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Old 02-18-2012, 09:39 PM   #2656
Klay
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Originally Posted by EvilGenius View Post
I.E. they don't rotate up or down.
But their pitch is variable, obviously.
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Old 02-18-2012, 09:40 PM   #2657
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Can someone in the know explain the point or philosophy of having the outboard props? Speed? Cruising efficiency?
Yaw control
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Old 02-18-2012, 09:47 PM   #2658
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Quote:
Originally Posted by Beach40 View Post
Can someone in the know explain the point or philosophy of having the outboard props? Speed? Cruising efficiency?
The quick answer is to delay the onset of retreating blade stall to increase cruise speed. I would give a more complete answer but I have to go.
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Old 02-18-2012, 10:22 PM   #2659
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Originally Posted by gbranham View Post
The quick answer is to delay the onset of retreating blade stall to increase cruise speed. I would give a more complete answer but I have to go.
Cheers. After reading this, I found this:

The X3 demonstrator is based on a Eurocopter EC155 helicopter with the addition of short span wings each fitted with a tractor propeller. The tractor propellers are gear driven from the two main turboshaft engines which also drive the five-bladed main rotor. The helicopter is designed to prove the concept of a high-speed helicopter which depends on the slowing down of the rotor speed to avoid drag from the advancing blade tip, and to avoid retreating blade stall by unloading the rotor while a small wing takes up lift instead.[2]
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Old 02-18-2012, 10:57 PM   #2660
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I could give a decent answer, but I'm extremely drunk right now.

Gimme 12 hours or so.
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Old 02-18-2012, 11:42 PM   #2661
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Originally Posted by EvilGenius View Post
I could give a decent answer, but I'm extremely drunk right now.

Gimme 12 hours or so.
Oh go on.... you know you want to...
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Old 02-18-2012, 11:49 PM   #2662
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Originally Posted by gbranham View Post
The quick answer is to delay the onset of retreating blade stall to increase cruise speed. I would give a more complete answer but I have to go.
That's why the main rotor is slowed down, not why the tractor propellors are outboard instead of on centerline.
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Old 02-19-2012, 06:21 AM   #2663
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Originally Posted by Klay View Post
That's why the main rotor is slowed down, not why the tractor propellors are outboard instead of on centerline.
You guys are getting your questions and answers mixed up.

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Old 02-19-2012, 07:39 AM   #2664
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Quote:
Originally Posted by Klay View Post
Yaw control
Yes, but only during the time that the main rotor is under power.

Quote:
Originally Posted by Beach40 View Post
Cheers. After reading this, I found this:

The X3 demonstrator is based on a Eurocopter EC155 helicopter with the addition of short span wings each fitted with a tractor propeller. The tractor propellers are gear driven from the two main turboshaft engines which also drive the five-bladed main rotor. The helicopter is designed to prove the concept of a high-speed helicopter which depends on the slowing down of the rotor speed to avoid drag from the advancing blade tip, and to avoid retreating blade stall by unloading the rotor while a small wing takes up lift instead.[2]
I doubt the "wings" do much lifting at all. A rotar system actually makes a great deal of lift in forward flight, but it doesn't have to be spinning at 100% to do that. So by letting the tractors take over forward thrust duty, they can slow the main rotor down which gives them the ability to go faster before hitting the wall off advancing blade breaking sound barrier and retreating blade stalling.

It's like a high speed billion dollar gyrocopter with the ability to hover.
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Old 02-19-2012, 08:26 AM   #2665
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Well...since we're discussing it...

Let me preface this question by saying that my understanding of helicopters is possibly greater than the average man-on-the-street, but obviously lacking by comparison to those who operate them regularly.

So how does slowing the main rotor, delay retreating blade stall?
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Old 02-19-2012, 08:30 AM   #2666
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Well...since we're discussing it...

Let me preface this question by saying that my understanding of helicopters is possibly greater than the average man-on-the-street, but obviously lacking by comparison to those who operate them regularly.

So how does slowing the main rotor, delay retreating blade stall?
Not much, but when you take the power away from an over head rotor and just let it spin it effectually becomes a large wing in forward flight (it does when under power too).

Theoretically it still matters, but not nearly as much as when youre using it to create lift by downward thrust.

At least, that's how I understand it.
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Old 02-19-2012, 09:00 AM   #2667
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So, how does this all work out if you put the helicopter on a treadmill?
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Old 02-19-2012, 10:41 AM   #2668
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http://www.cartercopters.com/

Everything you ever need to know about retreating blade stall and the why and how of making helicopters go fast.
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Old 02-19-2012, 11:20 AM   #2669
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http://www.cartercopters.com/

Everything you ever need to know about retreating blade stall and the why and how of making helicopters go fast.
Good link.

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Old 02-19-2012, 11:27 AM   #2670
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Originally Posted by Beach40 View Post
Cheers. After reading this, I found this:

The helicopter is designed to prove the concept of a high-speed helicopter which depends on the slowing down of the rotor speed to avoid drag from the advancing blade tip, and to avoid retreating blade stall by unloading the rotor while a small wing takes up lift instead.[2]
This sentence is poorly configured to explain what's going on here. The primary reason a helicopter is limited in its forward speed is retreating blade stall. The main rotor disc of a conventional helicopter is required to provide both lift and thrust in forward flight.

The rotor disc must compensate for dissymmetry of lift by decreasing the angle of attack on the advancing blade(s) and increasing the angle of attack on retreating blade(s).

As speed increases, the retreating blade's angle of attack must increase in order to maintain lift and thrust on its side of the rotor disc. Eventually the critical angle is reached where airflow separates from the upper camber of the blade with a decrease in lift and a large increase in drag. The result is a pitch up of the nose and a roll toward the side of the retreating blade and can be very violent.

In Eurocopter's inelegant solution they gave the thrust and anti torque duties to the tractor props. The main rotor only has to provide enough lift to maintain altitude so that even at very high speeds, where the stub wings become effective, the AoA of the retreating blade is still well short of the critical angle thus delaying the onset of retreating blade stall.

Sikorsky's X2 is a simple and more practical solution imo with a contra rotating main rotor eliminating anti torque requirements and tail mounted pusher to relieve the main rotor of thrust requirements.
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