Height vs. Velocity diagram explained

Jean Claude

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The windmilling is just beginning at this moment. But Increase the forward speed to 30 m/s and the engine will be driven by the propeller at say 2600 rpm (propeller at 1000 rpm). Then the representative blade element will have a circumferential speed of 65 m/s. Its angle of attack would be zero for a pitch of : Atan (30/65) = 25 degrees. This is about 5 degrees more than the pitch angle (Hence CL = - 0.5 ) and the propeller pull would then be about -350 N.
Such an increase in drag would be very noticeable

Sans titre.png

For simplicity, I have neglected the induced speed
 
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WaspAir

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Thanks - that agrees with my expectations.
 

Vance

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The Predator has a noticeably better glide with a stopped propeller compared to an idling propeller at 800 rpm direct drive.

I did not attempt to quantify it as I always assume a worse glide ratio than she has so I make it to my landing spot.

My GPS shows glide ratio and it can be anywhere from three to one to five to one depending on conditions.

The Propeller did not windmill at 50kts indicated airspeed.
 

Jean Claude

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The Propeller did not windmill at 50kts indicated airspeed.

Windmilling does not mean restarting the engine. It just means aerodynamic torque helping it rotation.
To restart the engine, a greater aerodynamic torque is needed to overcome the first compression.
Only then it can continue to run without any explosion
 
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querist

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Do gyros typically come with a Sprag clutch like helicopters do to prevent the rotor from turning in the wrong direction, thus enabling autorotative descent, or do they rely on the inertia to prevent the rotor from stopping or turning in the wrong direction when you reach zero forward airspeed?
 

GyroChuck

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No sprag clutch on gyros. If you are airborne and go to zero forward speed and no throttle. Then you are descending and the upward airflow thru your rotors keep them turning. Or as we say auto-rotating.

If your rotors stop in flight, most likely not because they will sail and cut off your tail first before stopping, then you will become a casualty.

The way you ask questions and reply on this forum. I think you will make an excellent Instructor.
 

Tyger

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Do gyros typically come with a Sprag clutch like helicopters do to prevent the rotor from turning in the wrong direction, thus enabling autorotative descent, or do they rely on the inertia to prevent the rotor from stopping or turning in the wrong direction when you reach zero forward airspeed?
Gyro rotors are always autorotating, so all descents (and climbs and everything else) are "autorotative" and it's never going to be turning in the "wrong" direction.
 

MikeBoyette

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The Predator has a noticeably better glide with a stopped propeller compared to an idling propeller at 800 rpm direct drive.

I did not attempt to quantify it as I always assume a worse glide ratio than she has so I make it to my landing spot.

My GPS shows glide ratio and it can be anywhere from three to one to five to one depending on conditions.

The Propeller did not windmill at 50kts indicated airspeed.
My Dad’s 0320 side by side wouldn’t wind mill most of the time. He had a skydive instructor with him at Bensen Days and wanted to give her a thrill so he went to 1500’ and said “ok we are going to simulate a power failure hold on the nose down attitude is pretty severe”. He had a moon roof in it and on a dead stick that where you saw the runway. He reached for the throttle and the engine quit. She said “I assume this is now real” he said “yes it is, here we go”. He lowered the nose and they came across the numbers at almost 120mph. That old Lycoming was windmilling like crazy. It seemed to take for ever for them to stop floating in ground affect and land. The little ball in the primer bulb turned sideways and shut the Fuel off. From then on we always put a bypass around the primer ball.
 

querist

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Gyro rotors are always autorotating, so all descents (and climbs and everything else) are "autorotative" and it's never going to be turning in the "wrong" direction.
As I thought, but a sprag clutch on many helicopters makes it nearly impossible for the rotor to turn in the wrong direction. I was just wondering if any gyros have that mechanism as well.
 

MikeBoyette

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As I thought, but a sprag clutch on many helicopters makes it nearly impossible for the rotor to turn in the wrong direction. I was just wondering if any gyros have that mechanism as well.
I sent you a PM. It is impossible while flying for the blades to stop much less turn the other direction. The rotor head has a main bearing that allows the blades to turn freely. They will turn as long as they are weight on them. If for some reason the bearing seized completely the blades would still turn. If for some reason it was able to stop them it would twist the mast right off. This is a problem that doesn’t exist so therefore nothing to worry about.
 

querist

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I sent you a PM. It is impossible while flying for the blades to stop much less turn the other direction. The rotor head has a main bearing that allows the blades to turn freely. They will turn as long as they are weight on them. If for some reason the bearing seized completely the blades would still turn. If for some reason it was able to stop them it would twist the mast right off. This is a problem that doesn’t exist so therefore nothing to worry about.
Good to know. It's amazing how these things work. My wife is the mechanical engineer. I just take it all on faith. :)
 

Tyger

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I don't know a lot about about helicopters, but my assumption is the sprag clutch is how power is delivered to the rotor, whilst allowing the rotor to freewheel should the engine stop turning. It's not to prevent the rotor turning the "wrong way"
 

GyroFI

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I don't know a lot about about helicopters, but my assumption is the sprag clutch is how power is delivered to the rotor, whilst allowing the rotor to freewheel should the engine stop turning. It's not to prevent the rotor turning the "wrong way"
All very interesting.
I agree with Tyger re the sprag clutch allowing power to the shaft and yet freewheeling should the engine power cease and this is exactly the same way that the sprag clutch works on the starter motor of the Rotax 91series and that also does not prevent the shaft being rotated the wrong way when gurgling (and possibly causing problems)
 

querist

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I don't know a lot about about helicopters, but my assumption is the sprag clutch is how power is delivered to the rotor, whilst allowing the rotor to freewheel should the engine stop turning. It's not to prevent the rotor turning the "wrong way"
https://www.rotorandwing.com/2010/02/01/autorotation-is-a-sprag-thing/

My understanding is that the sprag clutch will, along with the engine, prevent reverse rotation while allowing the rotors to rotate more quickly than the shaft (but only in the correct, lift-creating direction) if the engine quits, etc.
 

MikeBoyette

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https://www.rotorandwing.com/2010/02/01/autorotation-is-a-sprag-thing/

My understanding is that the sprag clutch will, along with the engine, prevent reverse rotation while allowing the rotors to rotate more quickly than the shaft (but only in the correct, lift-creating direction) if the engine quits, etc.
Ok so once again in a gyro (with one exception which is Dick DeGraw machines) the rotor blades have no permanent connection to the engine. The prerotator is only connected using a pulley pulled inward via a cable to cause the belt to turn. The belt turns a pulley operating a cable, a shaft, or a hydraulic pump. They go to a bendix that engages to the ring gear on the rotor head. Once the blades are up to speed it’s not engaged again until next take off run. Therefore no need for a sprague clutch on the rotor.
 

querist

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Ok so once again in a gyro (with one exception which is Dick DeGraw machines) the rotor blades have no permanent connection to the engine. The prerotator is only connected using a pulley pulled inward via a cable to cause the belt to turn. The belt turns a pulley operating a cable, a shaft, or a hydraulic pump. They go to a bendix that engages to the ring gear on the rotor head. Once the blades are up to speed it’s not engaged again until next take off run. Therefore no need for a sprague clutch on the rotor.
Yes. That's what I wanted to know. I know there is no connection to the engine, but a sprag clutch on a fixed shaft would still be able to prevent rotation in the wrong direction. However, given the explanations that others have offered here it seems that it is not an issue with gyros, so the point is moot.
 

WaspAir

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It's not an issue in helicopters, either. On the ground, with clutch not engaged, you can rotate the blades either way and it doesn't matter. On start up, while engaging the clutch, the engine will go in one direction only and the blades will follow suit. In flight, to reverse direction, you would first have to slow the rotor so much that the blades would cone up and collapse under the flight loads (like clapping your hands overhead) and you would hit the ground long before getting anywhere near reversal. Drop below about 80 percent rpm and you are in a parts-shedding projectile, not an aircraft. You'll never get to 0, much less a negative rotation rate, because blades need rapid rotation to have any stiffness in the rotor disc plane.

There was a Bond movie (Die Another Day?) in which he rides an engine-out NOTAR out from the back of a cargo jet, with blades fixed until he gets the engine started a few feet above the ground, and then spools up and away. I couldn't stop laughing, knowing that he would have broken off the blades long before.
 
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querist

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It's not an issue in helicopters, either. On the ground, with clutch not engaged, you can rotate the blades either way and it doesn't matter. On start up, while engaging the clutch, the engine will go in one direction only and the blades will follow suit. In flight, to reverse direction, you would first have to slow the rotor so much that the blades would cone up and collapse under the flight loads (like clapping your hands overhead) and you would hit the ground long before getting anywhere near reversal. Drop below about 80 percent rpm and you are in a parts-shedding projectile, not an aircraft. You'll never get to 0, much less a negative rotation rate, because blades need rapid rotation to have any stiffness in the rotor disc plane.

There was a Bond movie (Die Another Day?) in which he rides an engine-out NOTAR out from the back of a cargo jet, with blades fixed until he gets the engine started a few feet above the ground, and then spools up and away. I couldn't stop laughing, knowing that he would have briken off the blades long before.
That makes sense, and with the clutch engaged the blades cannot go backward. I did not realize that the blades would collapse before they got to that point, so I guess it doesn't really matter.
 

Tyger

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Yeah, I can't imagine anything actually making a rotor turn in reverse... maybe if it were falling upside-down?!

I think you are completely misunderstanding the purpose and action of the clutch. On a geared bicycle (see Rotax origins), the fact that the rear wheel can freewheel when you stop pedaling is not intended to prevent you from pushing your bike backward... nor does it.
 
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