A better arguement for a stable machine

[gyromike]

Tim

At the lower air speeds the HTl isn't as critical as they are at cruise. As long as the rotor is loaded and aircraft is close to the ground wot will just allow you to settle. The HTL is just more dangerous when speed is involved the rotors are unloaded and the thrust can over power the RTV.

If you are talking about keel mounted stabillizers Thom, the HTL is more critical at low speeds. The stabilizer needs airflow over it to produce downforce. That means airspeed.

Cut the speed in half and downforce is reduced by four. Now you are depending more on the rotor to counteract the HTL trying to rotate the frame nosedown.

How are the rotors more unloaded at high speeds vs. low speeds? The rotors are loaded the same whether you are going fast or slow. The AOA changes with speed, not the load.

 

[barnstorm2]

Would that HStab have to be in the prob-wash? Other wise at low airspeed the HStab would be ineffective and unable to counter a bunt?

Tim

At the lower air speeds the HTl isn't as critical as they are at cruise. As long as the rotor is loaded and aircraft is close to the ground wot will just allow you to settle. The HTL is just more dangerous when speed is involved the rotors are unloaded and the thrust can over power the RTV.

Humm.. I was under the impression that under certain conditions you can indeed unload the rotor during low airspeed conditions. If you have the engine running or apply power (which might be a knee-jerk reaction to 0-g) you would then bunt.

:sorry: Some of you guys need to wake up to reality. In 10 mph + gust spread all Gyroplanes will need input from a pilot to fly them. ......:

My CLT Aircommand would require very little input once trimmed gusts or thermals or smooth air.

 

[Doug Riley]

Steve Osborne, there is no need to chase the stick in turbulence in a stable gyro.

I've flown the tandem Dominator down in the mechanical-turbulence zone (couple hundred feet over hills and trees) in 30 mph winds. "Floating" the stick is counter-productive. Hold it still as usual and the machine holds airspeed very nicely. It noses smartly into up- and down-air. If you're used to an unstable machine (as I was), this activity will catch you by surprise. You'll soon realize, however, that the machine's responses are stable and helpful. If allowed to do what it wants, it will fly itself with stick fixed.

Once you cross the divide between stable and unstable, there are machines that go out of control and crash in a single unannounced forward flip, while others take a few more seconds about it. The latter should not be confused with stable craft.

 

[GyroRon]

If you are talking about keel mounted stabillizers Thom, the HTL is more critical at low speeds. The stabilizer needs airflow over it to produce downforce. That means airspeed.

Cut the speed in half and downforce is reduced by four. Now you are depending more on the rotor to counteract the HTL trying to rotate the frame nosedown.

How are the rotors more unloaded at high speeds vs. low speeds? The rotors are loaded the same whether you are going fast or slow. The AOA changes with speed, not the load.

That is just Thoms on going habit of stating what he believe as bonafide fact. He still has alot to learn.

 

[Doug Riley]

I don't know of anyone who has PPOed by hammering the throttle and throwing the stick forward at the bottom of a vertical descent. It probably could be done in a susceptible machine, though, especially if you were unlucky enough to get hit by a downdraft at the same time. If so, that would be a zero-airspeed PPO. Only a H-stab in the propwash would work at zero A.S.

An unscientific review of PPO accidents from nearly 38 years of PRA mags suggests that the majority of these accidents occur in the pattern, very often on climbout. The gyro is not going very fast if it's at best rate of climb. The throttle is wide-open and the pilot is probably not in the mood to slam the throttle shut at such a low altitude.

The HS absolutely MUST develop enough down-lift to counter any HTL effects at these low airspeeds. A real-world H-stab generates less than than four OUNCES of lift per degree of angle of attack at 40 mph. You have little choice but to immerse it in the propwash if you need down-lift.

A keel-mounted stab may be adequate for a CLT or LTL machine, but not for a HTL one.

 

[dragonflyerthom]

Doug

You have just reinforced my own understanding of the HTL machine at lower speed and descending. It does seem that this happens a lot on climb out and at the magical numbers of 60 MPH, This has pricked my interest and would like to research this problem. I am wondering why the last gyro was on climb out and then rolled to the right. Still no info on that accident.

 

[Doug Riley]

Thom: Does the prop on a RAF turn counter-clockwise? If so, the roll was likely torque reaction. Rotor thrust normally resists torque roll as well as PPO. Take away rotor thrust, and you'll roll over and pitch over at the same time unless you are lightning-fast at chopping throttle.

A related problem happens with low G in teetering-rotor helicopters. The tail rotor's thrust can cause a rolling action and mast bumping in low-G events.

The prop-roll effect can clearly be seen in the famous films of Pee Wee Judge's fatal crash. This accident happened in a Wallis gyro at the Farnborough airshow in England in the early 70's.

Tall tails and large full-span HS's substantially cancel torque roll without the need for rotor thrust.

 

[enewbold]

WELL, LET ME TELL YOU THIS...:tape: ...:blabla:...

Hehehe... Tell it like it is, Harry!

 

[dragonflyerthom]

Yes Doug

The prop on a RAF does turn counter clockwise from the rear,. But if you are asking from the front it turns clockwise. Sitting under the rotor it turns clockwise.

 

[Doug Riley]

Thom, if it turns CCW to a viewer standing behind and looking into the back of the aircraft, that's consistent with a torque-over to the right.

Torque-overs happen under the same circumstances as PPO (low G, high throttle). They are not brought about by HTL, however, but by an absence of fins that provide roll stability. The traditional Bensen hobbit-high tail does not provide roll stability. Tall tails and full-span immersed HS's do, to various extents.

 

[dragonflyerthom]

That makes sense. Seems something else that can go bad to a gyro pilot. I would think the tall tail would have counter torques on it giving mixed forces.

 

[Doug Riley]

Thom, picture that corkscrew propwash whirling back toward the tail. In a RAF with its CCW prop, the corkscrew turns (of course) CCW. That means the top half of the tall tail gets whacked by propwash on its right side. This makes the top half want to move to the left. The bottom half gets whacked on its left side, so it wants to move to the right. Both halves thus want to roll CCW, just like the propwash.

The engine's torque reaction, OTOH, is OPPOSITE the prop rotation. IOW, the torque tries to roll the gyro to the right. The tall tail catches propwash and tries to roll the gyro left. The two tend to cancel each other out.

In practice, a Dominator has almost no roll reaction or yaw reaction as it leaves the ground. Bensen-configuration gyros go through a little torque "wriggle" just as they un-stick.

I don't miss the "short-tail wriggle." I especially didn't miss it when I tried Rick Martin's Gyrobee at Bensen Days last year. I'd kinda forgotten about the "wriggle," but I certainly got reminded in a hurry. The Dom. gives you lazy feet.

 

[Bob Gregory]

Goal is to size a RAF stabilizer with 12 inch offset and 600 lbs thrust. Using a 72 inch propeller.

Will a two by five foot stabilizer set at one inch negative work if immersed in the propeller wash? Only if mounted five feet from the center of gravity?

Where is most efficient location for the stabilizer in the propeller wash?

Will a rotor experience a quick up moment before the frame turns down because of the stabilizer in an up gust?

I keep wondering if all this writing about RAF's is much over blown. I have always read the stabilizer is just a damper for gusts.

 

[StanFoster]

Doug: I most definately have noticed little to zero tendency to roll with my tall tail.

It took me awhile to get used to not having the nose go down with power reduction...and go up with applied power. I had to delearn some reflexes that my RAF had me acquire.

The RAF flew fine with these subtle reflexes....but now I dont need them and it is definately more hands free flying.

Both machines are fun to fly.......but my SH is the easiest thing I have ever flown.


Stan

 

[Fl90]

Good post, the difference with informative, simple, and nonargumentive statements.

Thanks Stan, Phil.

 

[Doug Riley]

Q:Goal is to size a RAF stabilizer with 12 inch offset and 600 lbs thrust. Using a 72 inch propeller.

Good luck; it is almost impossible and, in any case, costs you a severe performance penalty. With a five-foot lever arm, you need at least 120 lb. of down-load. That costs you the same amount of performance as 120 lb. of lead.

Q:Will a two by five foot stabilizer set at one inch negative work if immersed in the propeller wash? Only if mounted five feet from the center of gravity?

I'll assume you meant one degree. No, it won't be enough. It would need to be at 10 degrees or more; right near stall.

Q:Where is most efficient location for the stabilizer in the propeller wash?

We'll explore that in more detail at Bensen Days; most likely a little above or below the center.

Q:Will a rotor experience a quick up moment before the frame turns down because of the stabilizer in an up gust?

No. In my experience, a gyro with a proper H-stab immediately noses down like a dowser's rod, without at all going the "other way" first.

Q:I keep wondering if all this writing about RAF's is much over blown. I have always read the stabilizer is just a damper for gusts.

You're mixing two issues. A HS that is too small to do its primary job may still act as a damper to reduce the tendency to PIO, but its main function is to provide static angle-of-attack or G-load stability. This main function assures you that the gyro not only weathervanes into updrafts, but also will not pitch over as a result of moments induced by cabins, high thrustlines or other airframe features. The rotor cannot provide the force necessary to prevent a pitch-over if the rotor happens to be experiencing sudden low G.

 

[Bob Gregory]

Well, Doug Riley, if I use Chucks, Raghu's, and your ideas, formulas and philosophies and now are you saying they don't work. I incorporated all in the spreadsheet at #133. The example is in the lower right using the square of the length for which I can not find right now. Anyway you wrote it I think about the 8th. About the square of length of the cg to stabilizer needing less pounds. It did not sink in right away. It seems to work in the sheet. Sure appreciate coming back to me on this. If I am missing some concept of the formulas I sure want to know.
Thank you. BG

 

[Doug Riley]

Bob, I have not worked with that spreadsheet. However, Chuck, Udi, Raghu and all the others who understand this material are in substantial agreement. It's not new stuff at all; the principles were worked out before WWI. It's only new to homebuilt-gyro people.

Forget the spreadsheet for now. Your time is better spent working to UNDERSTAND the underlying principles and doing a few problems with a simple pocket calculator. Don't work blindly with a complex Excel program. H-stab lift is so simple that a program isn't necessary unless you need to run a large number of scenarios.

There are two stabilities to consider.

1. STATIC stability is the type that prevents the deadly power pushover (PPO) in HTL machines. The power of a H-stab to provide static stability does NOT increase as the square of the length of the moment arm. Rather, this power is a linear function of arm length, a linear function of H-stab area and a linear function of angle of attack. Twice the arm, twice the H-stab moment. Twice the area, twice the H-stab moment. Twice the angle of attack, twice the H-stab moment.

The only square function involved is that of airspeed. Twice the airspeed, FOUR times the H-stab moment if all the other factors stay the same.

To use the attached chart (based on actual H-stab lift tests), pick an airspeed, an angle of attack and a H-stab area. Read airspeed in mph down the left column and AOA across the top row. Multiply the resulting lift/sq. ft. by HS area and then by moment arm length to get your HS's moment in foot-pounds. For 600 lb. thrust on a 12" HTL, you'll need at least 600 foot-lb. of HS moment.

2. Dynamic stability (also know as damping) is the power of the H-stab to prevent or stop oscillations. This power IS a square function of arm length. It's a linear function of H-stab area. It's a square function of airspeed. A H-stab that provides positive static stability is almost certain to provide good damping as well. In practice, you size the HS for static stability at a reasonable AOA and dynamic stability is just about guaranteed.

Since the ability to provide static stability is a linear function of arm length, while dynamic stabilizing power is a square function of arm length, you will typically get more vigorous damping in a craft that gets its stab power from lots of arm length than one that gets it power from a huge stab area. That's probably why some people especially enjoy the feel of tractor gyros.

 

[scottessex]

The best way to put an end to all this is just fly something...like say a KB3, and then go fly a dominator. Then you can throw all the math and formulas out the window, and you will have your own proof.

Make sure that you throw in a couple of windy gusty days for your flight testing.

 

[Doug Riley]

That did it for me, Scott.