The catch 22 of autorotation on gyroplanes

[Aviator168]

I was just repeating a prof said back who know when which I had doubts about. I posted here to see what you guys think since that's what my instructor was saying. The prof's exact words "you want the mu to be kept to a certain number, so you want to the rotor to spin faster when you increase speed and the way to do that is to take a bigger bite of the air"

 

[ultracruiser41]

 

[Jean Claude]

Line 2 of table # 4 shows that mu could exceed .68 for the rotor of the PCA 2 (but of course L/D ratio decreases). This was achieved by relieving the rotor with the auxiliary wing.
Thus relieved, the elastic twisting of the blades decreased the pitch ajustment, which increasing both the mu max and the rpm.
Hence a high maximum speed (see column Dynamic pressure)

 

[C. Beaty]

The straight skinny from Shapiro

 

[C. Beaty]

Sorry about the half A posts; I'm using Linux Mint without knowing how.

 

[Jean Claude]

Chuck, The autorotation limits depends of the blade pitch.
Here, extract of Naca report 515

 

[WaspAir]

I was just repeating a prof said back who know when which I had doubts about. I posted here to see what you guys think since that's what my instructor was saying. The prof's exact words "you want the mu to be kept to a certain number, so you want to the rotor to spin faster when you increase speed and the way to do that is to take a bigger bite of the air"

Professor of what subject ?

 

[C. Beaty]

JC, a copy of Bensen metal rotor blades was installed on our first gyro, around 1968. They flew OK although not much better than wooden blades with external noseweights because of the segmented upper skins with air leaks due to gaps between skin segments. A good centrifugal air pump.

Anyhow, my partner and I, Bob Carbonell decided to experiment and find how much the blades could be increased in static pitch and still autorotate; the Bensen hub having adjustable pitch. We kept increasing the pitch until we reached the limit of the adjustment range and the blades could still be hand started with both of us pushing.

At the upper pitch limit, top speed was ~20 mph with the stick jammed hard up against the forward stop.

More power = climb; less power = descend. All at 20 mph. We didn’t have much instrumentation; homemade rotor tachometer using a bicycle alternator and voltmeter, airspeed indicator using a “wind meter”. a device with plastic ball in a tapered tube, etc.

It would have been nice to have had accurate measurements.

 

[Aviator168]

Professor of what subject ?

He was one of those that believed lift is generated because air has to go faster on top of the wing to meet up with the air travel on the bottom surface.

At the upper pitch limit, top speed was ~20 mph with the stick jammed hard up against the forward stop.

More power = climb; less power = descend. All at 20 mph. We didn’t have much instrumentation; homemade rotor tachometer using a bicycle alternator and voltmeter, airspeed indicator using a “wind meter”. a device with plastic ball in a tapered tube, etc.

It looks like when more power was applied, the mast (actually the whole plane) tilted back more and so was the rotor disk.


Edit
Don't over look that square tube mast and the equipments around the rotor head. If you put fairing around them, it will reduce a lot of drag. A lot of Ken Wallis' gyroplanes' structures use skinny round tubes.

 

[Eric S]

experiment and find how much the blades could be increased in static pitch

Interesting test.

These are the kind of flight tests I'd like to play with if I wasn't chicken.

How much effect does increasing the static pitch have on rotor speed?

What effect does it have on stick forces and maneuverability?

Eric

 

[Vance]

He was one of those that believed lift is generated because air has to go faster on top of the wing to meet up with the air travel on the bottom surface.


It looks like when more power was applied, the mast (actually the whole plane) tilted back more and so was the rotor disk.


Edit
Don't over look that square tube mast and the equipments around the rotor head. If you put fairing around them, it will reduce a lot of drag. A lot of Ken Wallis' gyroplanes' structures use skinny round tubes.

Most of the gyroplanes I have flown fly more nose down the faster I go.

In my opinion a round tube has more drag than a square tube of the same diameter.

 

[ferranrosello]

Eric, we know by experience in autorotating helicopters. More pitch implies lower rotor speed.

The question is for Chuck experience. Why? I think that with less rotor speed the flapping action will be bigger. Is this the reason? The rotor flaps so much that you run out of forward cyclic travel?.

Ferràn

 

[Tyger]

In my opinion a round tube has more drag than a square tube of the same diameter.

https://upload.wikimedia.org/wikipedia/commons/3/37/14ilf1l.svg

 

[C. Beaty]

Eric, we know by experience in autorotating helicopters. More pitch implies lower rotor speed.

The question is for Chuck experience. Why? I think that with less rotor speed the flapping action will be bigger. Is this the reason? The rotor flaps so much that you run out of forward cyclic travel?.

Ferràn

Of course; at maximum pitch setting, flapping would have been quite near the teeter stops.

But the stick being up against the forward stop didn’t come as a surprise to us; Bensen wood blades had excess reflex and dependent upon trim tab setting, the stick would often be against the forward stop at ~60 mph.

With correctly designed rotor blades, the pitching moment is zero and flapping angle is directly dependent on airspeed. Not so for rotor blades having built in pitching moments; dependent on torsional stiffness, blades with noseup pitching moments will have a larger flapping angle and will reach flap limits early; blades with nowedown pitching suppress cyclic flapping and may never reach flap stops, in fact, with large negative pitching moments combined with sufficient torsional flexibility, the stick may even move rearward with increasing forward speed.

Ferràn, I drove the length of Spain from Algeciras to the French border in the 1950s along the coastal route. I was a tech rep for the manufacturer of gyroscopic gunsights used on Jet fighters of that era and was transferred from USAF bases in Morocco to the US military advisory group in Denmark.

Spain, with its wood burning locomotives and charcoal burning automobiles was a different place in the 1950s.

 

[Jean Claude]

At the upper pitch limit, top speed was ~20 mph with the stick jammed hard up against the forward stop.

Chuck, what do you think mu was then?

 

[Inquiring Mind]

... in other words when forward speed equals 35% of tip speed. For example, at a tip speed of 400 fps, most efficient forward speed is 140 fps or 95 mph.
Top speed, limited by retreating blade stall is typically reached at a mu of 0.5.

So, for 8.8m diameter rotor at 360 RPM tip speed is 362MPH and most efficient forward speed is 127 MPH and top speed is 181MPH? Does that sound right?
Thanks.

 

[C. Beaty]

Those numbers are correct, showing such gyros don’t have sufficient rotor blade area.

Cierva arrived at a rotor blade loading of 35 lb/square foot for optimum results in gyroplanes operating around 100 mph. (blade loading, not disc loading)

My gyros, with an empty weight of ~ 250 lbs and with 7” x 23’ rotors, always operated near Cierva’s optimum and flew fairly well with a 447 Rotax.

This one had a 503:

 

[C. Beaty]

Chuck, what do you think mu was then?

JC, I wish we had made accurate measurements. With flapping limiting top speed, my guess is that mu would have been considerably higher than 0.5.

 

[Inquiring Mind]

Those numbers are correct, showing such gyros don’t have sufficient rotor blade area.

Nice flight!

How is rotor blade area accounted for when only rotor diameter and RRPM are needed to find rotor blade tip speed?
I'm not sure I understand how 8.8m Magni M-24 rotor "does not have sufficient rotor blade area". Could you elaborate, please?

 

[C. Beaty]

Rotor tip speed, ft/sec is equal to 66 x square root of blade loading; for example, if blade loading = 35 lb/square foot, then tip speed = 390.5 ft/sec.

As I’ve said before, most gyroplane “designers” aren’r rotorcraft engineers or any other kind of engineer; they’re “stylists” and are well advised not to monkey with rotors.