Rotorhead travel in pitch

[Mayfield]

Most light gyroplane rotor systems I have worked with are rigged to allow about 20 degrees travel in pitch. Usually about 18 degrees back and 1 or 2 degrees forward.

I have never approached one of these limits in flight.

I am curious about what the minimum safe back travel would be. The less travel I have to build in the higher my vertical tail can be.

R/S

Jim Mayfield

[chuter]

Jim,

Is your tractor going to be a tail dragger or tricycle?

This reason I ask is; Ron Herron pointed out to me that with a tail dragger you get a few more degrees of flair as the fuselage settles back onto the tail wheel.

I don’t remember him mentioning a definite number for degrees of travel.

He did say that with the stick all the way back and the centrifugal flap stops in place, he had about a half-inch of clearance between rotor blade and tail.

He said that if the flap stops were not in place the rotor blade would definitely contact the tail.

You should contact him to make sure my memory is not playing tricks on me.

Hope this is relevant,

[Doug Riley]

Jim, I only know of two gyro operations in which maximum back-travel really matters: (1) rotor starting and (2) landing flare.

With a prerotator strong enough to spin the rotor to the nosewheel-lift point (or whatever the equivalent is on a tractor) statically, function #1 becomes moot. I have yet to fly a gyro with a prerotator that consistently achieves this, but both the Wunderlich and the RFD hydraulic come very close on light machines. Beef up either of these units a bit and you're there.

Some gyro pilots routinely hit the rear stop on landing flare -- often after actually touching the ground. You can do a perfectly good landing with only a few feet of roll without flaring this dramatically, but if you want a zero-roll landing you'll need to bring up the rotor AOA to somewhere around 30-35 degrees somehow. I wonder if an auxiliary elevator that drops the tail rather than just the rotor might fit the bill. The Blackhawk has a huge flight-adjustable HS, alhough, ironically, it goes to full DOWN elevator on landing.... something like that, but going to UP elevator, might limit the amount of head travel you need.

After all, the early Ciervas had fixed heads and relied entirely on elevators.

Another way to skin this cat, at a cost in complexity, would be to employ collective pitch. Obviously, this would both "leverage" more RPM out of any existing prerotator and add another source of flare power. My personal dream gyro would have collective and possibly no direct cyclic!

[rehler]

Jim,

The "standard" for most Bensen type gyros (as opposed to the RAF) is 9 degrees tilt each way (in all directions) for a total of 18 degrees. If the mast is tilted back at 9 degrees (also "standard") the tilt is from zero (flat) forward to 18 degrees back.

[Doug Riley]

Ken, don't forget 5-10 degrees each for tailwheel rock-back and teeter hinge flapping (which is considerable -- right near the stops -- during the takeoff roll). That's where I came up with 30-35 deg. total rotor AOA. It might be more like 40 on some gyros.

A taildragger will have the "tailwheel rock-back" component permanently built into the landing gear, giving the designer something of a head start in the clearance department. Which he'll need with the long tailcone of a tail dragger.

[Mayfield]

Thanks Mike. It's conventional gear. Thanks Ken, that's in line with my memory of things.

Doug, I seem to remember (where's Al Hammer when you need him) you postulating a while back, that most gyros did not need the 18 or 20 degrees of head travel they have.

I don't think it had anything to do with clearance issues though.

Further complicating the layout is my inability, at this time, because of CG uncertainty, to positively identify the longitudinal position of the teeter point.

Just moving the teeter point a little back makes a huge difference in the clearance available.

Jim

[Doug Riley]

Jim, I hope I didn't say that. Blame it on bad office coffee if I did.

Some gimbal heads on the market (I think Rotor Hawk, for example) have less than 20 deg., but I'm all in favor of having as much aft-tilt (by hook or by crook) as possible, to help with rotor starting. At other end, you really do need to be able to level the rotor when you're operating in breezy weather.

Again, you can dispense with the extreme aft tilt if your prerotator is powerful enough. The photos of Cierva's machines make it look as if their rotors only went back 20 or so degrees relative to the ground when on the tailwheel. Guessing that this 3-point stance might be ten degrees aft of flight stance, and if these were fixed-head (aileron-elevator) jobs, then in flight stance the rotor would be tipped back ten degrees -- same as a Bensen rotor in flight. IOW, Cierva's geometry wasn't THAT much different from Bensen's -- except that a Bensen grabs an extra 8-10 degrees aft tip (ON TOP of the head's 20 degrees) once the craft rocks back, while Cierva uses that same "tailwheel increment" just to get to 20 degrees.

[Mayfield]

Doug,

I think (operative word, because I'm not yet sure) I am primarily interested in the tilt/flap of the rotor relative to the airframe.

Because I am swinging a 76 inch prop at the front and the gear is fairly far forward, the Gestalt will sit at a static deck angle of about 12 degrees. I would prefer 7-9 degrees but I am stuck with 12. Oh well, "S" turns while taxiing looks neat. So 18 degrees aft tilt plus 12 degree static deck angle plus 7 degrees or so of flapping could yield an initial angle of ~37 degrees.

The head/blades I am using have about 8 degrees of flapping range each way from neutral.

It is about 140 inches from my predicted teeter point to the highest point on my desired tail.

It is looking like I am going to have to use at least a 60 inch (above the top longerons) mast to get adequate clearance.

On the aft tilt requirement for landing, I have always thought of the aft tilt angle as a rate control more than a displacement control.

These are just thoughts. I appreciate all the comments and advice, and hope for more.

Jim

[Doug Riley]

Jim: A mast that tall sounds tippy and hard to store. If it were mine, I'd probably forego some aft travel, get the mast down to a more reasonable height and make sure the prerotator was up to snuff. You may need a featherable prop and/or up-elevator capacity to do that, though, without putting the craft over on its nose.

With no damping, I believe that the amount of rotor deflection controls the rate of pitch acceleration of the airframe -- your basic squirrelly ship, right? With a powerful HS quickly snubbing the acceleration, the amount of deflection should, as you say, control only the rate of rotation of the frame instead.

But in a flare in a direct-control craft with a powerful HS, frame rotation is not what matters -- it's how much the rotor tilts in relation to the (relatively steady) platform of the stabilized airframe that produces most of the flare. (This is what seems confuses high-time RAF pilots when they add a HS -- they're used to allowing for major frame rotation in the flare.)

The old NACA report on the wind-tunnel test of the PCA rotor at Langley gives some data on the lift curve of a gyro rotor and therefore how much flaring/braking force you get per degree of added AOA.

[Mayfield]

Doug,

I'm inclined to agree re: mast height. I am working at limiting the mast height to 48" above the top longeron.

I will have to be smart with my tail design, ensure that the centrifugal limiters work, and limit full aft head angle to about 15 degrees. No decisions yet. Still sketching/thinking.

Jim

[mceagle]

Jim, we have always manufactured our heads with 18 degrees minimum total travel. My first 532 powered two seater in 1887 used the full extremities but under less than usual conditions. When flown solo by my son it would run out of forward stick and climb at anything over 50mph. Flown solo by myself I could bump the joystick against the forward stop with little effect - it was only 1/2" from it. With a 300 lb student I would run out of rearward stick as the day warmed up.
Running out of rearward stick can also happen when doing some extreme mavouring at high power settings and little or no forward speed.
Back then we only ran a small horizontal stab - a larger one may have fixed the problem in both cases.
I too am in the process of designing a tractor gyro and am working through the same sort of problems. One of the problems I found with the taildragger was the extra 9 degrees of rear head angle while sitting stationary. In my opinion, this necessitated extra forward movement of 9 degrees to unload the rotor when landing into a strong wind. This extra movement was to be detented into the stick movement to isolate it from the normal flight movement. I finally decided on tricycle undercarriage so that the head movements were conventional to most other gyroplanes. This also has distinct advantages in ground handling and commonality to other gyros (ease of letting others have a fly)
I have managed to keep the mast height to 36" above the cabin (or 47" to the top of the head) and it dosen't look out of place. The head has full standard movement, including allowance for flapping.
I would like to speak to Ron H more but he seems to be kept very busy.

[GyroRon]

My rotorhead has alot more than 18 degrees total travel from forward stop to rear stop. I think it is around 30 degrees total.. it is a Dominator or RFD head. I like having the extra travel.

[Mayfield]

Hi Tim,

Yours must be a cabin ship. Out of curiosity, how far is it from your cabin floor to the top of the cabin. I'm assuming that you are mounting your mast to the cabin top like Ron Herron does.

Mine is open cockpit. From the cockpit floor to the upper longerons is 24 inches.

Jim

[Timchick]

Jim,
If you made a twin rudder tail could the height be shortened allowing more tail/rotor clearance?

[Brent_Brown]

That is what I am doing with the rudders. It is a shorter one seater and tri gear so it should not be an issue.

[Brent_Brown]

Here is what it looks like so far. That big black thing is the 15 gal. tank.

[Brent_Brown]

This is where I think the head will be and a slider type will be used. the mast will fold down for storage.

[chuter]

I have an RFD head on my tail dragger, and like Ron said it's got lots of extra travel.
This comes in handy for getting the blades level while on the ground with the stick forward, but still having plenty of back stick.

[mceagle]

Yes Jim - enclosed cabin.

Cabin floor to top of cabin - 45"
Cabin floor to top of mast - 80"
Cabin floor to upper longeron - 23"
Free mast (top longeron up) - 56"
Shed clearance = 119"
Overall length - 208"
Prop - 76"
All Alu. frame bolted and riveted, two side by side. Main u/c still problematic, may have to go composite.

Will try to post diag.

[Mayfield]

It looks like we are going to have a new crop of tractors soon.

Neat looking Brent.

Thanks Mike. I may have to limit travel.

Tim McClure, It is a good looking design.

The distance from the cabin floor to the top of the mast is not much different on our ships. About 80 inches. My "free" mast might end up being 60 inches.

I have not yet pinned down the longitudinal position of the landing gear, but the width is 84 inches.

Tim Chick, Yes, but I am trying to avoid it. It looks nice on Brent's aircraft, but I'm hoping to use a single surface.

Jim

[GyroRon]

Jim check out this thread and maybe give Joe a call or email... I test flew his tractor gyro and it was the most solid, stable gyro I have flown to date. His mast arrangement was also quite neat.

http://www.rotaryforum.com/forum/showthread.php?t=3577&page=1&pp=20

[Mayfield]

Thanks Ron. Joe's machine also looks impressive. I guess I have not been paying as much attention as I should. There are a bunch of tractors gestating or already flying.

Jim

[GyroRon]

Joes machine is built to the dimensions of a Little Wing, only he made it out of larger diamter steel tubing and his mast and rotorhead to mast arrangement is unique. It flew really nice but the engine overheated on the 2nd lap around the airport and I had to land. I can't vouch for any of the other tractor gyros around, as I have only flown this one and the Pitbull - which doesn't hold a candle to Joes.