Comments on: Need for speed

By: Jared

Jared — Thu, 16 Feb 2012 18:07:00 +0000

Why don’t we just buy a $150,000 fixed wing plane. I’ll never understand rotor heads. I work on them for a living and we all make fun of them. unless you are military, logging, or hauling injured people from accidents there is no need for a helicomputer.

By: Ehud Gavron

Ehud Gavron — Wed, 24 Mar 2010 14:52:31 +0000

I hesitate to sound like an Airwolf enthusiast… but there are three states of the MR’s lift:
1. The advancing blade is generating lift. The retreating blade is generating LESS lift.

2. The advancing blad is generating a LOT of lift. The retreating blade is not moving fast enough with respect to relative wind to create lift.

MIRACULOUSLY [by today's technology, but think JATO] the aircraft is accelerated MUCH FURTHER beyond point 2…

3. The “retreating blade” generates lift. It can do so either through going slow but being pitched opposite its normal direction, or it can do so through being very quick, and not having any part of it exceed supersonic speeds.

Personally I think that one day we will see MR systems where the blade on the not-in-the-direction-of-airspeed side (also known as the retreating blade) can be pitched AWAY from the direction it’s moving, and because the relative speed between it retreating and the advancing relative wind will be positive… it will have lift.

By: Rich

Rich — Fri, 19 Mar 2010 16:19:42 +0000

Justin, good question. The advancing blade has a speed limit which is generally the speed of sound. The speed of the advancing blade is the forward speed of the aircraft plus the rotor speed. As this approaches the speed of sound (a flight regime for which it’s not designed) an increase in drag occurs as well as problems with shock waves among other things. Also with the increase in drag comes an increase in power required, eventually something has to give.

Why not design the rotors to go faster? Basic aero problems, there is a reason the wing shapes of high speed aircraft and low speed aricraft are different.

By: Justin

Justin — Fri, 19 Mar 2010 15:16:55 +0000

I hate to post a dumb question, but why don’t they just spin the standard rotor faster and decrease the angle of attack? Does it have something to do with what Mr. McKillip was saying about hub drag?

By: Bob McKillip

Bob McKillip — Fri, 19 Mar 2010 12:59:07 +0000

It’s great to see a manufacturer doing such R&D to push the technology forward. The coax rotor, like all configurations, though, is no “free lunch” – the interference of the wake from the upper rotor on the lower creates losses on the order of what one sees with other configurations, be they tandems or main/tail rotor systems. Johnson (Helicopter Theory) discusses this aspect in his chapter on design.

The ABC has extremely stiff blades to prevent their flapping into each other at high speed. This was done to allow for a smaller hub length that separated the two rotors, which is important as hub drag is a BIG contributor to parasite power required at high speed.