MRB twist

[MLH]

All of the main rotor blades I am familiar with are designed with a linear negative twist from root to end in order to equalize span wise lift.


What would be the effect of flying a zero twist blade?

[CRAN]

About a 5-10% increase in power required to hover, would be the most noticeable.

It will also effect vibration in forward flight and autorotation performance.

[Ascend Charlie]

The tip will develop considerably more lift that it would if there was a twist, so the moment resulting from that will greatly increase forces along the blade, and on the grips and hub.

The twist is there to try to get a more even distribution of lift along the blade.

[AAKEE]

Originally Posted by CRAN

It will also effect vibration in forward flight and autorotation performance.

Yes. The center of "inertia" is normally about 2/3 från blade root and to keep everything in balance the center of lift should be on the same position. Without twist center of lift will be furter out, causing forces in the rotorsystem we dont want to have.

[chopjock]

Twisting a blade will improve efficiency when blowing air downwards, but would it be as efficient in auto rotation?

[[email protected]]

They used to taper the blades from root to tip to achieve the same thing in the olden days before manufacturing techniques improved.

Having an untwisted blade in autorotation would mean the blade operating at higher AoA on the outboard section, giving more driving force and higher Nr; for the same shape and size of blade, the untwisted version would have to be rigged at a lower pitch setting for auto I think.

[jellycopter]

The only helicopter I know of that doesn’t have washout on the blades is the Rotorway Exec/Talon. They were designed like this to ease the manufacturing process for the kit builder and the same design has been maintained by the factory since they offered fully built blades. With a few hundred hours on type, my observations of no twist are;

1. Excellent NR recovery in auto (2degs negative pitch also helps, as does the semi-symmetrical profile).

2. A very marked flap-back tendency; I believe this is due to the uneven ‘toroidal’ lift distribution causing the onset of translational lift to be abrupt and non-linear. This often manifests itself in porpoising in the cruise on gusty days by inexperienced pilots.

3. Drop-off in aft cyclic authority when hovering with a tailwind as the down wash is heavily focused towards the tip and ‘falls off’ the horizontal stabilizer abruptly as the negative airspeed builds. It’s just not as progressive as a ‘conventional’ blade. (Exacerbated by the limited control power of the teetering head).

4. Interestingly, the uneven lift distribution isn’t readily seen when observing the coning/bending of a hovering helicopter. This is probably due the centrifugal force being relatively greater than the bending forces produced by lift having a ‘straightening’ effect plus the fact that the Rotorway blades are very stiff.

JJ