Checkerboard
Topic Author
Posts: 33
Joined: Fri Jul 27, 2001 10:07 pm

Best Gliding Speed

Mon Jul 28, 2008 3:18 pm

Hi folks,

From my understanding, in order to cover the longest distance on gliding, the best gliding speed has to be maintained. And the best gliding speed depends on the weight of the plane, which in turn will let the plane fly at the AoA with the best L/D ratio.

My questions are: 1. Is airspeed always be the reference for glider pilots for the best L/D ratio? 2. If the L/D ratio depends on the AoA, why airspeed is always the reference for pilots to achieve the longest gliding distance instead of using the AoA directly?

Thanks a lot!

Cheers.
 
jrheilig
Posts: 9
Joined: Sun Feb 12, 2006 11:38 am

RE: Best Gliding Speed

Mon Jul 28, 2008 5:48 pm

I think it's because a) many airplanes do not have any kind of AoA indication in the cockpit, and b) at a given weight and a given AoA, you can compute the speed that gives the best l/d ratio. Thus, it's easier to simply give a pilot an airspeed to fly.

JH
 
YWG
Posts: 1055
Joined: Wed Feb 07, 2001 11:29 am

RE: Best Gliding Speed

Mon Jul 28, 2008 11:15 pm



Quoting Checkerboard (Thread starter):
1. Is airspeed always be the reference for glider pilots for the best L/D ratio?

Yes, a glider pilot will fly that speed if he wants to remain up there as long as possible. But if you have the altitude and are within range of the field, fly to Vne.

Quoting Checkerboard (Thread starter):
2. If the L/D ratio depends on the AoA, why airspeed is always the reference for pilots to achieve the longest gliding distance instead of using the AoA directly?

In a glider, pitch controls airspeed thus changing the AoA.
Contact Winnipeg center now on 134.4, good day.
 
411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Best Gliding Speed

Wed Jul 30, 2008 3:28 am

Min drag speed on many types of swept wing jet transport aircraft approximates V2+110.
 
rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

RE: Best Gliding Speed

Wed Jul 30, 2008 7:19 am



Quoting Checkerboard (Thread starter):
From my understanding, in order to cover the longest distance on gliding, the best gliding speed has to be maintained. And the best gliding speed depends on the weight of the plane, which in turn will let the plane fly at the AoA with the best L/D ratio.

My questions are: 1. Is airspeed always be the reference for glider pilots for the best L/D ratio? 2. If the L/D ratio depends on the AoA, why airspeed is always the reference for pilots to achieve the longest gliding distance instead of using the AoA directly?

The L/D is not simply related to either AoA or airspeed, but the relationship to airspeed is better.

Unless otherwise specified, I'm assuming lift is constant...

The total drag is the sum of parasitic and induced drag. The latter decreases with AoA (which in turn decreases with an increase in airspeed) and with the square of the airspeed. So the best induced drag numbers will happen at the maximum possible airspeed. The parasitic drag increases with the square of the airspeed (and is basically unrelated to the AoA), and is "best" at zero airspeed. The best L/D occurs where those curves sum to the minimum value (and given how steep the curves invariably are in that region, it's where the curves cross).

The L/D curve changes as weight changes (either because you're turning and experiencing more than one G, or have loaded more mass into the glider), but the maximum L/D usually stays fairly close over the normal range of operating weights (at a given G loading), but happens at higher or lower airspeeds depending on the weight of the glider. For example, the best L/D of a light Centrair 101 Pegasus (365kg) is 40:1 at about 90km/h, but at maximum weight (505kg), it's still about 40:1, but at about 107km/h.

So in practical terms, one of the two drag components is strictly related to airspeed, and the other is indirectly related to airspeed, so if you wanted to reduce that to a single number, airspeed is the choice.

Which is not to say that an AoA indicator shouldn't be a standard feature in every cockpit, but it is, alas, not. That is a different discussion, however.

Quoting YWG (Reply 2):
Yes, a glider pilot will fly that speed if he wants to remain up there as long as possible. But if you have the altitude and are within range of the field, fly to Vne.

The best L/D speed gets you the best range. The minimum sink speed, which gets you the longest time in the air (assuming you don't find any lift) tends to happen at a fair bit slower airspeed, and with a significantly worse L/D.
 
Buzz
Posts: 694
Joined: Sun Nov 21, 1999 11:44 pm

RE: Best Gliding Speed

Wed Jul 30, 2008 12:18 pm

Hi Checkerboard, Buzz here. In the previous century when I was working my way through college I had to type up a paper. So I chose a similar topic which involved the best speed to minimize drag.
I was thinking of light aircraft like Cessnas and Pipers... what happens if you have an engine failure, or need to loiter while something is being cleared off the only runway available to you.

The short version is to fly the speed between the best rate of climb, and best angle of climb. Best angle is where airframe drag is the lowest. For me, it's a handy number that most light aircraft pilots already know. And when the pressure is on... the less calculating the better.

Yes, an AOA indexer would be a great thing. The US Navy flies all their carrier aircraft final approaches with the AOA indexer as the airspeed reference. Not many non - Navy aircraft have them... such is life in the real world.

g'day
 
bio15
Posts: 1048
Joined: Fri Mar 23, 2001 8:10 am

RE: Best Gliding Speed

Mon Aug 04, 2008 11:12 pm



Quoting Checkerboard (Thread starter):
why airspeed is always the reference for pilots to achieve the longest gliding distance instead of using the AoA directly?

Hi Checkerboard.


It's a more practical approach to quickly finding the best glide condition, as it has already been mentioned.

Flying the airplane to a specific AOA can be more tricky than getting to a specified airspeed, mainly because AOA is an instantaneous, and hence more sensible, indication of the airplane condition. Think of it as flying with either the VSI or the Altimeter, where one is the cause of the other. Higher AOA, more drag, airspeed goes down. Finding an airspeed can usually be quickly solved by getting to the required airspeed and trimming. On the other case you could find yourself "flying the AOA needle" with the yoke using an AOA indicator, and the final result is an airspeed very similar to Vbr. It's a more precise indication, but you have to be more precise too.

On light airplanes the difference of using AOA or airspeed is not that significant. Even more on a glider where there's no weight-change issue, so airspeed and AOA maintain a constant relationship for straight flight.

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