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Off the wall thread

Old 08-03-2007, 05:09 AM
  #1  
John Boy
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Default Off the wall thread

I have a couple of questions about helicopters that will stress the engineers out there. I am not a helicopter person (rc or real). However, I had professional reasons for knowing in great detail how real helicopter blades work. One of the more interesting facts is that real helicopter blades are spun at a constant rate with the outer edge travel at just below the speed of sound. This produces the maximum amount of lift for energy expended. This is also why jet airliners are designed to fly up to but not beyond the speed of sound, maximum lift occurs just before the wing (or prop) goes super sonic.

So, is this the holy grail for model helicopters? Or is safely keeping the blade speeds much lower?

A related question comes from observing model helicopters. The blades are extremely stiff compared to the typical commercial helicopter blade. Models seem to have stiff short blades akin to what you see on an attack helicopter where damage is a big concern. Is this just a limitation of plastic blades? Is it intentional? Wouldnít flexible blades help with stability in turbulence?

Similar questions could be asked of the airplane crowd where I reside. Given that real airplanes have very limited prop rotation rates and all high end planes use variable pitch props and not variable speed props why donít we do the same? I believe the answer is weight and complexity of the mechanism would drive the cost far to high to accurately scale.

Or to put it another way, does it bother anybody else that the speed of sound does not scale? Well, it is keeping me up tonight and I thought I might share this with you.
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Old 08-03-2007, 05:11 PM
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Murdock1
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I don't know the answers to any of those questions. I certainly agree with your statement of full size helicopter rotor blade flexability though. I've got some stills of an HH 60-J CG helicopter during a rescue. According to the crew they were about 100 ft. above the sea and the seas right then were 15-20 ft. The wind and turbulence were very strong and the helicopter was pointing dead into it. This helicopter has 4 rotor blades and you can plainly see that in addition to the fact all 4 of them are in different planes, they are bent up and down to varying degrees at the blade mid points, in addition to being slightly twisted along the length. It looks as if something is violently wrong with them but I've been told that's normal and per design.
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Old 09-03-2007, 05:54 PM
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p901
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Hello John Boy, just ran across your thread and realized I had those questions myself.
Don`t know if you are into reading books that are more formula than text but I was fortunate to find a book called Rotary Wing Aerodynamics by Stepniewski and Keys. The answers are in there if you are willing to take the time to read into the formulas and thankfully for me there is a bit of text that backs up some of the math. Reading into the explanations it will also make clear why the blades appear to be pitched and bent and coned all over the place as was shown in the still photos described.
As far as the question with the controllable props on planes you might think of it as changing gears in a car. Once the plane is up to cruising speed selecting more pitch allows the prop to maintain its degree of bite into the oncoming air. Don`t know if it actually has a direct relationship of coming near the speed of sound but has more to do with the drag on the rotating component building by the square root so that at some point in time an inordinate or impractical amount of power will be required to gain very little additional speed.
The designers realized that as long as you don`t select too much pitch and overload the engine by watching the intake manifold pressure then you can in a sense put the plane in overdrive and save a significant amount of fuel which over time outweighs the cost of the additional mechanisms.
Hope this helps, andrew
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Old 09-03-2007, 11:51 PM
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HX3D014
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I have a couple of questions about helicopters that will stress the engineers out there. I am not a helicopter person (rc or real). However, I had professional reasons for knowing in great detail how real helicopter blades work. One of the more interesting facts is that real helicopter blades are spun at a constant rate with the outer edge travel at just below the speed of sound. This produces the maximum amount of lift for energy expended. This is also why jet airliners are designed to fly up to but not beyond the speed of sound, maximum lift occurs just before the wing (or prop) goes super sonic.

So, is this the holy grail for model helicopters? Or is safely keeping the blade speeds much lower?

A related question comes from observing model helicopters. The blades are extremely stiff compared to the typical commercial helicopter blade. Models seem to have stiff short blades akin to what you see on an attack helicopter where damage is a big concern. Is this just a limitation of plastic blades? Is it intentional? Wouldnít flexible blades help with stability in turbulence?

Similar questions could be asked of the airplane crowd where I reside. Given that real airplanes have very limited prop rotation rates and all high end planes use variable pitch props and not variable speed props why donít we do the same? I believe the answer is weight and complexity of the mechanism would drive the cost far to high to accurately scale.

Or to put it another way, does it bother anybody else that the speed of sound does not scale? Well, it is keeping me up tonight and I thought I might share this with you.
Wow. you got lots of Questions and they are not basic ones either

One of the more interesting facts is that real helicopter blades are spun at a constant rate with the outer edge travel at just below the speed of sound
correct. RRPM is constant
Correct tip speed is keept below Sonic (With forward speed added)
however. if it dose go any faster it will suffer.
A hover rotor speed is not near sonic. not all heli are the same. some are fare less and some come very close and can excede (With forward speed)

here is the formula to find tip speed
first find the circumference
2xπxr
π (Pi) = 3.1415926535897932385...
r (Radius)

lets use my little Heli
Radius of 27cm
RRPM of 2600

hover tip speed is

2 x 3.1415926535897932385... x 27 = 169.646 cm circumference

now it is doing this 170cm 2600 times a min.
so 170 x 2600 = 442000cm/min
or
4420m/min
or
4.42km/min
or
times now by 60 min to get an hour = 265.2kph
this is a micro electric. and it is an easy speed that could be reached in a car.

why only this speed ?

2 things.
Centrepetal forces on the hub and bolts there.
Drag

lift is increased with speed
but drag is also increased with speed
so it comes to a matter of Power and Efficiency.

there is a chart that shows a lift and drag relation. and real heli operators like to operate within a good Lift/drag ratio.

A heli can go faster but it is waisting power to do so. so there is a point where it can operate with maximum efficency.

when a blade comes very close to Sonic it experencs Sonic buffeting. and it causes great stress on the blades.


blades are extremely stiff compared to the typical commercial helicopter blade
stiffer blades and head give Agility but cost in comfort and smoothness in gusty conditions etc.
RC heli are being thrown around the sky at rates that would destroy a real heli. so they need the rigity to be able to do so. the stiffer the blade the Quicker the responce (But more twitcy, which could be good or could be bad)

there are blades of differend design and some are flexable and some are symetrical and some are Flat bottom or have a Camber. it depends on what you want to do with you heli.

VPitch props work on efficency and allow the engin to operate in its best RPM range.
there is a problem with VP props.
the more pitch you pull. the less efficent it is becoming. so again there is a point where you need to work out the best performance of speed and pitch to get better ecconomy.
as you increase the pitch. the individual prop is moveing its lift vector back from the forward direction and this results in an increase in torque stress on the engin.

Digest that and get back with more.
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