does rotor size effect stability ?

JETLAG03

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Conversation with a gyro pilot yesterday and he proposed that

"in any given situation a larger rotor will result in less stability"

I asked him to expand but he simply repeated the same thing.
 

Resasi

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What he could be talking about is if the disc loading gets too low with a too long rotor for a given all up weight the rotor will be turning slower and updrafts could cause it to slow even further. A slow rotor isn't as rigid and can flex more, not really a good thing flexing rotors can possibly strike tails/airframe.

Typical figures I think are around 1.2-1.4, below 1.2 and the rotor will be more susceptible to slowing in turbulent air. Also with a powerful engine and a steep climb you are unloading your rotors and slowing them.

Long rotors are heavy and droop more when not in use? There is a table that Chuck Beaty did for Dragon wings that showed disc loading for that blade profile/length to let you choose what disc loading you wanted.

There were some interesting observations on this thread.

 
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JETLAG03

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I have a cruise RRPM of 330
 

Mike G

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Phil
Could you be more specific about what you and/or your fellow pilot mean by "stability"?
Mike G
 

Doug Riley

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"Stability" in the engineering sense isn't the same thing as "stability" as pilots experience it.

From the informal, pilot's viewpoint, a slower (lower-RRPM) rotor will generally feel more stable than a fast (higher-RRPM) one. That's because a slower rotor takes just a bit longer to respond to a cyclic pitch change.

No rotor changes its orbit instantly when a cyclic pitch change occurs. Instead, the rotor stays in its old plane of rotation for a measurable interval of time -- the slower the rotor, the longer this time will be for a rotor of a given mass. During this "lag" time, the rotor's thrust tries to pull the spindle back to its old position. The pilot feels this as a pressure in the control stick resisting a control input ("stick feel"). Most people will experience this resistance as a form of stability. The term for this behavior in engineering lingo is "rotor damping."

Retreating blade stall isn't a sudden event in a gyro, as a stall in a FW plane would be. A portion of the retreating blade (normally inboard) is always stalled. This stalled portion expands outward as the gyro's airspeed increases. This expanded stalled area, in turn triggers more cyclic flapping and a higher blowback angle. (This angle may be 3 degrees at cruise, but 4, 5 or 6 degrees at higher-than cruise speeds.) IOW, as the gyro speeds up, the rotor disk tilts more and more aft relative to the spindle (even though the spindle hasn't moved). This increased "un-squareness" of the disk to the spindle means that the disk will pull back harder and harder on the spindle, slowing the gyro back down unless the pilot adds forward stick. The pilot may even "run out" of forward stick. Most pilots, again, will interpret this activity as "stability." A slower rotor behaves more strongly in this way than a fast one.

Engineers define stability much more precisely. They use the word to identify a device's tendency to return to the status quo when a particular aspect of the device's environment changes. In this sense, stability breaks down broadly into static and dynamic stability. Dynamic INstability (the tendency to oscillate more and more violently) is actually caused by static stability! And on and on ... huge topic.

Both of the above "stabilities" are examples of static stability, the first with respect to cyclic pitch changes, the second with respect to airspeed. In those two respects, at least, a big rotor is more stable (in the formal engineering sense) than a small one, other things being equal. But other things are rarely equal!
 

Martin W.

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.

Yes to what Doug says .... in the case of helicopters a Bell heavy 2-blade (slower) rotor feels "more stable" to the pilot because there is a small delay before the rotor reacts to pilot inputs or gusts.

Compared to the smaller diameter 4 blade , (much faster rotation) of say the Hughes 500 which is very twitchy and sensitive to inputs.

But the Hughes attributes are very beneficial for doing precise up-close work ... such as a man sitting on a skid repairing a hydro line ... or putting a bottle opener on a skid and opening a beer bottle .... there is a video of it somewhere.

.
 

DonBishop

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But the Hughes attributes are very beneficial for doing precise up-close work ... such as a man sitting on a skid repairing a hydro line ... or putting a bottle opener on a skid and opening a beer bottle .... there is a video of it somewhere.
All I could think of is, "I need a Hughes 500 right now. My beer won't open itself, you know!"
 

Doug Riley

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Again from the pilot's viewpoint, "stability" and "agility" are often perceived as opposites (though from the engineering viewpoint they are not, exactly).

For example, the stock Bensen/Brock gyro was beloved by many hot-doggers for its quick control response and light stick pressures. These qualities stemmed, I think, from the lightweight, high RRPM rotors that Bensen used (RRPM > 400 at 1G). Such a rotor has low rotor damping and indeed the rotor follows control inputs quickly.

It also, on the downside, does not resist PIO and PPO as well as a gyro with a slower, heavier rotor. say , for example, that an unintended nose-down rotation starts (whether in PIO or PPO). If the pilot holds the stick still, the light, fast rotor just follows right along instead of resisting a bit.

In contrast, a rotor such as a McCutchen, with more diameter, lower RRPM and more mass, has more rotor damping (lag) than a Bensen. It's not unusual for a McC. rotor to lag enough that the pilot gets 2/rev slap-back in the stick when he/she makes rapid, reversing lateral inputs. The rotor can't get out of the way of the moving rotor spindle and so hits the teeter stops (the teeter stops travel with the spindle).

DON'T make rapid, reversing fore-aft inputs, please! And keep in mind that rotor damping is proportional to rotor thrust. No thrust, no damping, no matter how fast the rotor aligns itself with the spindle. That's one reason that rotor damping alone is not a foolproof PPO-preventer.
 
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