Vy= Least Drag?

[uwochris]

Hey guys,

I read somewhere that when you climb at Vy, you are maintaining the AOA where the L/D ratio maximized, providing the least amount of drag relative to lift. Also, at this point, excess power is maximized.

At first, this seemed to make sense... least drag= least resistance= best rate of climb.

However, the author then goes on to say that in the power off condition, Vy represents the best endurance glide. He also says that the best glide speed is close to Vx.

This seems contradictory... if maintaining Vy with power on does in fact give the AOA for best L/D ratio, then this AOA should also apply for the best glide speed. But it is Vx that corresponds to the best glide speed with power off.

I am confused, please help

ps- this came from "see how it flies" by John Denker.

 

[jayme]

When the nose is above the horizon, there is a vertical component of power, that translates into more lift. That changes everything. Best glide will not directly correspond to Vx or Vy, it will only approximate it.

 

[Mu2Driver]

Vx is the speed at which L/D is maximized

 

[midlifeflyer]

Vy is the airspeed in which you have the greatest difference between the power that is necessary for level flight and the power available to the airplane.

If you took the power available vs power required graph for an airplane and superimposed it on the L/D graph, you'd find that Vy, although close to Vg is a bit higher.

 

[Aerosmith]

First of all, lets not mix apples & oranges.

Max L/D equals Vx ( or speed for best gradient or speed for minimum drag (JET))
It also gives Max JET endurance/Max PROP range. Finally it equals max glide range, BOTH JET & PROP.
Notice the difference in best glide RANGE and glide ENDURANCE.

 

[uwochris]

Thanks for the responses.

Now that I think about it, it seems to make more sense that Vx be associated with max L/D (i.e. minimum drag) rather than Vy.

Drag is a force, and when climbing for Vx, we want excess thrust, not excess power. The point where the excess thrust (thrust-drag) will be minimized should occur where drag is low.

I was confused because in his book, Denker refers to the Vy climb as climbing in "the low rent district." He goes on to say that rent is synonomous with drag, which led to the confusion.

If anyone has anything else to add, please do.

Chris.

 

[uwochris]

... another thing to add.

In the POH for the Katana, it quotes:

- Best glide= 73
- Vy= 75
- Vx= 60

This is a reason why I was confused. My instructors also told me that it is Vy that corresponds to L/D max; looking at the number above, this appears to make sense.

 

[Aerosmith]

Dont forget that speed for best L/D aka Vx changes with weight.
Vx or best glide is given for one weight only in your manual. This does not have to be the same. Again, best glide is on some aircraft a combination of best glide (longest distance over ground) and best glide endurance (longest TIME in the air) hence the 3 knots difference.

Here is a good site:

http://142.26.194.131/aerodynamics1/Performance/Page10.html

 

[Mr. Cole]

Also notice that Aerosmith emphasized the differences between Props and Jets. The reason Vx occurs at the same point as (L/D)max, as was already mentioned, is because you're trying to find the velocity that maximizes T-D. For jets, thrust is nearly constant with velocity for most regions of interest. If T is constant, then you just have to find the velocity for which D is minimized, which is best glide speed, or (L/D) max. It's the same exercise that midlifeflyer mentioned for Vy, which the exception that Vy is a function of power required and power available due to thrust and drag, and Vx is a function of thrust and drag. For props, the thrust tends to decrease with increasing velocity, so you can't take the minimum drag point like you can with jets.

Dave

 

[enigma]

Great topic (as usual) Chris!. I had forgotten how much I have forgotten

enigma

 

[uwochris]

Once again, thanks for the replies.

However, I glanced at the link that Aerosmith provided, and the author mentions that it is Vy that is usually close to the Best Range speed for props.

This contradicts what was previously said... ie) that Vx should approximate Vg.

It seems the more I read on this, the more I get confused

 

[Aerosmith]

Dont mix PROPS & JETS.
Best JET endurance is Vmin drag (Vmd).
Best PROP endurance is Vmin power (Vmp)(approx. Vy)

Best range is Vmd for PROP and Vmd x 1.32 for JETS (approx Vy)

Remember that fuel flow on a JET is proportional to THRUST (instant output), whereas on a PROP it is proportional to POWER (work over time)

Thrust output on a PROP decreases rapidly with speed, but on a JET it is fairly constant