Vertipedia currently assumes (see Glossary) that 'wash-out' (local pitch angle reduces towards the tip) is entered as a positive number. This was chosen as the vast majority of rotors have 'wash-out' (some autogiros have 'wash-in', where the local pitch increases towards the tip).

From a pure mathematical perspective, it is understandable that 'wash-out' should be quoted as -ve as the derivative '(change in pitch)/(change in radius)' would be -ve.

Also, twist is currently entered in the unit of radians rather than degrees as the database is based on the SI standard.

It is not clear if there is a VTOL industry standard, so this discussion has been raised to gain feedback from members on the preferred standards to be used in Vertipedia. Please note that whatever is chosen, there will be no change to the data within Vertipedia other than (potentially) changes to the displayed values and the definition in the Glossary.
* Last updated by: comodo15 on 7/20/2016 @ 6:58 PM *

"Twist on a wing is most usually defined as positive nose up from root to tip; this is called wash-in. Most wings in practice have negative twist from root to tip, which is called wash-out. The positive sign convention for the twist angle of a wing follows directly from the definition of a positive angle of attack. The same convention is usually, but not always, adopted for rotor blades, so negative values of twist rate (i.e., wash-out) are the more commonly quoted “twist” values for rotor blades. In most of the US rotorcraft publications, nose down blade twist will be defined as having negative values. Twist rates can be defined with dimensions of angular displacement per unit length or angular displacement per rotor radius.

The twist is not always defined from the "rotor hub c/l to blade tip." In fact, the use of "c/l [centerline]" is misleading and what is meant here is the rotational axis of the rotor. However, twist can be measured from any datum point and the rotational axis is not always used. In fact, blade pitch (twist) angle referenced to 75% rotor radius is often used as a datum. Sometimes, for the purposes of analysis, a known blade twist distribution will be converted into an equivalent linear twist, which may be measured relative to the 75% radius position. More commonly, as in the blade element types of analysis, the local blade twist is just specified at each and every blade station relative to the specified datum (reference point) using the adopted sign convention."

Gordon Leishman
Professor, Aerospace Engineering, Embry-Riddle Aeronautical University
AHS Member since 1986

Transcribed by AHS Staff Betty Chen
* Last updated by: VertipediaTeam on 8/11/2016 @ 8:24 PM *