ryan1234

I was always under the assumption from several text books/manuals that best glide speed (indicated) should change with wind factors. This does not seem to be the popular view of many, if not all, flight instructors/pilots around my parts. I was wondering what the general take is on this, or if this is alien to most:

- Maximum glide range in a no-wind condition will be done at (L/D)max - or the POH value for best glide speed.

- In a headwind, glide speed needs to be increased for best range.

- In a tailwind, glide speed needs to be decreased for best range.

Planespotta

Forget a published airspeed - L/Dmax is what you are aiming for, and because of this, Vglide is hardly ever its published value. I have always been taught (and will teach) to change speed when you have a head or tailwind. This has been confirmed by every DPE (4-5) I've asked about the subject, for whatever that's worth. You are trying to glide a certain distance over the ground, and hence your indicated airspeed should be adjusted to obtain a groundspeed that will produce favorable results.
Ever lesser known is to decrease Vglide by 5% for every 10% below max gross weight. Just a handy and surprisingly accurate rule of thumb. Make sure you know if Vglide has been calculated with or with out flaps, gear, etc., too.

CaptFuzz

You're right. Best speed for max glide range over the earth changes depending on the wind

As an example, consider a small plane with a published best glide of 50 knots. If you are flying into a 50 knot headwind, your ground speed would be nothing. If you fly faster than 50 knots, you will have a positive groundspeed, and therefore have a better glide distance.

Published glides speeds assume a particular weight and give you best distance through your air mass, not best distance over the ground. Similar things apply to best range speeds.

ryan1234

Spotta,

Interesting enough (L/D)max will only produce best glide in no-wind conditions. The change in the indicated airspeed changes the L/D. My understanding is that for best range the object is to lessen the time effect of an adverse wind condition (as it will relate to glide speed) - so increase the airspeed.

The airspeed/weight/wind for glide calculation is also a bit interesting and sorta ties into what you said.
- No wind - weight has no effect on range
- Tail wind - jettison weight for best range
- Head wind - retain weight for best range

oldveedubs

L/Dmax is not a function or weight, wind, or airspeed....it is a function of aerofoil. If the aerofoil isn't changing, drag polar stays the same.

That being said, wind will change your "speed to fly" based off how a tangent line intersects with the polar.

In very basic terms, you add 1/2 the headwind to your L/Dmax speed and subtract 1/4 tailwind...this is known as your speed to fly, talk to some glider pilots.

ryan1234

(L/D)max, won't be affected by wind. If all other factors (i.e. airspeed, etc) remain constant, any one factor (i.e. weight) increase/decrease will make the L/D shift.

The point being though that sometimes (L/D)max does not equate to the best glide speed for range if you're in a headwind or tailwind.... which is why glider pilot increase/decrease their airspeed like you said.

shdw

A couple random things first:

L/Dmax = A given AOA based on airfoil. (I suspect this is what oldveedubs was getting at)

L/Dmax changes with weight based on: sqrt(Weight_actual/Weight_maxGross) * BestGlide_maxGross

Simple formula: [(Weight_change/Weight_gross) * 100] / 2



This is certainly true. However, I have yet to read any mathematical proof as to the pilot "rules of thumb" being accurate. That is, increasing by 1/2 the headwind. I doubt it is linear, as the rule of thumb suggests.

I am more comfortable agreeing with the decrease in airspeed for a tailwind, but again only to a point. That point is clearly defined as min sink or minimum power required.

FWIW I use this: 1/2 headwind up to a 10 knots increase of bg and if you have a tailwind of greater than 10 knots use min sink.

In all honesty though, when in doubt fly best glide. You have enough to worry about when your engine shuts off. Thinking about all of this crap for an extra 1/4 mile will likely leave you flying by perfectly good landing sites for something only marginally better. This stuff is great to discuss and debate, but unless you practice it constantly (glider pilots) you're likely better flying best glide and expending brain power elsewhere.

rickair7777

Never mind.

nciflyer

Vg is the speed that will give you the best L/D aerodynamically in a given aircraft configuration and does not include environmental factors.

L/D is simply the aircrafts (coefficient of lift / coefficient of drag), and is only airfoil related. The best glide angle is not necessarily best L/D because now we are speaking in terms of distance over the ground vs altitude. The best glide speed & angle is all inclusive and factors in wind, lift/sink, L/D, & weight. The overall best glide speed is not necessarily Vg.

Actually, I should correct myself... L/D can be purely dealing with airfoils but in the terms we are speaking about it it is really calculated from flight test data... The flight test accounts for the aircrafts overall L/D which also includes lift and drag produced by the fuselage, etc. The test involves doing sawtooth climbs/descents and power off descents...

Some interesting reading...

Talks about glide angle regarding speed & lift/sink. It doesn't talk about wind, but to factor for wind all you do is move the origin of the line in the x direction.
Glider Performance Airspeeds

Detailed paper that debunks and confirms 1/2 wind speed rule of thumb. Talks about about best range speed but can apply to best glide as well. The author, David Rogers, also uses his E33A Bonanza to address some other interesting things.
(Enter the site and find the article "Wind Effects on Maximum Range")
Prof Rogers

Sawtooth climb flight test...
http://www.dfrc.nasa.gov/Education/O...f/sawtooth.pdf

NoyGonnaDoIt

Every time I see one of these discussions I try to picture a solo pilot, faced with a sudden powerplant failure, pulling out his E6B and calculator so that he can accurately figure out the precise indicated airspeed to use to maximize his glide...

... for the 30 seconds since a minute later, the airplane will be lower, maybe turning toward a landing site (although maybe not with the pilot's head in the cockpit figuring out best speed) and the calculation will need to be done all over again.

I know. I know. I like an academic discussion as much as the next guy, but still...