07 September 2009

Compound Helicopters



We've discussed before how the forward speed of helicopters is limited by the speed of sound and by a phenomenon called "retreating blade stall".
As the rotor approaches Mach I a shock wave begins to build in front of it and drag increases exponentially.

The relative wind flowing across the advancing rotor blade is the sum of the forward blade speed and the speed of the aircraft, while the speed of the relative wind on the retreating blade is reduced because it is running away from the forward speed of the aircraft. Therefore, the retreating rotor blade has to have a higher angle of attack in order to produce the same lift as the advancing blade. I'm still amazed helicopters even fly.

So the speed of sound is a physical limitation for true helicopters and in order for rotary wing aircraft to go faster, we'll have to explore technologies that make the aircraft something other than a true helicopter.

The V-22 Osprey is an expensive, complicated (and controversial) way to attack the problem.
Compound helicopters are another, less complex way to increase forward speed...
By adding a wing, the angle of attack on the rotor can be reduced because the wing is assisting in production of lift.
The speed where "retreating blade stall" is encountered is thereby increased, which allows the aircraft to fly faster before encountering that problem.

This video shows flight testing on such a machine...
You can see the stub wings on the fuselage, and the ducted fan and rudder-like device that replaces the tail rotor. It's an exciting concept, and may help bump up the speed at which rotorcraft can fly before encountering aerodynamic limits.

Exciting stuff!
(Hat tip to friend Terry for the link!)

7 comments:

Cissy Apple said...

Hell, I can't drive my car without careening through the half-opened garage door! Don't think I could manage a copter.

Greybeard said...

Cissy, I COULD teach you to fly copters.
But situational awareness is a little more important in a copter than it is in a car, so maybe it wouldn't be your best bet for longevity!

cj said...

Hey, look! A flying post!

(Sorry, couldn't resist...)

cary said...

Not only a flying post, but as is the case with GB's "flying posts" he learns us non-pilots a thing or two about them thar heliumwhopters...

Thanks for the lesson, Greybeard!

Greybeard said...

De nada por nada, mi amigo.
And your comment made me smile...
I'm bitter, remember?
Merriment just isn't my style man.

cary said...

Whoa - sorry, GB - didn't mean to make you break character or anything. Go ahead on back to being a grumpy old fart, we understand - you have a "rep" to maintain...

Cody said...

Wow! That was a spectacular! I really love to be in that Helicopter. It really amaze me. I was in a helicopter before and it was a great ride.:D