Importance of rotor size at altitude

[mdalcant]

I am in the process of building a gyro, and I have a question regarding the length of rotor I should use. I live at altitude (6000 ft) and I regularly fly between 6000 and 12000 ft. In the summer the density altitude can easily hit 10000 on the runway. My gyro will have the 915i Rotax engine. I have the choice of an 8.6meter rotor or an 8.8meter rotor. I would like the opinion of people that have actually used one or the other in conditions similar to mine. 8.6 or 8.8? Pros and Cons for both, please.
Thanks,
Mauro

 

[Smack]

I believe Chuck Beaty has posted a lift equation previously. Regardless I think knowing the all-up weight is also required.
Brian

 

[Vance]

Without knowing what the all up weight of your gyroplane is the recommendations can’t be too specific.

If your rotor disk not big enough the rotor will spin too fast and waste power at higher density altitudes.

My 9.1 meter blades on a 1,150 takeoff weight gyroplane spin to 330 rotor rpm at 1,000 feet density altitude and spin at 400 rotor rpm at sea level increase sing the total rotor drag by 20%.

A larger rotor disk will give you more cushion at high density altitude.

The down side to a larger rotor disk is it takes longer to get the rotor up to speed and the gyroplane tends to have a heavier stick.

The best thing you can do is speak with the rotor system manufacture for their advice.

 

[mark treidel]

As Vance replied; without knowing more specific information about your build, it would only be an educated guess.
Is your gyro a 2 place? Most 'euro-blades' in meters come in 8, or 8.5" chord widths. That makes a difference
also at your altitude.
I fly an AR-1 with 8.8 (29') Averso blades. At my altitude (very close to yours), the extra length is not only a plus,
it's a necessity. S/L rrpm's are ± 360. More on a high d/a day or w/2up. Very minute difference in stick pressures between
the two sizes. At altitude, length is crucial in that it compensates for the lack of heavier air. One caveat however, make sure
you have enough mast height to make up for retreating blade ground clearance in a high flare landing (full back stick).

 

[mdalcant]

As Vance replied; without knowing more specific information about your build, it would only be an educated guess.
Is your gyro a 2 place? Most 'euro-blades' in meters come in 8, or 8.5" chord widths. That makes a difference
also at your altitude.
I fly an AR-1 with 8.8 (29') Averso blades. At my altitude (very close to yours), the extra length is not only a plus,
it's a necessity. S/L rrpm's are ± 360. More on a high d/a day or w/2up. Very minute difference in stick pressures between
the two sizes. At altitude, length is crucial in that it compensates for the lack of heavier air. One caveat however, make sure
you have enough mast height to make up for retreating blade ground clearance in a high flare landing (full back stick).

As it happens mine will also be an AR-1. I weigh 150 lbs and will fly with no passenger most of the time. Does your mast have the standard height for the AR-1, or you had it made taller?
Thanks.

 

[mark treidel]

Mine is the folding mast. I was forced to do that to transport the gyro from the factory that way.
I don't believe the mast height is affected either way. I have plenty of retreating blade clearance when landing.
I do not know what Abid estimates the AUW to be with the 915, but mine came in 780# dry w/914.
There should not be much appreciable difference. Still, I would opt for the 8.8 blades. I'm certainly not
sorry I did and if they made a 30' one, I would have used that. You should probably expect 340/350 rrpm
at your altitude/weight. That would be a ball park figure. You can drop 30rrpm off of that at sea level.
I'm sure they mount the engine in such a manner as to compensate for some P factor but with the extra
hp, the longer blades help maintain gyroscopic stability. That thing should climb like a homesick angel.
I doubt you will need full throttle on take off and I would be very careful doing that. 'Sneak up' on it a little at
a time on each takeoff to get the proper feel for the throttle. Don't be afraid to use up some runway and ease in
the gas. Fine line there.
Also, with the heavier blades, I normally don't force high power into the spin up to keep from over-torquing the
mast. Usually, just pre rotate to about 180-200 and let the wind to the rest.

 

[mdalcant]

Mine is the folding mast. I was forced to do that to transport the gyro from the factory that way.
I don't believe the mast height is affected either way. I have plenty of retreating blade clearance when landing.
I do not know what Abid estimates the AUW to be with the 915, but mine came in 780# dry w/914.
There should not be much appreciable difference. Still, I would opt for the 8.8 blades. I'm certainly not
sorry I did and if they made a 30' one, I would have used that. You should probably expect 340/350 rrpm
at your altitude/weight. That would be a ball park figure. You can drop 30rrpm off of that at sea level.
I'm sure they mount the engine in such a manner as to compensate for some P factor but with the extra
hp, the longer blades help maintain gyroscopic stability. That thing should climb like a homesick angel.
I doubt you will need full throttle on take off and I would be very careful doing that. 'Sneak up' on it a little at
a time on each takeoff to get the proper feel for the throttle. Don't be afraid to use up some runway and ease in
the gas. Fine line there.
Also, with the heavier blades, I normally don't force high power into the spin up to keep from over-torquing the
mast. Usually, just pre rotate to about 180-200 and let the wind to the rest.

Thank you Mark for your advice. I am looking forward to having this machine. I flew in a Cavalon for a couple of years with a 914 engine. Even with my light weight, climbing in the summer was a chore. It had shorter rotor blades.
Thanks again,