LDmax in helicopters

[JB77UK]

In airplanes LDmax (best lift to drag ratio) is the point where the wing's angle of attack is at a 'sweet spot' where it provides the most lift to drag. In an airplane that translates into the best glide speed (See FAA Airplane Flying Handbook pg3-16 for more details).

But in the helicopter, with the aerodynamic forces involved in an autorotation I'm not sure that things are the same. I have been told that LDmax for a helicopter is actually Vy speed - best rate of climb.

So I fly an R22, and I am sure most rotorheads know that helicopter, so lets take that as an example.

In the R22:


Vy (best rate of climb) is 53 KIAS
Vg (best glide in an autorotation) is 75 KIAS

So which (if any of those) is LDmax?

As a comparison, a Diamond 20 single engine airplane:


Vy 75 KIAS
Vg 73 KIAS (also identified as LDmax)

Many thanks for all contributions

[rotorfossil]

75 kt is best range speed in auto not minimum R of D speed which would be about 55 kt.

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JB - you can't consider an individual blade and use L/D as a measure, you have to look at the power required curve which is essentially U-shaped - at the bottom of the U the power available is at a maximum.

Induced power requirement is high at zero speed and reduces quite quickly with forward speed, parasite power (to overcome drag) is zero at zero speed and increases rapidly with forward speed. The best rate of climb speed and the minimum rate of descent speed in autorotation will both therefore be at the bottom of the U and within a few knots of each other - in your example 53 KIAS for Vy and 55 KIAS (from rotorfossil) for min RoD.

The 75 KIAS is for range and, as Um-lifting says, is a tangent from the origin to the power curve and will be higher than min RoD speed. It is a trade-off for extra distance covered against an increase in RoD.

[JB77UK]

Thank you guys.

So basically LDmax doesn't really apply to helicopters as a whole as you cannot isolate the individual blade AoA and airspeed for a given helicopter airspeed?

And you can't say that Vy (53 knots) in a helicopter IS LDmax - right?

[zudhir]

A better coefficient would be CT/Sigma