Engine power for a ultralight

[XXavier]

I agree with J.R. and Antony.

The definition I use for Lift to drag ratio (L/D) is the lift of an airfoil divided by the drag of the airfoil.

In my opinion it does not go to zero because a gyroplane is not moving forward (a vertical descent) because part of the airfoil is still providing lift and the airfoil still has drag as it provides lift.

The definition I use for Glide Ratio is the distance the aircraft flies forward divided by the sink.

There is much more involved in Glide Ratio than the L/D ratio of the airfoil.

In my opinion Glide Ratio is what goes to zero in a vertical descent.

Glide ratio and L/D are numerically identical for any aircraft.

Think of that aircraft falling vertically. There is an aerodynamic force acting on it. Now, the conventional definition of lift is the component of that force perpendicular to the relative wind. It's quite obvious that, in the case of any object falling in vertical direction, that componente is zero, because if it weren't zero, the fall wouldn't be vertical...

Well, we now now that L is zero. Hence, L/D must be zero... And the glide ratio, being numerically identical to L/D, is zero too...

Yes, there may be parts of the falling object, like blades or fins, where –locally– a certain lift does exist, but globally considered, the whole lift of that falling object is zero.

 

[Vance]

Glide ratio and L/D are numerically identical for any aircraft.

In my opinion an unwillingness to separate the L/D of the airfoil (rotor blade) from the glide ratio of the aircraft limits an understanding of how a rotorcraft flies.

Most of the helicopter books I have do not use the terms L/D and glide ratio interchangeably.

I can fly two gyroplanes with identical L/D of the rotor system and the one with less fuselage drag (coefficient of drag times area) will have a much better glide ratio.

 

[Jean Claude]

Words and definitions changes depending on the messages. For example when you say that in vertical descent the speed forward is zero.
Since "forward" speed refers to movement, then "forward" is below in a vertical descent and forward speed is the speed of descent.

 

[XXavier]

In my opinion an unwillingness to separate the L/D of the airfoil (rotor blade) from the glide ratio of the aircraft limits an understanding of how a rotorcraft flies.

Most of the helicopter books I have do not use the terms L/D and glide ratio interchangeably.

I can fly two gyroplanes with identical L/D of the rotor system and the one with less fuselage drag (coefficient of drag times area) will have a much better glide ratio.

If the L/D ratio is considered –as it's usually done– for the entire aircraft, and not only for the wings or rotors, then L/D and glide ratio are numerically identical. In that case, and concerning only the numerical value, not the concept, L/D and glide ratio are identical.

 

[Jean Claude]

Yes, undoubtedly

 

[WaspAir]

If the L/D ratio is considered –as it's usually done– for the entire aircraft, and not only for the wings or rotors, then L/D and glide ratio are numerically identical.

Where we differ is only in assuming (or not assuming) that "usually" also applies to rotorcraft (as contrasted with fixed wing aircraft that do not ever drop vertically in a level attitude), not in our understanding of the things to be measured.

 

[XXavier]

Where we differ is only in assuming (or not assuming) that "usually" also applies to rotorcraft (as contrasted with fixed wing aircraft that do not ever drop vertically in a level attitude), not in our understanding of the things to be measured.

FWs can descend vertically, if properly induced to do so.

From Kermode's 'Mechanics of Flight':

 

[WaspAir]

FWs can descend vertically, if properly induced to do so.

Not in a level attitude, as I said. The only way that happens is if you retract the landing gear while sitting on the ground (ouch!), or maybe under a deployed BRS.

I think we're pretty far off the thread by now.