Which v-speeds go down with weight?

kath

kath

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Katherine
Hi, all,

Reading Kershner and studying my aerodynamics. Again.
The lift force goes as the square of the speed, and also goes linearly with coefficient of lift which is related to AOA . So if you need the lift force to be a certain number of pounds = some factor times the aircraft weight, and the AOA to do this is fixed, then the necessary speed will go like the sqrt(weight). Slower when you're lighter. So this applies to things related to stall speed like Vs0 and Vs1 and Va. OK, that makes sense.

What about best glide? Quoth Kershner: the glide ratio of your airplane is independent of weight, but the speed that you use to fly it decreases with decreasing weight. And there are rules of thumb like "1.3 times the flaps-up stall speed" which suggest the same thing. So, I don't fully understand why this is, but it sounds plausible.

What about Vx and Vy? These have to do with excess thrust, or excess power, or some such that is making my brain explode. Do they change with weight, and if so, is it also a sqrt(weight) thing?

What about approach speeds in the pattern? I'd guess that these would be linked to stall speed as well, so sqrt(weight) again. What do y'all do, do you adjust?

Other than Va, these are listed in the POH only for max gross, which is why I'm asking.

Edit: Subject of thread should read "which v-speeds go down with *decreased* weight"
 
For single engine: Va, Vx, Vy, Vref, Vs, Vso, Vbg, Vbe
 
Almost all of them go up or down with and increase or decrease in weight. Some that don't:

Never exceed, flap, gear...
 
kath said:
Hi, all,

Reading Kershner and studying my aerodynamics. Again.
The lift force goes as the square of the speed, and also goes linearly with coefficient of lift which is related to AOA . So if you need the lift force to be a certain number of pounds = some factor times the aircraft weight, and the AOA to do this is fixed, then the necessary speed will go like the sqrt(weight). Slower when you're lighter. So this applies to things related to stall speed like Vs0 and Vs1 and Va. OK, that makes sense.

What about best glide? Quoth Kershner: the glide ratio of your airplane is independent of weight, but the speed that you use to fly it decreases with decreasing weight. And there are rules of thumb like "1.3 times the flaps-up stall speed" which suggest the same thing. So, I don't fully understand why this is, but it sounds plausible.

What about Vx and Vy? These have to do with excess thrust, or excess power, or some such that is making my brain explode. Do they change with weight, and if so, is it also a sqrt(weight) thing?

What about approach speeds in the pattern? I'd guess that these would be linked to stall speed as well, so sqrt(weight) again. What do y'all do, do you adjust?

Other than Va, these are listed in the POH only for max gross, which is why I'm asking.

Edit: Subject of thread should read "which v-speeds go down with *decreased* weight"
Click to expand...

Most speeds decline with decreasing weight. Some POHs will give you numbers for different weights. In addition to weight, CG will also affect the performance. You will have an infinite number of combinations. Unless you are counting on getting the maximum published performance (which is unwise, in my opinion), this is not worth sweating over in a small aircraft. Perhaps the only exception is Va.
 
sarangan said:
Most speeds decline with decreasing weight. Some POHs will give you numbers for different weights. In addition to weight, CG will also affect the performance. You will have an infinite number of combinations. Unless you are counting on getting the maximum published performance (which is unwise, in my opinion), this is not worth sweating over in a small aircraft. Perhaps the only exception is Va.
Click to expand...

Well it can make a huge difference in certain airplanes for landings. Mooneys and Comanches can eat a lot of runway when you come in ~10% above Vref (can happen if you are REALLY light). I always adjust for that because I can run out of runway in a hurry if I don't.
 
EdFred said:
Well it can make a huge difference in certain airplanes for landings. Mooneys and Comanches can eat a lot of runway when you come in ~10% above Vref (can happen if you are REALLY light). I always adjust for that because I can run out of runway in a hurry if I don't.
Click to expand...

Yep! Very correct! Every extra knot is 100 more feet of runway I float past.

Published stall speeds are at gross, that's all that's in my Owners Manual. Since final approach speed is 1.3 X stall speed, and stall speed is lower at lower weights, this must be corrected. So I reduce my target airspeed by 5 mph for every 300 lb the plane is below gross for that landing. 90 mph downwind and base, 85 mph on final, slowing to (75 - weight correction) on short final--it's even easy to do in my head.

Note that this correction factor will vary by aircraft model, and may be ignored for very draggy types.
 
Hank S said:
Yep! Very correct! Every extra knot is 100 more feet of runway I float past.

Published stall speeds are at gross, that's all that's in my Owners Manual. Since final approach speed is 1.3 X stall speed, and stall speed is lower at lower weights, this must be corrected. So I reduce my target airspeed by 5 mph for every 300 lb the plane is below gross for that landing. 90 mph downwind and base, 85 mph on final, slowing to (75 - weight correction) on short final--it's even easy to do in my head.

Note that this correction factor will vary by aircraft model, and may be ignored for very draggy types.
Click to expand...

Gross is 3000 in mine. Me and minimum fuel in the PA24 I can be at or under 2000. So Imagine flying that at book numbers. 20% fast? Oh thats no problem.:rolleyes::rolleyes::eek::eek:
 
Clip4 said:
Or it could be your airspeed indicator is indicating too low.
Click to expand...

Not according to my biennial pitot static checks . . .

One speed works well at a heavier weight, but is too fast at a much lower weight. That ain't instrument error.
 
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