Gyroplane Thrustlines vs. Center of Gravity

GrantR

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In the rotorcraft flying handbook is states that
if you have a gyroplane where the CG is below the propeller thrust line, the propeller thrust gives your aircraft a nose down pitching moment when power is applied. To compensate for this pitching moment, the CG on this type of gyroplane is usually located behind the rotor force line. This location produces a nose up pitching moment.

Then it goes on to say fuselage drag, pitch inertia and the addition of a Horizontal Stab can alter when the CG is placed.

A high TL cause a nose down pitching so the rotor has to be tilted back more to overcome this nose down pitch or the gyro will dive. So when the rotor is tilted back that automatically puts the RTV further forward and ahead of the CG thus making it tail heavy. This is how it compensates for this nose down pitching moment. With no horizontal stabilizer tilting the rotor back is the only way to over come this.

So in a high thrust line Gyro the engine thrust is fighting with the rotor thrust. The rotor wants to keep the nose up and the engine wants to put the nose down. When one variable changes whether rotor thrust or engine thrust it causes pitching, nose up or down in the airframe since the change causes an imbalance in the opposing forces. This plus being tail heavy makes the aircraft unstable.


Now I am going out on a limb and say it seems to me a HTL is safe with a large HS that is trimmed and capable of creating enough down force on the tail of the gyro to offset the nose down pitching. It seems if its trimmed just right it could even cause a nose up pitching with increased engine thrust. Now if the gyro sees 0 or negative Gs this is where it gets confusing. I know from reading the H stab mods for one type of HTL enclosed Gyro is not big enough to offset the massive amount of nose down pitching the HTL causes.


Using an effective H stab at full power, is there a bunt possibility since the H stab is trimmed to create enough down thrust to offset the nose down pitching? Or At full power is there a bunt possibility since the H stab is trimmed to create enough down thrust to offset the nose down pitching in normal flight with a loaded rotor?
 

Terry

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Rtl?

Rtl?

I am really struggling with that article Chuck.

I can find no reason to believe there is a forward component to the rotor on an autogyro as 4b. seems to indicate. It looks like Voodoo.

Also, while the article seems to want to debunk the idea there is a sum drag component to the rotor, it continues, textually, to affirm that it exists.

I understand that the rotor blades have a forward component of lift in relation to their plane of rotation, but it is still negative in relation to the angle of incidence of the rotorplane and aircraft, and working contrary to propeller thrust.

So, it seems for the entire aircraft in straight and level flight there does exist rotor drag <b>and</b> structural drag which are aerodynamically balanced against propeller thrust or drag. When the rotor is then unloaded, the drag component of the rotor diminishes rapidly as the rotor plane rotates toward parallel with the wind.

It seems to me that both are correct. Stability issues are both that of balancing thrust near the center of mass, and about balancing thrust near the center of drag. Or, at least, about balancing these two balances against each other. The only problem being that the center of drag moves dramatically, vertically dependent upon angle of incidences, and is not as stability seeking as conventional aircraft.

Adding to this confusion it seems powered gyroplane design not only needs to consider propeller thrust but propeller drag when powered down; more so than conventional aircraft because of the misalignment of all these offset forces.

As a newbie, it sounds like this idea of "rotor thrust line" is a carryover from helicoptereze and, because it is so esoteric, causes confusion in the discussion of autogyros.

Terry Graham
:yo:

Moved by willed purpose.
 

Mike Schallmann

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Oh Terry -you have so much to learn --all of this has been hashed out and discussed over many years both on this forum and the previous one- ( Norms)- start by doing a lot of research on the subject -- you cant change the laws of Physic no matter how hard you try--The statements you made display your lack of knowledge -- study hard young grasshopper and you to can become a master--live long and prosper--
 

All_In

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Hi Terry
I'm still learning it myself.

Your confusing a few things that's all (Or more probably I’m confuse as to what you really meant!). But you’re on the right track!!!
This thread is a good one to read from start to finish and there are several others.
There is no quick answer, except the math, so for most the only way to really learn it is to read all of the threads and then the light begins to shine.
 
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Terry

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You're right Mike, I do have much to learn.

Yet what I said seems to also coincide with the ASTMs posted on the PRA site. Is it my terminology or what?
 

GrantR

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Doug,

I am correct in my interpretation of text I referenced from the rotorcraft flying handbook?
 

mitt11

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Excellent info on gyros for a newbee before any purchase is done. Thanks to barnstorm2.
 

Doug Riley

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Grant: Much-delayed answer: It's possible to make a HTL gyro PPO-proof with enough H-stab power. With a large HTL, however, this kind of compensation becomes impractical.

A prop thrustline around 2-3" above the CG is manageable.

The 12- and 14-inch HTLs, OTOH, require an unwieldy amount of HS down-load that badly compromises performance. To the rotor, 100 lb. of HS down-load looks just like 100 lb. of lead. For the HS to make the download, it has to create both profile drag and induced drag, further degrading the gyro's efficiency. Taken to the silly extreme, it's like designing a table with little rocket motors in place of its legs. It'll work, but it has disadvantages.

If this is what you are getting out of the Rotorcraft Handbook, then you're reading it correctly.
 

Doug Riley

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Mike, a slightly low thrustline augments the rotor's own contribution to airspeed stability. When you add power to a LTL gyro, the nose rises. If the stick is held still, the effect of the nose rising is an increase in rotor AOA. This, turn, prevents the gyro from speeding up in reaction to the addition of power. Instead, the gyro initiates a climb at its original airspeed.

This behavior is "static stability of airspeed with respect to power setting" and is generally a good thing. Among other benefits, this type of stability causes the gyro to drop its nose gently and preserve its airspeed if the engine quits. An uncompensated HTL gyro, in contrast, tends to execute a mid-air flare when power is cut, resulting in a loss of airspeed just when you need it. Bensens used to do that.

Too much of a good thing can give you a bellyache, though. Moderately excess LTL causes the gyro to slow down when power is added with stick fixed (Dominators often do this). You must re-trim or push forward when adding power to avoid slowing down. In moderation, this is merely extra work for the pilot. The same LTL setup also causes the aircraft to pick up speed when power is cut. You must pull back to avoid an accelerating dive should the engine quit. Again, extra work.

Extreme LTL takes this slight annoyance to a possibly dangerous point. If the engine quits on such a craft, some combinations of rotor, airframe and limited HS power may react badly to the sudden, extreme nose drop that will occur when power is cut. It's possible (on paper anyway) for an extreme LTL gyro with a light, fast rotor to trip over its own airframe and tumble if power is chopped at high airspeed. There's no documented case of this happening to such an extent that it caused an accident, but one or two unexplained accidents may have been caused by it, and at least one pilot has reported being scared by the excess nose drop.

Thrustline plus or minus 2" of CG in the vertical axis, with a large HS having some propwash over it, is the consensus rule of thumb. It still works well. Other combinations can be made to work as well, but require more sophisticated design work to avoid creating a "coffin corner" (or simply a pig of an aircraft).
 

Earthboundmisft

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Doug, excellent explanation. Makes perfect sense. The projected thrustline of my machine extends through my hip, when seated in the gyro. I have not done the double hang test yet, but plan to soon. Also, my thrustline is set to be level, as is my horizontal stab, when the mast is at nine degrees tilted rearward. Thanks, Mike.
 

Vance

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Observations on low thrust line gyrplanes

Observations on low thrust line gyrplanes

I present my observations on gyroplane thrust line offset.

The two place tandem gyroplane I fly, the Predator, flew like the description of a low thrust line gyroplane. I don’t feel that I have ever properly and consistently determined the vertical center of gravity so I am not able to share that information.

I recently changed the engine and propeller and raised the engine almost two inches. I will try to describe the difference. I feel it is probably still close to center line thrust depending on the fuel load and passenger weight.

As Doug described she used to raise her nose when I would increase power and drop her nose when I would reduce the power. The faster I changed the power the more the change in pitch. The SparrowHawks and modified RAFs I have flown did the same thing only more so. The two place Dominator I flew briefly seemed to do the same thing. In most operations this did not have much effect on the flight path and I though little of it. I prefer it to the nose drop on the back side of a thermal with the high thrust line gyroplanes I have flown. I have also found that the faster I go in a high thrust line gyroplane the more the nose drops. I find this disquieting.

Previously on takeoff under full power her nose would come up very quickly and it was hard to catch her before the tail wheel hit. Now when the nose comes up it is more gradual and it is easier to keep the nose wheel near the ground.

Previously when I would back off the power too quickly at the top of my climb the nose would drop and it seemed like there was an extra step in the process. Now as I reduce power there is almost no change in pitch and the transition seems seamless. She just climbs less.

Previously when I would reduce power to land the nose would drop and she would tend to pick up speed if I didn’t make cyclic adjustments. I found that waiting until the pitch excursion had finished before I turned base made things more precise. Now the process seems to flow together and it does not feel segmented.

Previously when I would add power to cushion the landing the nose would pitch up and it reduced my ability to control the aircraft near the ground. Now when I add power near the ground she just floats along with no pitch excursions. It is easy to fly very near the ground and I have more ways to save an inelegant landing or manage gusting conditions.

I can now go from wide open throttle to full back without any pitch excursions.

In my opinion, because the Predator has a very large horizontal stabilizer set well back from the center of gravity, any pitch excursion is a very casual experience.

I feel that it is important to adjust the throttle smoothly and I am not advocating rapid throttle movements. I am only reporting on my observations based on my level of experience and my piloting skills. I prefer a gyroplane where the throttle and pitch are less connected.

Thank you, Vance
 

Earthboundmisft

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Vance, your results with your machine before, and after the mods seem to reinforce Dougs experience. Sounds like you changed for the better.

Pete, greetings. I tweek on the Missfit almost nightly. I look forward to my wrench sessions, working down my squak sheet. The plan is to have Dave eyeball it, and hopefully take her around the Wauchula patch 12/30/09. Im nervous as a cat on a hot tin roof.
MY first solo is undetermined as of yet. I am feeling confident, but I need more hours.
Got quite a few beans invested, dont want to wad it up. Later, Mike.
 

kolibri282

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JAL

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Thanks for posting the paper as it an interesting read. On the section of rotor teeter motion they stated that when quickly moving the stick forward the rotor sped up signifcantly (which is what the instruments indicated) I was wondering why? is just that the drag is reduced but the same amount of air is flowing through the rotor.

Also I found it interesting that they had shown the teeter angle is excessive and close to chopping the prop with the rotor when carrying out the recovery stated in the tests They stated within 5cm). Would this suggest that at least some of the PIO accidents may be the result of the recovery action of the pilot and not the continuation of the tumble. What I mean is that if the pilot starts to nose over and pulls too hard on the stick to recover and exceeds the safe teeter angle and chops the tail or propellor off?
 

Jean Claude

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In my opinion, the analysis from experimenter is bad: an increase of rpm can only come from an overload of the rotor (not with the stick forward). What is surprising is that the experimenter is not surprised!
The slow increase observed up to 7 ° (fig. 13), associated with increased rpm (from 360 rpm to 460 rpm), are caused by a disk delay / head when the nose quickly up (40-60°/s ?). Under these conditions, the tail is far from the disk and the blades can not touch the propeller.

Jean Claude
 
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Doug Riley

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While valuable, the article does not deal with the effects of immersion of the H-stab in the propwash. In fact, it states specifically that the H-stab is only effective at high forward airspeeds. That statement assumes that the H-stab experiences only the airspeed that the aircraft itself experiences.

A H-stab immersed in the porpwash experiences airspeeds of 80-100 mph or more at wide open throttle, even when the aircraft's forward airspeed is zero. Therefore, the statement in the article about the limited effect of horizontal stabilizers is correct for non-immersed stabilizers only.

A gyro with a high thrustline should have some degree of immersion of its horizontal stabilizer. The stab should be sized and mounted with sufficient negative incidence so that the aircraft does not change its airspeed when the throttle setting is changed.

By "manageable," I mean that a 3" HTL is sufficently small that a practical H-stab can achieve these goals. If this is done, the aircraft will not be pitch-unstable and will not be at all difficult to handle. In fact, I have not noticed any difference in handling between a craft with a small amount of HTL, properly compensated by immersed H-stab with negative incidence, and a LTL craft with an immersed H-stab having zero incidence.
 

Jean Claude

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By "manageable," I mean that a 3" HTL is sufficently small that a practical H-stab can achieve these goals. If this is done, the aircraft will not be pitch-unstable and will not be at all difficult to handle. In fact, I have not noticed any difference in handling between a craft with a small amount of HTL, properly compensated by immersed H-stab with negative incidence, and a LTL craft with an immersed H-stab having zero incidence.

Exactly Doug, but it is a false HTL: propellers axis is high, but the resulting thrust is centred.

Jean Claude
 
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Doug Riley

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Jean Claude: Yes, that's true. "False HTL" is a colorful way to describe it.

Of course, if you add the rotor's thrust (behind the CG) to your drawing, you will have static equilibrium.

More important, though, is that, when the rotor's thrust disappears, the "net thrust" (or "sum of moments") causes a nose-up rotational acceleration. This condition is the exact opposite of PPO. Unlike PPO, it tends to restore positive G on the rotor, rather than causing the the aircraft to tumble -- even though the propeller's thrust line is above the CG.
 
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