Techy Induced Drag Question

[Alex Whittingham]

I'm having a problem sorting out the exact shape of the induced drag curve and relating it to the induced power curve. The theory suggests that induced drag is at a maximum in the hover and reduces with forward speed at quite a high initial rate which then reduces as the rotor disc has to be tilted forward more at higher speeds to overcome the parasite drag. Some diagrams show exactly this (Wagtendonk page 34).

The RAF and RN books (AP3456, AP129, Mathieson) don't show drag curves but skip instead to power required curves. AP3456 and Mathieson show induced power required out of ground effect at any steady forward airspeed as a constantly reducing line at low forward speed, a reflection of the drag line from Wagtendonk.

So far, so good, but AP129 has a diagram in it that shows induced power out of ground effect as constant initially, then reducing. Now AP129 is usually very exact, and I'm not sure whether to rely on the modern books or whether they are simplifying the situation to relate it to fixed wing induced drag, which is a slightly different beast.

Can anyone clarify this please?

[heedm]

The curve has to be flat at zero speed. The curve can be extended into negative airspeeds, and for low negative airspeed it should be a mirror image of the low positive airspeeds. Continuity of the curve requires it to be flat at zero speed.

What happens between zero speed and translational lift speed is a decrease in induced drag, since the forward speed increases efficiency of the disk. How much the efficiency increases will spell the difference between a fairly constant line and a sharply decreasing line.

It's important to remember that the points on the curve are for steady state. I confused myself once thinking you created more induced drag as airspeed increased from zero because you still need power to maintain level, but you also need power to accelerate. That is not a steady state thus I confused myself.

SUM UP!! Level at zero, decreases towards TL, gradually decreases until you get into higher speed compressibility issues.

[Shawn Coyle]

Where can one get a copy of AP129?

[Alex Whittingham]

Of course, thanks. I copied the steady state bit but, being a plank driver, didn't think of negative airspeeds.

The RAF Museum at Hendon keep all the old APs and will copy them for you for a small charge, they're very helpful. We asked for the last edition of AP129 just before it became AP3456.

[212man]

You can now buy AP3456 commercially. Transair sell it or you can order it directly from the editorial office at RAFC Cranwell (address not to hand I'm afraid but a google search will bring it up I'm sure).

[Shawn Coyle]

We have AP3456, just thought AP129 might be something more modern and more helicopter specific.
Thanks.

[Alex Whittingham]

No, AP129 is an older publication. AP3456 stands accused of being 'dumbed down' over the years. The older publications tend to be more detailed in many areas.

[212man]

I'd agree with that; it certainly doesn't look as detailed as when I last saw it 16 years ago, and I don't think it's anything to do with me knowing more now. It's far more like an ab-initio study guide and hardly A2 swatting material.