practical best range application?

On Mon, 31 Jan 2005, xerj wrote:

Wouldn't it simply be your airplane's best glide speed?

It would be the same AoA, but not necessarily the same speed. This speed
also varies with density altitude and weight.

hmm, AoA... Well just off the top of my head here, someone please correct
me if I'm wrong...

Stall speed, of course, also depends on AoA, not speed. So, if you have
an idea what stall speed does at different weights & density altitudes,
best glide speed should follow the same curve. Best glide is the point at
which overall drag is lowest, so it stands to reason it's also where max
range would be. If you can't avoid a headwind you may need to speed up a
bit to get the best ground speed per fuel burn, but that's a simple
calculation using ground speed & a close estimate of fuel burn.

So it seems, in practice, one should be able to get really close to max
range speed very quickly without any complicated calculations.

-Dan

Wouldn't it simply be your airplane's best glide speed?

It would be the same AoA, but not necessarily the same speed. This speed
also varies with density altitude and weight.

hmm, AoA... Well just off the top of my head here, someone please correct
me if I'm wrong...

Stall speed, of course, also depends on AoA, not speed. So, if you have
an idea what stall speed does at different weights & density altitudes,
best glide speed should follow the same curve. Best glide is the point at
which overall drag is lowest, so it stands to reason it's also where max
range would be. If you can't avoid a headwind you may need to speed up a
bit to get the best ground speed per fuel burn, but that's a simple
calculation using ground speed & a close estimate of fuel burn.

So it seems, in practice, one should be able to get really close to max
range speed very quickly without any complicated calculations.

-Dan


You are correct in your implicit suggestion that these airspeeds are based upon
angle-of-attack. Maximum range glide speed and maximum endurance speed are the
same since they both occur at (C_L/C_D)max AOA. However, your statement, " . .
.. so it stands to reason it's also where max range would be . . . " is
incorrect. Maximum range speed occurs at ((C_L)^1/2 / C_D)max AOA. Thus, it
is higher than (C_L/C_D)max airspeed. Maximum range and maximum endurance
airspeeds do not occur on the same point on the performance chart.



Kurt Todoroff


Markets, not mandates and mob rule.
Consent, not compulsion.

On Tue, 1 Feb 2005 at 19:44:36 in message
, Kurt R. Todoroff
-ON wrote:
You are correct in your implicit suggestion that these airspeeds are based upon
angle-of-attack. Maximum range glide speed and maximum endurance speed are the
same since they both occur at (C_L/C_D)max AOA. However, your statement, " . .
. so it stands to reason it's also where max range would be . . . " is
incorrect. Maximum range speed occurs at ((C_L)^1/2 / C_D)max AOA. Thus, it
is higher than (C_L/C_D)max airspeed. Maximum range and maximum endurance
airspeeds do not occur on the same point on the performance chart.

This interests me. Can you point me to the maths that produces this
result? I cannot see at the moment why maximum range should not occur
at maximum lift/drag: apart from some smaller effects like the effect of
engine thrust on lift etc. and any effects where engine and/or propellor
efficiency has a significant effect.

Maximum endurance for a glider occurs at minimum sinking speed which is
normally closer to the stall AoA than maximum Lift/Drag. Maximum range
for a glider occurs at maximum Lift/Drag as you say (I think that's what
you mean), but maximum endurance does not, as far as I can see.

I presume the calculation assumes a linear relation between AoA and CL?

Your last sentence in your paragraph above, with which I agree, seems to
contradict your second sentence.
--
David CL Francis