Is a tail rotor really needed?
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Senis Semper Fidelis
Joined: Feb 2001
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From: Lancashire U K
Baranfin, Good Morning,
Some were in the dim and distant past , I can remember seeing a Documentry and then reading about ducted air is cleaner and subject to less turbulance than open air and therfore requires less power to create a lot of push. Whilst on this ducted theory, is it certain that rotor tips in a fenestron create tip vortices?
Some were in the dim and distant past , I can remember seeing a Documentry and then reading about ducted air is cleaner and subject to less turbulance than open air and therfore requires less power to create a lot of push. Whilst on this ducted theory, is it certain that rotor tips in a fenestron create tip vortices?
Joined: Jan 2000
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From: Derby
Back to the original question, and a slight skew on it, and to spark a bit more dicussion:
Replace the spinning mass with a cylinder with "buckets" (along the lines of an anemometer, except driven rather than being driven by the wind), the open end of the buckets pointing in the direction of rotation, which would be counter to the direction of rotation of the rotor. Varying the size of the bucket mouths, or the speed of rotation, or the size of an exit hole in the buckets would vary the drag of the system. You could maybe arrange it so the variation in mouth/exit hole size occurred in a cyclic manner so the aircraft would have some form of yaw control.
Replace the spinning mass with a cylinder with "buckets" (along the lines of an anemometer, except driven rather than being driven by the wind), the open end of the buckets pointing in the direction of rotation, which would be counter to the direction of rotation of the rotor. Varying the size of the bucket mouths, or the speed of rotation, or the size of an exit hole in the buckets would vary the drag of the system. You could maybe arrange it so the variation in mouth/exit hole size occurred in a cyclic manner so the aircraft would have some form of yaw control.
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Baranfin asked:
I remember reading somewhere that ducted fans produce more thrust for their size than un-shrouded. Is this true? why?
Nick sez:
the shroud and the duct are carefully shaped to control the flow and cut tip losses. Tip losses are worth 5 to 10% of the thrust, because they allow air to escape around the edges of the blades, dumping lift. The close shroud makes the whole blade work for a living.
The shape of the shroud and duct is like a rocket nozzel, so it carefully allows the air to fan out after the disk, making it cover a larger area in smooth flow. This means that the duct creates a larger equivalent disk, so the small fan is as efficient as a larger disk. The Sikorsky Fantail demonstrater that proved the fan we eventually put on Comanche used exactly the same horsepower in a hover as the S-76 tail rotor, even though it had a 3.5 foot diameter as compared to the 8 foot diameter of the 76 tail rotor.
Of course, the duct and fan support mechanism weighs more and the thick duct has more drag, so you don't get something for nothing.
The great strength of the fan is that it does not know when it is being operated in a sideslip at high speed. This is somewhat bad because it does not push the nose back straight again, so the fan is a poor yaw stability device. That is why fenestron helos have big tail surfaces, to make up for the weak directional stability. On the plus side, the fan does not mind big sideslips, where tail rotors get very high stresses, so the fan allows big snap turns at high speed. Comanche can turn around its mast at 80 to 100 knots, shoot, then turn back forward again.
I remember reading somewhere that ducted fans produce more thrust for their size than un-shrouded. Is this true? why?
Nick sez:
the shroud and the duct are carefully shaped to control the flow and cut tip losses. Tip losses are worth 5 to 10% of the thrust, because they allow air to escape around the edges of the blades, dumping lift. The close shroud makes the whole blade work for a living.
The shape of the shroud and duct is like a rocket nozzel, so it carefully allows the air to fan out after the disk, making it cover a larger area in smooth flow. This means that the duct creates a larger equivalent disk, so the small fan is as efficient as a larger disk. The Sikorsky Fantail demonstrater that proved the fan we eventually put on Comanche used exactly the same horsepower in a hover as the S-76 tail rotor, even though it had a 3.5 foot diameter as compared to the 8 foot diameter of the 76 tail rotor.
Of course, the duct and fan support mechanism weighs more and the thick duct has more drag, so you don't get something for nothing.
The great strength of the fan is that it does not know when it is being operated in a sideslip at high speed. This is somewhat bad because it does not push the nose back straight again, so the fan is a poor yaw stability device. That is why fenestron helos have big tail surfaces, to make up for the weak directional stability. On the plus side, the fan does not mind big sideslips, where tail rotors get very high stresses, so the fan allows big snap turns at high speed. Comanche can turn around its mast at 80 to 100 knots, shoot, then turn back forward again.
Joined: Feb 2000
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From: UK (Wilts)
Re Fenestron:
Theoretically (and in practise) a tail rotor needs to be 1.4 times (root 2 in fact) the size of the equivalent a fenstron. Interestingly it is the shape of the duct that does half the work - if you put a fenestron fan in a straight tube it is much less efficient then 2nd generation fenestrons.
Theoretically (and in practise) a tail rotor needs to be 1.4 times (root 2 in fact) the size of the equivalent a fenstron. Interestingly it is the shape of the duct that does half the work - if you put a fenestron fan in a straight tube it is much less efficient then 2nd generation fenestrons.
Thread Starter
Senis Semper Fidelis
Joined: Feb 2001
Posts: 1,288
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From: Lancashire U K
Hey Uncle Frank Robbie,
What about a Fenestron fitted to the R22, take away a lot of danger from flying/being out of balance, Would it??
(Now reaching for the Tin Hat MkII)
Have a nice weekend Rotorheads!
What about a Fenestron fitted to the R22, take away a lot of danger from flying/being out of balance, Would it??
(Now reaching for the Tin Hat MkII)
Have a nice weekend Rotorheads!
