Rotor blade chord

Aviator168

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Why majority of the rotor blades have a constant chord from the root to the tip? Since the air speed variation root is highest, you would expect that region will have a shorter chord, no?
 

Vance

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Why majority of the rotor blades have a constant chord from the root to the tip? Since the air speed variation root is highest, you would expect that region will have a shorter chord, no?
No; I would not expect the chord of a gyroplane rotor blade to change chord from the root to the tip.

In my opinion a rotor blade with a changing chord would be very difficult to fabricate for very little if any benefit.

Reasoning that the blade in the stalled region of the rotor does little to benefit performance NACA did tests on a gyroplane rotor in a wind tunnel eliminating the rotor blade airfoil near the hub and lost performance.
 

WaspAir

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Long hub bars are the closest you are likely to see to a small chord at the root, but there has been a fair bit of work on tip shapes for helicopters.

https://en.m.wikipedia.org/wiki/BERP_rotor

Optimising twist is usually worth the effort but chord variation doesn't pay off as well.
 

fara

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Long hub bars are the closest you are likely to see to a small chord at the root, but there has been a fair bit of work on tip shapes for helicopters.

https://en.m.wikipedia.org/wiki/BERP_rotor

Optimising twist is usually worth the effort but chord variation doesn't pay off as well.

Twist would help in hover but it can hurt in forward flight (increases blade loads as well as vibration). We do not hover in gyroplanes and are not concerned with figure of merit. Twist is thus not very useful in gyroplanes in general. It is in helicopters.
 

Jean Claude

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Compressibility phenomena are not worrying on slow blade tips of our gyros unlike fast helicopters.
We better worry flow separations on the rear parts of the fuselage. Much more to gain with less research.
Unfortunately, we look only the cosmetic of nose .
 

Aviator168

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We better worry flow separations on the rear parts of the fuselage. Much more to gain with less research.
Unfortunately, we look only the cosmetic of nose .
We are flying less than 100mph. Does it matter? The only way to deal with post fuselage separations is with a tail.
 
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Jean Claude

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Just change Cd*S from 1 (with no fairing) to 0.35 (about clean) at 50 mph allows 25% power less in cruise.
The only way to deal with post fuselage separations is with a tail.
Are you sure that everything is done to avoid the separations of the flow behind the pod?
Sans titre.png
 

XXavier

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In one of the few recent technical books on modern gyros, 'Flugphysik der Tragschrauber' https://b-ok.cc/book/2870637/39d9da ), the authors assume a Cd of 1 for the MTOsport (similar to the open ELAs and Magnis). I suppose that the more streamlined Magni M24 and specially the new ELA Eclipse may be around 0,7 ... 0,5 or perhaps even less. But that's just a personal estimation. I would like to see wind tunnel values somewhere...
 

Aviator168

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Just change Cd*S from 1 (with no fairing) to 0.35 (about clean) at 50 mph allows 25% power less in cruise.

Are you sure that everything is done to avoid the separations of the flow behind the pod?
View attachment 1146151
Still, majority of the drag is coming from the rotor. Not much you can do about that.

Regarding separations. Most gyros did little if any at all about separantions. This is an exception
 

Vance

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Still, majority of the drag is coming from the rotor. Not much you can do about that.

Regarding separations. Most gyros did little if any at all about separantions. This is an exception
Cavalon, Calidus, Magni Orion ELA Eclipse and the various iterations of the Xeon all make efforts to manage airflow at the rear.

As the speed picks up a greater portion of drag comes from parasitic drag.

It appears to me at 85kts less than thirty percent of the total aerodynamic drag comes from the rotor in the gyroplane I fly (The Predator).
 

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Aviator168

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It appears to me at 85kts less than thirty percent of the total aerodynamic drag comes from the rotor in the gyroplane I fly (The Predator).
I like to some real measurement on this. Compare to a fixed wing LSA, a gyro seems to be a lot slower with the same power.
 

XXavier

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Cavalon, Calidus, Magni Orion ELA Eclipse and the various iterations of the Xeon all make efforts to manage airflow at the rear.

As the speed picks up a greater portion of drag comes from parasitic drag.

It appears to me at 85kts less than thirty percent of the total aerodynamic drag comes from the rotor in the gyroplane I fly (The Predator).
Just for comparison... IIRC, at optimal L/D, induced and parasitic drags are equal. In the MTOSport, that L/D is 4,3 at 90 km/h = 49 knots. At 125 km/h = 67 knots, nearly 30% of the drag of the MTOSport is induced drag at an L/D somewhat higher than 3...
(It's all 'in the book' the reference of which I gave above. Pages 112-113...)
 

Jean Claude

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Just for comparison... IIRC, at optimal L/D, induced and parasitic drags are equal.
(It's all 'in the book' the reference of which I gave above. Pages 112-113...)
This is only true for an FW, but false for a gyro because the profile power of a rotor keep almost constant .

In my opinion, relying on the cD min of 8H12 measured in a wind tunnel with low turbulence, H. Dudda underestimates the drag of the rotors . Thus, to justify the power absorbed in level flight, it overestimates the parasitic drag.
 
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Vance

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The power required to cause the rotor to auto rotate is significant. I wish there is a way to determine that.
I tried to post a spread sheet but was not able.

Horsepower and drag on The Predator according to a spread sheet I use at various speeds:

at 26kts induced drag 25hp, Total drag 55 hp & 689 pounds.

at 57kts induced drag 11hp Total drag 42hp & 242 pounds

at 87kts induced drag 7.4hp Total drag 38hp & 142 pounds

If you send me an email I will send you the spread sheet. It is easy to change the numbers for different gyroplanes.
 

Jean Claude

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I also can't post an Excel file, while I did in the past
A possible formula for profile power when forward speed is nul is: P = ¼ Cd.ρ.c.ω^3. R^4 with Cd = 0.011

So, when ρ = 1.225 kg/m3, c= 0.18 m R= 3.5 m and ω = 36.6 rd/s, then Profile power = 4400 w, let's say 4800 w at 20 m/s forward



 

XXavier

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Vance, thanks in advance. I would like to experiement with different number. I will private post my email to you.

With the figures given by Vance, and an on-line curve fit service, it's easy to draw the curves. The fit is perfect, since there are three points and it's a 2nd degree curve...

Captura de pantalla 2020-01-14 a las 10.25.06.pngCaptura de pantalla 2020-01-14 a las 10.27.49.png
 

XXavier

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This is only true for an FW, but false for a gyro because the profile power of a rotor keep almost constant .

In my opinion, relying on the cD min of 8H12 measured in a wind tunnel with low turbulence, H. Dudda underestimates the drag of the rotors . Thus, to justify the power absorbed in level flight, it overestimates the parasitic drag.

So, if I get it right, and in the case of the MTOsport (and similar designs) the optimal finesse isn't at 90 km/h but at a higher airspeed...

Much higher?
 

Jean Claude

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Entering the characteristics of MTOsport (392 kg) in my spreadsheet, I obtain a maximum L/D of 3.7 at 29 m/s
At this moment,
Parasitic drag = 360N
Rotor profile drag = 426 N
Induced drag = 250 N
 
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