These numbers are a bit odd. Disk loadings are within 1% of each other but rrpm differs by about 27%? Currently I have no idea how to explain this.
This is getting off topic, but...
Disk loading is just one variable. There are many other variables to take into account, this is totally apples to oranges in Vance's case.
Here's a mind-bender example, my personal experience: Two 28' RFD rotors, same tip weights, profile, chord, length/diameter, disk loading, and installed on the SAME gyro - at different times, obviously. Guess what? One spins and flies at 280 RRPM, the other at 320.
Why? Angle of attack, it is the only variable not accounted for, above. One had more twist at the tips than the other.
In Vance's case, you are talking different EVERYTHING, and trying to account for such different RRPMs is chasing your shadow until you have all the variables accounted for: blade pitch + disk pitch (together = AOA), weights and disk loading, blade mass and tip weights, chord and lifting surface area, profile (lift and drag coefficients), RRPM x diameter = tip speed, etc.
And don't forget drag/lift are a function of speed SQUARED, so differences in tip speed drastically affect lift.
Look at how RRPM factors in to produce similar tip velocity in three common size rotors:
RRPM ______ 414 ____ 340____ 317.5
Diameter ___ 23 _____ 28 ____ 30
Tip, MPH ___ 340 ____ 340 ____ 340
I've always been fond of the way the equation for tip speed works out for 28" rotors. RRPM = tip speed MPH.
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I admit to having read just a few posts on this thread...it is about ground effects, the old discussion of whether or not gyroplanes are affected by it, right?
It's useful to think in terms of Frisbee aerodynamics, not propeller dynamics. There can be no doubt as you experiment with tossing a frisbee close to the pavement that ground effects do affect the vehicle.
Why get all caught up with helicopter downwash vs. autogiros? Does a FW produce helicopter-type downwash? No, yet it is accepted by all that ground effects do come into play when landing a FW, right? Why should it be any different for a rotating wing?
Further, in the end, both helos and gyros have powered rotor systems - believe it or nuts. Not in terms of a direct-drive link to a motor, but in terms of the fact that it takes power input to get a lift component output which flies the vehicle. What are you doing when you are standing on the ground, hand spinning up your Bensen, other than powering the blades? Are you going to tell me that as those blades are turning, even at 5 RRPMs, that they produce no downwash, no vertical lift, and no forward lift? Of course not.
In the case of a helo, it is obvious where the power is coming from. In the case of a gyro, it is more subtle, less so, but is there, nonetheless. The vehicle is powered forward by either gravity in a glide, or a prop when the throttle is up, and instead of a drive shaft coupling power to the rotor the apparent wind is the coupling between gravity/engine power and rotor.
Either way, an autogiro rotor is powered in the end.
Lift is perpendicular to its surface of origin. EVERY wing has a certain component of forward lift. This is very easy to demonstrate using a foam wing. Drop a section of foam wing, perfectly parallel to earth, and it will move FORWARD some good distance before it drops its leading edge and gathers speed as it continues its descent to the ground due to gravity, and perhaps noses back up, stalls, etc - depending on its shape and balance.
Always.
It will ALWAYS move forward, flat and level, before it nose-dives at all.
That demonstrates how an autogiro rotor works, very well.
It is thought that the outer 1/3 dia. of the rotor "powers" the lifting area of the rotor - the center 1/3 dia. of the disk. I wonder if this is true. I heard of a story of a famous gyro builder/designer/pilot/test pilot that goes like this (This is only what I heard, so take it for what it's worth and dont' shoot the messenger).
An untested, never-before-tried set of rotor blades was being flown for the first time. Wide-cord, rather flexible spar and chord-wise twisting rotors were installed on a gyrocopter. Pilot flew the thing, and at some point the blades twisted chord-wise to the point that the tips no longer had the 3-degrees pitch they needed to continue to create vertical lift. With the rear-ward component of drag reduced, the forward component of lift was no longer in balance with that drag, and the rotors sped up...which caused more untwisting, which created more forward speed, and the closed-loop system caused the gyro to produce too much forward lift and not enough vertical lift until the vehicle plummeted to the ground, killing the test pilot.
If this account of events is in fact true, then we need to rethink our notion of how the autogiro rotor "engine" actually works. For if, as the tips UNWIND, they then create LOTS of forward lift, and lots of forward blade speed, while the middle section of the rotors remains somewhat close to the same AOA, then that gyro should have experienced INCREASED vertical lift, not loss of vertical lift. The blades were a progressive spring of which the OUTER portion untwisted a LOT more than the area closer to the root, after all.
It is therefore a mistake to think in terms of the typical FAA Rotorcraft Handbook notion of the outer 1/3 areas of a disk powering a gyrocopter's inner second third of the disk's so-called "lifting area".
The outer third of the rotor is just as critical for vertical lift as it is for forward lift - the engine's power connection to the autogiro's rotor blades. And it is far more critical for vertical lift than the second third of the disk diameter. The accident described above proves it beyond a doubt.