A aviation & planes forum. AviationBanter

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » AviationBanter forum » rec.aviation newsgroups » Piloting
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

Glide distance vs. weight



 
 
Thread Tools Display Modes
  #1  
Old February 27th 05, 10:36 PM
Larry Dighera
external usenet poster
 
Posts: n/a
Default

On Sun, 27 Feb 2005 21:14:25 GMT, "Julian Scarfe"
wrote in ::

"Larry Dighera" wrote in message
news
While your statement above is generally accurate, it's not absolutely
true (as was pointed out to me by a glider pilot in e-mail). Here's
some empirical evidence of L/D changing with a change in weight (note
the right hand polar graph under 'Technical data'):
http://www.dianasailplanes.com/szd55.html


The data there indicates an L/D of 51 at higher weights, 49 at lower (about
50%). That seems consistent with the idea that at higher Reynolds numbers
(in effect, higher speeds) the skin friction drag coefficient reduces a
little.


Reynolds number: http://aerodyn.org/Frames/1flight.html

Given the "clean" design of the glider, the increase in parasitic
drag at higher speeds is probably insignificant compared to the "skin
friction drag" reduction.

  #2  
Old February 27th 05, 11:03 PM
Julian Scarfe
external usenet poster
 
Posts: n/a
Default

"Larry Dighera" wrote in message
...

Given the "clean" design of the glider, the increase in parasitic
drag at higher speeds is probably insignificant compared to the "skin
friction drag" reduction.


I think so. Words for drag vary, and I've always used parasite drag to
include skin friction, but I think we mean the same thing.

For a laminar boundary layer, skin friction is proportional to Re^-0.5, and
for a turbulent boundary layer to Re^-0.2. If skin friction drag is about
2/3 of the total parasite drag (by which I mean skin friction + form drag),
which in turn is 1/2 the total drag at best glide, that would suggest that
the L/D should improve by between 1/15 and 1/6 of the increase in speed.
The data you quoted, with a 50% speed difference for a 2-3% difference in
L/D suggest something like the 1/15 expected of a turbulent boundary layer.

Julian


  #3  
Old February 27th 05, 11:04 PM
Stefan
external usenet poster
 
Posts: n/a
Default

Larry Dighera wrote:

Given the "clean" design of the glider, the increase in parasitic
drag at higher speeds is probably insignificant compared to the "skin
friction drag" reduction.


But given that the original poster was most probably talking of
airplanes with noisemakers, I suspect that for him, best glide gets
dramatically worse at higher speeds. As I always say: Airplanes don't
*need* airbrakes because the whole plane *is* just one huge airbrake.

Stefan
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
more radial fans like fw190? jt Military Aviation 51 August 28th 04 04:22 AM
Landing and T/O distances (Was Cold War ALternate Basing) Guy Alcala Military Aviation 3 August 13th 04 12:18 PM
Buying an L-2 Robert M. Gary Piloting 13 May 25th 04 04:03 AM
"I Want To FLY!"-(Youth) My store to raise funds for flying lessons Curtl33 General Aviation 7 January 9th 04 11:35 PM
Angle of climb at Vx and glide angle when "overweight": five questions Koopas Ly Piloting 16 November 29th 03 10:01 PM


All times are GMT +1. The time now is 08:24 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
Copyright ©2004-2025 AviationBanter.
The comments are property of their posters.