Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Best Gliding Speed  
User currently offlineCheckerboard From Hong Kong, joined Jul 2001, 33 posts, RR: 0
Posted (6 years 1 month 1 day 21 hours ago) and read 4642 times:

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.

6 replies: All unread, jump to last
 
User currently offlineJrheilig From United States of America, joined Feb 2006, 13 posts, RR: 0
Reply 1, posted (6 years 1 month 1 day 19 hours ago) and read 4621 times:

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


User currently offlineYWG From Canada, joined Feb 2001, 1146 posts, RR: 2
Reply 2, posted (6 years 1 month 1 day 13 hours ago) and read 4558 times:



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.
User currently offline411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 3, posted (6 years 1 month 9 hours ago) and read 4466 times:

Min drag speed on many types of swept wing jet transport aircraft approximates V2+110.

User currently offlineRwessel From United States of America, joined Jan 2007, 2346 posts, RR: 2
Reply 4, posted (6 years 1 month 5 hours ago) and read 4435 times:
Support Airliners.net - become a First Class Member!



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.


User currently offlineBuzz From United States of America, joined Nov 1999, 697 posts, RR: 21
Reply 5, posted (6 years 1 month ago) and read 4404 times:

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


User currently offlineBio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Reply 6, posted (6 years 3 weeks 3 days 13 hours ago) and read 4238 times:



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.


Top Of Page
Forum Index

Reply To This Topic Best Gliding Speed
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
Crosswind Landing-- Slip Vs Stall Speed posted Tue Jul 22 2008 07:54:06 by Dakota123
Jet Transport Cruise Speed History posted Sun Jul 13 2008 11:04:27 by OldAeroGuy
737-800 Landing Speed? posted Thu Jun 12 2008 18:15:15 by UltimateDelta
Typical Ldg Speed Of Embraer EMB 120 Brasilia posted Tue Jun 10 2008 18:17:34 by HeyWhaTheHay
DC-8 Landing Speed posted Tue Apr 29 2008 20:40:47 by Blackbird
Avro RJ / Bae 146 Speed Brake posted Thu Apr 3 2008 14:28:00 by Sandyb123
Are There Taxiing Speed Limits? posted Thu Feb 28 2008 21:14:17 by SAN787
SAN-LHR: Planes With Best Economics posted Sat Feb 9 2008 10:27:14 by DL767captain
What Is An Emergency Landing "At Speed"? posted Sun Feb 3 2008 13:18:26 by AirCatalonia
Global Express XRS Stall Speed (dirty)?! posted Thu Jan 17 2008 05:31:45 by INNflight

Sponsor Message:
Printer friendly format