Vyse change with weight?

[hugh flung_dung]

This might be difficult to explain using text ... but ...
I've always assumed that, to a first approximation, Vyse will vary with weight (and therefore Lift); so Vyse at Weight W1 can be estimated by modifying the Vyse at MAUW by the ratio of the SQRTs of W1 and MAUW. Obviously there will be a small form drag element that's not lift-related, but what else is significant?

If it were only weight related, a Vyse of 90 at 4200 becomes a Vyse of 78 at 3200 - and I don't believe that!

HFD

[machonepointone]

HFD,

The figure is correct. The maths involved are as follows-

New Speed = Old Speed x Square root of new weight divided by the square root of the old weight. Give or take a few decimal points the answer really is 78 knots.

[Stan Evil]

Some manufacturers, Cessna for one, actually have a graph of Vyse vs weight in the POH. The rest just quote Vyse for max auw. As has been said, it varies with the square root of weight ratio.

[A and C]

Its not just Vyse that will change with weight ( or should that be mass with the JAA ) Vy will but I can see why you would be so interested in the case of Vyse !

[mad_jock]

Instead of thinking about weights and Speeds

Trying thinking that the coeffecient of lift is constant for a particular flight performance.

So it will give you 2 equations that equal each other. Then all you need to do is remember the one equation instead of lots.

Another note is that there is actually 2 Vyse either side of the drag curve for some types. Due to ram effect on turbines. Both will give you the same performance but one higher speed. The reason why you don't hear of the second one is because Vyse is most critical near the ground where you don't have enough room to accerate to the second Vyse. On my type this is shown by the fact that you will get exactly the same performance in climb by doing 170knts@ FL100 instead of the book figure of 155knts this is 2 engines running but the same still applies because the only thing that is different is the co of lift. which i might add is different for both speeds.

MJ

Ps I am of course talking real life ops with the above not the nice stuff they would like you to believe in JAR. Performance by Swatton is the bible on all this stuff.

[bookworm]

I've always assumed that, to a first approximation, Vyse will vary with weight (and therefore Lift); so Vyse at Weight W1 can be estimated by modifying the Vyse at MAUW by the ratio of the SQRTs of W1 and MAUW. Obviously there will be a small form drag element that's not lift-related, but what else is significant?

Engine power variation with speed.

Your calculation will correctly assess the ratio of drag and power required. Thus for glide angle and sink rate minimization, it works. However, as soon as you start adding speed-dependent power, it shifts the minimum of the excess power curve.