc.

Udi

[C. Beaty]

Rotor thrust remains normal to the rotor plane as the result of cyclic feathering/flapping.

Yawed flow entering a propeller generates a precessional moment that tends to return it to pure axial flow.

Autogiro histories are usually written by journalists without a background in mechanics. The rigid rotor machines that preceded the invention of flap hinges would have pitched nose-up rather than rolling to one side. Same principle that governs the flight of a boomerang.

[Aussie_Paul]

.... as to offer my, practical rather than the pure theory, thoughts.

The greater the thrust line(T/L) to the CoM offset the greater the work being done by a h/stab. A stab in the prop wash is almost a neccessity with large offsets like the Raf. It would help in keeping the stab to a "less than barn door size".

With the T/L to CoM of a couple of inches or less, the stab is more of a trimming device for airframe reaction to tubulence. In this situation I have found that having a 2' section of extruded rotor blade, with the spar removed, mounted upside down on each side of the rear keel works quite efficiently. Not the lightest method BUT ideal for someone who is not comfortable working with composites.

Aussie Paul.

[Udi]

Yawed flow entering a propeller generates a precessional moment that tends to return it to pure axial flow.

Chuck – it took me almost an hour of trying to prove you wrong to finally understand what you actually meant! I know you want to spend your time on articles for Rotorcaft, but come on!!!

The P-factor is taught in flying ground school but in real life this effect is completely negligible. Not only is it negligible moment-wise, but also the real effect of the P-factor, as you said in so FEW words , is a de-stabilizing moment in a tractor aircraft and a stabilizing moment in a pusher aircraft.

I understand what you said, but maybe you can be a little more generous for the general public.

Your comment regarding autogiro historians is really interesting. I guess I too was fooled into thinking that a rigid rotor system would result in left roll... Thanks for the insight! From now on I would have to change my "story" regarding the purpose of the teetering rotor. The gyro would loop, not roll!

Udi

[C. Beaty]

When I was a kid, Udi, roll roofing (mineralized felt) came with saucer shaped sheet metal stampings at each end to protect the edges. Made great frisbees before frisbees had been invented.

I used to cut radial slits around the periphery with tin snips and twist the segments with pliers for a propeller effect.

To my amazement, when tossed with a spin like a frisbee, the things would loop.

It was not until years later that I understood why.

Simply the law of gyroscopic precession; displacement lags force by 90º.

[mrford61]

Correct or not, I have come to the conclusion from reading stability tests is that if sudden full power on brings the nose up, and sudden power off does the opposite, then a gyro,s thrustline is fundamentally ok ??

If that is in any way correct, then changing my gyro from the heavy "Mitre 10 aviation" fin, rudder and large HS on the keel, to a Tall Tail with HS on the thrustline, changed my gyro from unstable to stable.

Mark Clifford.

[Fliterisk1]

My question concering thrustline change came out of Newton's law of action/reaction. If the tail plane will feel a larger push then normal, then shouldn't the prop feel a larger pull then normal to develop that pushing force; and from the same direction as the relative wind?

As applied to Udi's thoughts on the rotor, this force would be considered as part of the rotor's drag, not lift. And if it exists then it would negate the drag to allow forward motion.

As to whether it exists or not, (and how strong it would be), this could be determined by monitoring the loads on a prop in a wind tunnel. Or maybe measuring the loads on the second fan in the two-fan experiment. I thought this test might already have been done, and could be used as proof to the initial arguement that prop wash increases HS response.

Also, with regard to Mr. Beaty's comments on the yawed flow. Okay, P factor doesn't exist as described (it should actually lower the nose to point it into the wind). So, as Udi mentioned, Cierva's aircraft actually suffered dynamic rollover on takeoff from too slow of a rotor.
Finally, I thought cyclic flapping actually shifted thrust to different portions of the disk, rather than shifting the thrustline itself. The flapping forces the disk to align with the rotor hub, and doesn't necessarily care whether the actual thrustline is normal to the rotor disk plane?
--
Fliterisk1