low tail horizontal stablizer on gyros

mikegrady

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Joined
Oct 11, 2005
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Location
South Chareston, WV
I have heard some talk about the centerline thrust gyros and the effect that the prop has when placed at the aircraft's vertical center of gravity. I noticed that most just state "center of gravity" but that certainly isn't the case...... I have noticed that the rear horizontal stablizers are much lower than the vertical center of gravity. The air traveling over this should have a pretty large effect. Placing this well below the vertical center of gravity seems to go against all the other information that I see so many people talking about.....especially since the wing is so far behind the actual center of gravity.
What is your take on this and what provisions should I consider in a future project?
Thanks,
Mike Grady
 
I will have a crack at it for you Mike.
mikegrady said:
Placing (the horizontal stabilizer) well below the vertical center of gravity seems to go against all the other information that I see so many people talking about....
It matters little whether the stabilizer is above the centre of mass or below it - the effectivness of the stabilizer depends primarily on its airspeed, area, and its distance from the C of M. You will notice on "T" tail aircraft like the Piper Tomahawk that the stabilizer is a considerable distance above the C of M.
mikegrady said:
especially since the wing is so far behind the actual center of gravity.
As stated above, the further from the C of M, the better.

There are many other considerations in stabilizer locations that should be considered. Some prefer it to be in the accelerated propeller slipstream to gain an advantage from extra airspeed. It can also cancel out some of the "torque roll".
Others prefer it to be on the keel and further back, where it can gain an advantage from its distance from the C of M, and operate in the same relative airflow that the rotor sees (not propeller influenced airflow).
There are also others with combinations of the above.

It basically comes down to a matter of personal choice.
 
Tim is correct! The further from the center of mass, in the longitudinal direction, the better. It is strictly a matter of leverage or, as it is often stated here, the horizontal stabilizer 'moment'. The center of mass is what all the 'moments' (rudder, prop thrust, rotor thrust, aerodynamic) act on or around.

The placement in the longitudinal direction affects pitch stability while the placement of the horizontal stabilizer, in relation to the prop, brings other factors into play as Tim has mentioned. It doesn't matter whether the horizontal stabilizer is above or below the center of mass. There are advantages and disadvantages in the different configurations and, as is usually the case, choices/compromises have to be made to match the mission or desires of the designer.
 
Righto tim !
in my adventures with the verious HS configurations I have made and studied on or about i have found that eather in the center line of the thrust or at the half way point between centerline and the keel in the fastest air possable were the best configurations .... I believe however like you say Tim its a matter of prefrence
the safty ishue comes into play if the HS is to small ot too close to the COG....
by all means figure out the sqr.footage of the HS and see if it is big enough , most are marginal at best .
I use HScalc2.xls to calculate mine... so far so good
I can't remember where i down loaded it from but its an Excell spred sheet program and seams to be acurate.

My HS is 18" off the keel in the fastest air it can be put into and is 7.4 sqr ft in size .... its Over kill .
just my 2 coppers !

Bob........
 
I was hoping to get the compromises.......which way does what. I understand the leverage effect you will see by having the tail further from the CG.....not necessarily always good....depending on how much it is being loaded. How much it is being loaded is the question I can't answer. We obviously don't load it nearly as much as a typical aircraft as we don't reach their velocity. The more info you can provide, the more I can understand what I should focus on and what I shouldn't. One of the reasons this is so appealing to me is that most of these aircraft are being built by individuals. Slight differnces can make a big difference in some cases.....others not. I am just trying to figure out wher I can play and where I shouldn't.
Thanks,
Mike Grady
 
Mike: The load on the HS is not a function only of the gyro's airspeed. There are other considerations.

The HS will be affected to some degree by the prop slipstream. The airspeed of the slipstream is in the vicinity of 100 mph at wide-open throttle, even for a slow gyro. If the HS is fully immersed in the slipstream, the forces on it will be determined by the speed of the slipstream, not the speed of the gyro. (For example, the HS could "see" 100 mph of airspeed at wide-open throttle, even if the gyro were on the ground and tied to a tree.) The HS will be "fully immersed" if it's placed near the centerline of the prop disk. The HS will be at least partly immersed almost anywhere you put it.

The HS load is also affected by the HS's angle of attack (AOA). Like any other wing, the HS may have an AOA that's positive, negative or zero. The AOA, in turn, is a function of the HS's incidence -- the angle the HS makes with respect to the horizontal datum line of the aircraft. People often assume that AOA and incidence are the same, but they aren't necessarily. They will usually be close, IF the aircraft is rigged to fly with its airframe level. An aircraft rigged to fly very nose-down or nose-up will have a HS AOA that's significantly different from its incidence.

Whether a gyro's HS is loaded more or less than a FW plane's is a choice to be made by the designer. Zero load or a very light down-load in steady flight is normally all that's needed for an open-frame CLT gyro. A gyro that has a cabin and/or a thrustline slightly above the CG will likely require more down-load. A gyro with a thrustline 5, 6 or more inches above the CG will require a HS that delivers several dozen pounds of down-load continually. At some point, the thrustline misalignment can be so large that you can't get the necessary power out of the HS -- or you can, but the resulting loss of performance becomes intolerable.

That's one good reason for CLT in the first place.
 
Doug,
Thanks for the info. I actually follow your logic pretty well. I had a feeling that the prop would be a big player in airspeed.....not the total gyro but at the wing. The acceleration of the air (and resultant elevated velocity) at the prop and the rotor is why we have that low pressure area that allows us to leave the surface of the earth.

Let's go the next step in this investigation and education of a rookie.... What gyros are CLT designs? Is there a place on here or elsewhere that discusses the fine points of the most recognized gyro designs?

I guess what sparks my interest the most is wondering who the designers are and where they came from with regards to their experience. The open air gyros seem awefully simplified BUT I am just smart enough to know I need to be VERY careful when playing around with this stuff.....basicly because I know I don't know enough.

I came from racing karts. I raced all over the east coast in the top series BUT in the past year, it was becoming obvious that I needed to make a change. I am glad my friend flys radio controlled aircraft. If his words hadn't sparked my interest, I wouldn't have seen that gyro with the little guy in it on the internet......turned out to be a real guy in a human sized aircraft....LOL. That pretty much sold me.

Thanks for the input,
Mike Grady
 
Mike (and others in the same boat): Here are a couple sources of non-partisan technical info about today's gyros:

FAA Rotorcraft Flying Handbook (includes a section about gyros). Available from mailorder companies such as Aircraft Spruce and Wicks Aircraft.

Articles by Greg Gremminger -- First published in the PRA magazine and reprinted at the U.S. dealer section of the Magni Gyros web site.

As for whose design is CLT -- none of them is perfectly CLT, nor do they need to be. As long as the CG is reasonably close to the prop thrustline (no more than 3" off for most designs), the HS can take care of fine-tuning the remaining pitch effects of the thrustline location. The HS also must be set up to counter other pitching tendencies, such as those arising from landing gear, windshields, cabins and the like. The key to pitch stability is not just CLT, but the craft's ability, on its own, to remain balanced in pitch when disturbed (whether by an airspeed change, turbulence, maneuver or throttle change).

Among the aircraft that I BELIEVE meet this criterion of balance-in-aerodynamic-pitch are the Magni, Sparrowhawk (and Sparrowhawk's RAF conversion), Dominator, Gyrobee, Air Command (new series), Butterfly and Little Wing. This list is not exhaustive! It's based only on what one guy has happened to run across.

Most gyro designers do not have engineering training. That's simply a fact to be lived with. A few do, and a few more have submitted their designs to experts for review. Some also have flight-tested their craft to the Light Sport Gyro standards prepared by the industry in connection with the Sport Pilot regulations. In each, case, ask!
 
Perhaps I’m dense but I don’t understand why you would put a HS in the prop wash.

The air in the wash is moving in a vortex. Not n straight stream.
Then if you look at the air hitting leading edge of an HS. That is in the wash. You will find the air is not a constant stream but a series of micro bursts. Each burst caused by the spinning propeller

It seems to me this would cause a great deal of vibration. I always thought that vibration equals stress on the frame and pilot.

The purpose of an HS is to help prevent PIO.
The motor and prop don’t know if the nose is up or down.
Only the air less affected by the prop has the ability to stabilize the craft.

That is why I think a HS out of the wash is best. Am I missing something?
 
One benefit: Placing the HS in the propwash creates a direct link between down-load and throttle setting - which is important if you're trying to compensate for a high thrustline and reduce the risk of PPO. To that extent, you can think of the HS as deflecting the propwash upwards, with the consequent downward force on the HS.

As the overturning moment from the prop increases, so does the compensating moment from the stabiliser.
 
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John: Yes, you are missing something. You've joined the discussion after the end of a very long exploration of just this topic that extended over several years. We've beaten this topic to death.

First, the vibration problem... isn't. You'll just have to take our collective word on that. Scores, if not hundreds, of gyros have deeply immersed HS's, as do such warhorse airplanes as the Seabee, Cessna Skymaster and a few dozen other models... plus the type-certificated production gyros such as the McCulloch J2 and Air and Space 18A. It isn't difficult to make the structure adequately stiff to handle the propwash turbulence without a problem.

Second, it's an easy mistake to make to assume that the direction of the propwash is unrelated to the direction of the freestream. In fact, the propwash is a horizontal tornado that "bends" in sympathy with changes in direction of the surrounding flow. Later, I'll describe a simple experiment to prove this, in case you might still be skeptical.

Third, the propwash does rotate (although it's not quite the tight swirling corkscrew that some people envision). The HS receives this swirling air... and straightens it out. The reaction on the HS produced by this "straightening" process is a torque on the airframe that COUNTERS the torque produced by the engine. As a result, a large, centered HS helps to reduce torque roll and yaw effects. When combined with a full-span vertical fin-rudder as seen on the Dominator, Sparrowhawk and many other gyros, the tail essentially eliminates these effects. This dramatically reduces pilot workload.

Now, the experiment for the skeptics. Obtain two electric fans. One can be a big box fan; the other should be small enough for you to lift and move about easily. Tie some yarn tufts to the big fan's grille at various locations and turn it on. The tufts will stream out, of course, and they'll also lean a little in the direction of the fan blades' rotation, showing you that the slipstream does rotate. Now for the cool part. Take the smaller fan in hand, turn it on and aim ITS slipstream into the INTAKE side of the big fan. Move and tip it in any direction you like -- you'll see the tufts on the EXIT side of the big fan move in response!

What this demonstrates is that the orientation of the outflow is a function of the orientation of the inflow. IOW, the prop slipstream's direction follows the direction of the surrounding flow. Even if you're not relying on the HS to counteract engine-related pitching effects, the HS is still more effective when inside the propwash than outside it (the lift of an aerodynamic surface varies as the SQUARE of its airspeed).

The main fly in the ointment with this setup is that the HS loses power when you throttle back or turn off the engine. You feel this in the controls as an increasing mushiness or "rubbery" feel when doing idle-power landing approaches.
 
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Dang Doug
You should have seen the look my daughter gave me as I hauled the fans out of the basement. It’s about 55 f out side to day.
Your post was great but her puzzled look was priceless.
 
I got funny looks when I tried it, too. In fact, we never did take the yarn off that fan.

You're never too old for a science fair project.
 
You're never too old for a science fair project.

You're never too old for a science fair project.

Doug
The science fair project got me thinking. Witch then turned in to a larger since fair project. After four hours I have some good pictures that show the forces a HS work in.

I show both totally immersed and partially immersed HS. You can see how they both respond in level flight and in PIO situation.

But I can’t get the pictures to down load.
There were two threads this week that had instructions on how to do it. But the moderator must have cleared them.
I could just kick myself for not coping them.
Could you send me a personal message on how to down load pictures?
Thanks
John
 
John, when you have a question for the group then the first thing you should do is a 'search'. There are few topics that haven't been covered here. It is likely that what you will find will raise specific questions to clarify what you find. I did a search on 'picture upload' and found the tutorial that Mike Gaspard put together. You can find it here https://www.rotaryforum.com/forum/showthread.php?t=1274&highlight=picture+upload

Todd, Mike, I thought this had been made a sticky. If it hasn't then I would suggest that it becomes one since this question pops up frequently.
 
blind

blind

THANKS for the info
I did a search but I’m blind in one eye and can't see out the other.
Great pilot material don’t you think.
 
You'll be disgusted to know the RAF has got yet another ,reconfigured stab onit Doug. :D ;)
 
Birdy: Is it a real HS (frame mounted) or the aerodynamic trim spring/whale tail device?

Any stability device* that's rigged to manipulate the rotor misses the whole point. The rotor DISAPPEARS aerodynamically during zero G.

* "Device" can mean trim spring, offset gimbal head, pivoting mast, whale tail, computer autopilot or even just a human pilot who's had "training, training and more training." No amount of training or gadgetry can repeal the laws of physics.
 
yes Doug, its a real stab. I woulda called it sumthn else if it was sumthn else. ;)
 
Birdy: When you mentioned "disgusted," I first thought you meant the factory gave in on the issue and mounted a HS... but perhaps you mean your own personal RAF? A HS can't cure a stock one altogether, but it's bound to help.
 
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