Extra large rotor for STO(L) performance?

Jason312

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I know the gyro can land on a "dime" but the takeoff roll typically bars it from being classified as a STOL aircraft for off field ops.
I know the AR1 can have the 8.8m rotor for high density altitude.
What about an even larger i.e. 12m rotor for better SHORT TAKE OFF AND CLIMB PERFORMANCE. Obviously there would be more drag so top/cruise speed would be reduced.

I would happily sacrifice those for better off field performance. If I wanted to do cross country flights I could easily swap to the standard rotor.

I'm obviously not an aerodynamics engineer and Abid is great at responding :)

Is this possible?
 
The A&S 18A has a rotor of nearly 11 meters diameter (3 blades) to support an 1800 pound gross weight, so large diameters are definitely possible, but generally aren't used for STOL purposes. One consequence of long blades is that rotor rpm is substantially reduced, keeping the tip speed in a comfortable and efficient speed range (for structural and drag reasons, the tips can't go arbitrarily fast). Higher rpm is a consequence of reduced air density at altitude, but tip speed must still be limited. Longer blades can help with that.

I would not expect meaningful improvement in short takeoff and climb performance from greatly increased span alone.

[The 18A does the ultimate short take off by jumping, but that's a result of storing energy in a high inertia rotor system and controlling collective pitch, not span alone.]
 
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Thanks for your input.
Jump takeoff would be great but appears to have more complexities than most want to deal with.

Longer blades also require longer hanger space and have a larger disc area which can be a downside.
Perhaps they could be made 2x the width??

Of course more Horse Power helps but the 915is appears to already be putting a taxing amount of torque effect on the airframe.
 
Do you mean twice the blade chord, increasing solidity ratio but not changing disc loading?
 
My latest gyro has 29' rotors on a single seat and can get off fairly short, low wing loading for sure helps get the most out of a gyro as far as stol go's.

wolfy
 
As I understand it there are definite drawbacks to lowering the wing loading below a certain point. More susceptibility to rotor deceleration when subjected to updrafts was one I believe. Does it not also result in a lower rotor RPM?

Chuck Beaty warned against having too low a wing loading, I need to refresh my memory on why but for the time being believe that there is an optimum below which there start to be safety/stability problems.

Will definitely be interested to hear from those more versed than I on rotor aerodynamics, what an optimum is, and why.

In the meantime from the quick search I did do found this.

CAA Paper 2009/02 The Aerodynamics of Gyroplanes.
During testing...

"Disc loading was varied by reducing blade radius by 0.3 m; and increasing it by the same amount relative to the standard aircraft. This results in very considerably different rotor and blade loadings. The small rotor has a blade loading in these simulations of 248.4 kg/m2, and a disc loading of 9.1 kg/m2. By contrast the large rotor blade and disc loadings are 214.6 kg/m2 and 6.8 kg/m2. The scale of the following differences should therefore be seen in this context.
The trim results (Figure 7.28) show, as might be expected, a large difference in the rotorspeed of 60 rpm, and a 2 deg difference in pitch attitude; the more lightly loaded rotor operates at a lower angle of attack.
The dynamic stability results (Figure 7.29) show that the aircraft with the smaller loadings has better short-period mode damping, but poorer phugoid damping to the extent that it only becomes Section T compliant above 40 mph. The better short- period mode damping is due to the greater pitch damping which comes from a lower rotorspeed (the disc will precess more in response to pitch rate disturbances). The time histories (Figure 7.30) quantify the magnitude of the difference in phugoid damping, showing that the response is more oscillatory with the larger and therefore more lightly-loaded rotor."

The whole article was in this. A study that was done a while back and probably needs revising but pretty comprehensive for the time.


CAA Paper 2009/02
The Aerodynamics of Gyroplanes.
 
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WaspAir
I see your point. I have to think in terms of disc area not blade area.

Wolfy
Yes, I believe lower disc loading would shorten takeoff distance.

Resasi
I figured there would be a point of diminishing returns and as you point out may infact create stability and rpm issues.
Thanks for the article :)

As a helicopter guy I really like the ability to land and takeoff in areas airplanes cannot. However, the price of helicopters is restrictive.
 
Increasing the diameter inevitably entails side effects that can also affect takeoff distance, such as the inertia of longer blades, and the lower initial rpm if the same pre-launcher power.

In my comparison I assumed the same coning for the same vibration 2/rev. which requires heavier blades, therefore the take-off weight is increased by the same amount to retain the same payload.

With D = 6.45 m c = 0.18 m Blade mass = 7.5 kg, Flight rpm: 395 at 90 km/h Total mass = 216 kg Pre-launcher power: 2 kW giving 260 rpm
Calculated rolling distance: 82 m

With D = 7.45 m c = 0.18 m Blade mass = 10 kg Flight rpm 323 at 90 km/h Total mass = 221 kg Pre-launcher power: 2 kW giving 215 rpm due to increased diameter
Calculated rolling distance: 79 m

Insignificant difference !
Sans titre.png
Insignificant difference

Data used:
Chord : 0.18 m
Aerodynamic pitch setting: 3.5 degrees
Static propeller thrust: 1200 N
Frame drag area (S.Cd): 0.4 m2
Rolling coefficient on grass: 4%.
Rotor shaft: +20 degrees /ground
 
Thank you Jean Claude.

That is interesting to see the effects on RPM and rolling distance. It appears you get just under 4% gain in rolling distance. Definitely not a drastic increase but it did improve.
Perhaps an even larger diameter would be beneficial if you could increase the Pre-launcher power.
Where did you derive the 2 kW for Pre-launcher power and how does that compare to what the Rotax 915is delivers?

Is there a flight rpm range that is optimum, I.e. is 395rpm better than 323rpm or visa versa? I know we don't want too high of an RPM or blade tip speed will reach the the sound barrier. Is there a minimum RPM?
I assume more weight in the rotor = more energy in the event of power loss for emergency landings which would be a good thing.
 
Jason,
You may want to research disk solidity and how it affects performance. My Dad started making 8.5 inch blades with intent of marketing them to the European gyro owners here in the US. He had great success with his own testing however, he found a almost die hard allegiance to the manufactures by their customers. This made the venture not worth the investment. It's too bad the ones we talked to wouldn't even try them. They would have liked them. Joe Piers has one of only a few sets ever sold.
Here is a video of him flying them. BTW they were dead smooth.
 
That is interesting to see the effects on RPM and rolling distance. It appears you get just under 4% gain in rolling distance. Definitely not a drastic increase but it did improve. Perhaps an even larger diameter would be beneficial if you could increase the Pre-launcher power.
A very small difference for such a large difference in diameter shows that this is not a good way to shorten the take-off.


Where did you derive the 2 kW for Pre-launcher power and how does that compare to what the Rotax 915is delivers?
2 kW is the launcher power required to reach 66% of the cruise rpm of a single-seater, i.e. almost the limit for shortening (about70%).
It is not related to rotor diameter or propeller thrust.
 
You do not need 12 meter rotors. It really depends on the gross weight. You want to fall within a range of blade loading. Increasing even 6 inches of diameter of rotor has a significant effect. Peter Kalev is on here. Ask him of the difference between 8.6m versus 8.8m rotor. It is noticeable and the ill effects are minor. A 9 meter rotor will shorten the takeoff roll significantly on something like an AR-1. I clear a 50 foot obstacle by 850 feet consistently but a lot depends on the pilot. Takeoff ground roll for me with 8.8 m rotors one up is around 250 feet. The best investment a pilot can make to get shorter takeoff roll is stop eating at McDonalds. Option cost is $0.
 
Increasing the diameter inevitably entails side effects that can also affect takeoff distance, such as the inertia of longer blades, and the lower initial rpm if the same pre-launcher power.

In my comparison I assumed the same coning for the same vibration 2/rev. which requires heavier blades, therefore the take-off weight is increased by the same amount to retain the same payload.

With D = 6.45 m c = 0.18 m Blade mass = 7.5 kg, Flight rpm: 395 at 90 km/h Total mass = 216 kg Pre-launcher power: 2 kW giving 260 rpm
Calculated rolling distance: 82 m

With D = 7.45 m c = 0.18 m Blade mass = 10 kg Flight rpm 323 at 90 km/h Total mass = 221 kg Pre-launcher power: 2 kW giving 215 rpm due to increased diameter
Calculated rolling distance: 79 m

Insignificant difference !
View attachment 1160550
Insignificant difference

Data used:
Chord : 0.18 m
Aerodynamic pitch setting: 3.5 degrees
Static propeller thrust: 1200 N
Frame drag area (S.Cd): 0.4 m2
Rolling coefficient on grass: 4%.
Rotor shaft: +20 degrees /ground

Not to my testing. There is a difference between 8.4, 8.6 and 8.8 meter rotors from Averso on AR-1 and the difference is noticeable. Certainly not 3 meters between 8.4 and 8.8 meter rotors. Climb rates are different too. The pitch angle needs to reach 19-20 degrees back for the rotor disc
 
Abid,
If you change the diameter of the blades coming from the same manufacturer, i.e. same mass per meter, then the coning is also modified and the conclusion is different, as I had specified.
You can't make useful comparisons if you change all the parameters at once.

The same applies if you launch a larger rotor at the same rrpm, as this increases the launch power, which generally shortens the run on ground.
Then you can't tell which parameter is involved: Rotor diameter or prelaunch power?
 
The pitch angle needs to reach 19-20 degrees back for the rotor disc
Yes, as indicated in my post #8, the rotor bearing was tilted back by 20° (ie from 20.5 to 22.5 degrées of the disc due to longitudinal flapping)
 
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Abid,
If you change the diameter of the blades coming from the same manufacturer, i.e. same mass per meter, then the coning is also modified and the conclusion is different, as I had specified.
You can't make useful comparisons if you change all the parameters at once.

The same applies if you launch a larger rotor at the same rrpm, as this increases the launch power, which generally shortens the run on ground.
Then you can't tell which parameter is involved: Rotor diameter or prelaunch power?

Thanks for clarifying it for me Jean. Yes I only tested the same rotor just larger. I was able to get both sizes to about the same rotor RPM in pre-rotation as well.
 
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