Advantages and disadvantages of larger rotor

MonkeyClaw

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Apr 10, 2019
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15
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Sedona, AZ
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Columbia 400, AR-1c on order
I've placed the order for my AR-1 from Silverlight Aviation. One of the options was in regards to rotor size. They have two options - 8.6m and 8.8m. Since I live at just over 4500', I went with the larger rotor. As I look through the documentation, it appears that the larger rotor is better for high-altitude ops, better for fuel economy, better for low-speed flight, and pretty much anything else I want to do.

So this begs the question - why would I want the smaller rotor at all? Obviously there are limitations as the rotor gets bigger. But between these two choices, what am I giving up by getting the slightly larger rotor?
 

XXavier

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I've placed the order for my AR-1 from Silverlight Aviation. One of the options was in regards to rotor size. They have two options - 8.6m and 8.8m. Since I live at just over 4500', I went with the larger rotor. As I look through the documentation, it appears that the larger rotor is better for high-altitude ops, better for fuel economy, better for low-speed flight, and pretty much anything else I want to do.

So this begs the question - why would I want the smaller rotor at all? Obviously there are limitations as the rotor gets bigger. But between these two choices, what am I giving up by getting the slightly larger rotor?
Easier hangar fit...
 

PeterFromLA

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May 15, 2019
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Los Angeles
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American Ranger AR-1, Kallithea
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I've placed the order for my AR-1 from Silverlight Aviation. One of the options was in regards to rotor size. They have two options - 8.6m and 8.8m. Since I live at just over 4500', I went with the larger rotor. As I look through the documentation, it appears that the larger rotor is better for high-altitude ops, better for fuel economy, better for low-speed flight, and pretty much anything else I want to do.

I have an AR-1 with the 8.8 m rotor... and I love it. Maybe I just don't know better, but I love the way the gyro flies. In 2020 I flew almost identical machine with an 8.6m rotor and it felt weird - did not climb as fast, did not float on landing as I am used to. The only drawback you may consider is the pre-rotation being a bit sluggish when compared to other gyros. I am used to it and the technique I use is adequate to get me off the ground effortlessly, but the argument can be made, so just be aware.

So, enjoy your gyro... :) ...and come visit us in L.A.
 

Abid

AR-1 gyro manufacturer
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The larger rotor is a bit more floaty. People used to flying with smaller rotors would say it gets effected by turbulence more perhaps because with thermals it will go up and down more when loaded lightly with one person. Of course to me personally most gyroplane guys don't even know what they are talking about in reference to going up and down. Try flying a soaring trike or a Pipestrel motor glider, that will build up your stomach a bit.

The other difference would be perceived in responsiveness. A smaller rotor would feel a bit more zippy in that sense just as an airplane with smaller wing span would. But a larger wingspan and wing surface area is always better for STOL. Of course you want to maintain a minimum disc loading
 

Kevin_Richey

Yamaha gyro...Oregon, USA
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You've going to need every bit of the 8.8 meter rotor disc @ over 4,800' El. where Sedona is situated. You'll likely need a larger system than that if you want to fly a passenger in your AR-1 @ those altitudes.

I flew @ those type of density altitudes & higher using a set of 25' rotors. But I had plenty of horses (Yamaha 3 cyl.) & was only a single seater gyro.
 

Abid

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You've going to need every bit of the 8.8 meter rotor disc @ over 4,800' El. where Sedona is situated. You'll likely need a larger system than that if you want to fly a passenger in your AR-1 @ those altitudes.

I flew @ those type of density altitudes & higher using a set of 25' rotors. But I had plenty of horses (Yamaha 3 cyl.) & was only a single seater gyro.

His order is with 915iS. It is normalized to 16000 feet.
 

MonkeyClaw

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Apr 10, 2019
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Location
Sedona, AZ
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Columbia 400, AR-1c on order
Yes, I went with the big engine and big rotors (and big wheels). The plan is definitely to eventually carry passengers.

It sounds like many of the advantages to the larger rotor are similar to having a bigger, higher-aspect wing in a FW. Along those lines, should I expect a slightly lower min, max, and cruise speed (or at least economy cruise)? Also, if you float a little more on landing, should the landing speed be lowered? I'm sure a lot of this will be figured out during the phase 1 flights or are already known.

I'm also curious how the H/V curve will be affected.
 

MonkeyClaw

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Apr 10, 2019
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Sedona, AZ
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Columbia 400, AR-1c on order
I have an AR-1 with the 8.8 m rotor... and I love it. Maybe I just don't know better, but I love the way the gyro flies. In 2020 I flew almost identical machine with an 8.6m rotor and it felt weird - did not climb as fast, did not float on landing as I am used to. The only drawback you may consider is the pre-rotation being a bit sluggish when compared to other gyros. I am used to it and the technique I use is adequate to get me off the ground effortlessly, but the argument can be made, so just be aware.

So, enjoy your gyro... :) ...and come visit us in L.A.
I watch your videos on YouTube :) I've thought about getting out there to take a flight with you. If I can find the time, I'll be reaching out! I look very closely at your gyro every chance I get!
 

Abid

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Yes, I went with the big engine and big rotors (and big wheels). The plan is definitely to eventually carry passengers.

It sounds like many of the advantages to the larger rotor are similar to having a bigger, higher-aspect wing in a FW. Along those lines, should I expect a slightly lower min, max, and cruise speed (or at least economy cruise)? Also, if you float a little more on landing, should the landing speed be lowered? I'm sure a lot of this will be figured out during the phase 1 flights or are already known.

I'm also curious how the H/V curve will be affected.

Official H/V curve pretty much for all these machines is going to be give you 400 to 500 feet as the safe height to recover from starting at 0 airspeed. Its true that lower disc loading of 8.8 m rotor versus 8.6 m rotor will give you about app. 40 feet extra on H/V curve.

I usually use 500 feet AGL as a rule for safety from 0 indicated airspeed (and engine idle/out) to be able to recover, flare and land safely for both sizes and that works well for me.
 

Doug Riley

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6,710
The size of the rotor doesn't, in itself, affect the gyro's speed range. With other things equal, however, RRPM does.

The NACA studies of autogyros back in the 1930's resulted in a statistic that the researchers dubbed the mu ratio (the Greek letter "mu" is pronounced like a kitten's noise: "myoo"). This is simply the ratio of the tip speed of the blades to the forward speed of the gyro. The ratio helps us tease out the drag compromise between high RRPM (limiting retreating-blade stall but risking high advancing-blade drag) and low RRPM (limiting the blades' profile drag but creating a larger retreating-blades-stalled area).

The sweet spot turned out to be around 0.35-0.4. I.e. the blades' tip speed should be about 2.5 times the aircraft's forward speed for maximum efficiency.

Increasing the rotor diameter (with the same blade design and pitch) on a given machine will make the rotor turn slower. This means that the larger-rotor version will hit its maximum efficiency at a lower airspeed.

Of course, by "tip speed" we mean the average tip airspeed or, saying the same thing, the geometric tip speed (RRPM x rotor circumference).

High airframe drag can overwhelm our attempts to optimize mu ratio. For example, the original Bensen B-8M had a 22-foot rotor turning around 400 RPM. Tip speed was therefore 460 ft./sec. or 314 mph. Multiply by an ideal mu of 0.35 and you get an ideal cruise speed of 110 mph! That's only from the rotor's viewpoint, however. The B-8M's open frame is so draggy that almost no amount of horsepower can push it that fast, no matter what the rotor would prefer. Bottom line: for body-drag reasons, Bensens had least-drag airspeeds of 45-50, corresponding to mu=0.2 or less.

IOW, un-streamlined gyros are inefficient. Surprise! We can make even a Bensen a little less inefficient by slowing its rotor down -- which many of us did, back in the day, by pitching them up past the scribe marks and/or swapping for a longer hub bar.

The original Gyrobee takes these tactics a bit farther, using a 24-to-25-foot rotor on a featherweight machine. The resulting gain in efficiency is enough to make a fairly decent gyro using only the 40 hp Rotax engine. Of course, the Rotax's redrive, allowing a big prop, helps, too.
 

Brian Jackson

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The size of the rotor doesn't, in itself, affect the gyro's speed range. With other things equal, however, RRPM does.

The NACA studies of autogyros back in the 1930's resulted in a statistic that the researchers dubbed the mu ratio (the Greek letter "mu" is pronounced like a kitten's noise: "myoo"). This is simply the ratio of the tip speed of the blades to the forward speed of the gyro. The ratio helps us tease out the drag compromise between high RRPM (limiting retreating-blade stall but risking high advancing-blade drag) and low RRPM (limiting the blades' profile drag but creating a larger retreating-blades-stalled area).

The sweet spot turned out to be around 0.35-0.4. I.e. the blades' tip speed should be about 2.5 times the aircraft's forward speed for maximum efficiency.

Increasing the rotor diameter (with the same blade design and pitch) on a given machine will make the rotor turn slower. This means that the larger-rotor version will hit its maximum efficiency at a lower airspeed.

Of course, by "tip speed" we mean the average tip airspeed or, saying the same thing, the geometric tip speed (RRPM x rotor circumference).

High airframe drag can overwhelm our attempts to optimize mu ratio. For example, the original Bensen B-8M had a 22-foot rotor turning around 400 RPM. Tip speed was therefore 460 ft./sec. or 314 mph. Multiply by an ideal mu of 0.35 and you get an ideal cruise speed of 110 mph! That's only from the rotor's viewpoint, however. The B-8M's open frame is so draggy that almost no amount of horsepower can push it that fast, no matter what the rotor would prefer. Bottom line: for body-drag reasons, Bensens had least-drag airspeeds of 45-50, corresponding to mu=0.2 or less.

IOW, un-streamlined gyros are inefficient. Surprise! We can make even a Bensen a little less inefficient by slowing its rotor down -- which many of us did, back in the day, by pitching them up past the scribe marks and/or swapping for a longer hub bar.

The original Gyrobee takes these tactics a bit farther, using a 24-to-25-foot rotor on a featherweight machine. The resulting gain in efficiency is enough to make a fairly decent gyro using only the 40 hp Rotax engine. Of course, the Rotax's redrive, allowing a big prop, helps, too.
I love learning, and this was explained very well. Thanks, Doug.
 
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