Best Glide Speed
 

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.

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.

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.

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

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.

(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.

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.

Never mind.

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

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...

Everyone here is pretty much in agreement with the notion to increase airspeed in a head wind for best range -

For anyone wanting an in-depth description of "descent performance", There are two good sources on it:

USN TPS Fixed Wing Performance, Flight Test Manual (Chapter 8 'Descent Performance') - which you can download online

Aerodynamics for Naval Aviators


shdw,

I can understand how you feel about rules of thumb - this older, airline captain whose a great pilot (and seaplane pilot) had an interesting rule of thumb about fuel consumption - he said that if you divide the max HP of recip engines by 20, you can usually get a ballpark figure of fuel consumption in GPH at normal cruise. Seems to be pretty close from a 172 to a T-6G Texan. It left me scratching my head...

I know what you mean... however... as luck would have it I found myself about 4 months ago with a pretty serious fuel valve problem (which resulted in less than 4 gal in the functioning tank) at around 12,500ft in a 172 out over ocean with the nearest airport (and land) being Punta Cana (90nm)... there was a tailwind to Punta Cana, but a headwind to Aguadilla... but... luckily I leaned it out enough and started a descent rate to make it 'just in case', the engine coughed while turning base for the runway in Punta Cana and shut off on the apron...

Now not everyone will find themselves in that type of position... but I haven't been flying for too, too long and it happened to me - while this conversation is less than practical - there's always the possibility :) - it's nice to know what can help you/hurt you.

You never know what kind of situation you'll find yourself in....

Not sure what you mean exactly, unless you are saying that L/D max is independent of environmental factors due to being based solely on angle of attack, and that Vg does vary with environmental factors such as density, wind, and weight.

No! The best glide angle for range purposes is based on L/D max, period. It does not vary with anything else. It is a known value for a particular airplane design, and it does not have anything to do with weight, density, or wind conditions at the time.

Ok, except pull the " & angle" out of this sentence lest it be incorrect. The angle is exclusive to any factor except L/D and in the case of maximum range, L/D max. Best glide speed Vg varies (only), and here's the equation:

http://i284.photobucket.com/albums/l...glidespeed.jpg

Yes it is. Perhaps you mean the "published" Vg is not always correct, and the true Vg may differ due to circumstances- and I agree with that. The airplane will not go as far through the air without being at L/D max. Ground location is another issue altogether.

Yes, drag polars are made from flight test data usually because when the aircraft exists it is easy to obtain them. Preliminary polars are made using hand calcs and CFD. Actual drag polars are very hard to get hold of because they tell you so much about an airplane that manufacturers won't release them if they can avoid it.

As far as the wind speed issue goes, it is the sum of the speeds that counts in the above equation. So if the airplane is going 150 ft/s in still air and a 20 ft/s headwind bumps this figure up to 170 ft/s, then the airplane needs to slow down by that amount to remain at L/D max.

I am not in agreement. I consulted a couple of standard engineering texts on this and unless I am missing something, then the speed through the air the airplane "sees" is what counts. Ground speed has nothing to do with it. If the airplane does happen to "see" a higher speed due to headwinds, then it needs to adjust to bring it back to the calculated Vg speed based on the equation shown in my earlier post. Perhaps the confusion here is that if this speeds drops or increases by the action of local wind, then the airplane may be climbed or descended a little to account for the change in total airspeed. This would be done to bring the airspeed back to the correct Vg for best range. Otherwise the airplane is not at L/D max and is not going as far due to the excess drag.

Garmin G1000 and some other panels do it for you. I get the student to check the airspeed tape for the location of the Vg mark and have them fly that.

I think this is a valid discussion that has practical use. By studying and understanding the math, you learn what variables to pay attention to and how much those variables will affect your results.

You are right, in the real world you will be ball parking it, but if you did a lot of practice calculations on the ground beforehand, your ball park will be a lot smaller.

Cub,

The two references I gave say it verbatim ...the idea is that by lessening the time effects of the adverse wind, you'll have a net gain in distance.

Captfuzz had a good analogy

Capt Fuzz is Right
 

Cub:

Fuzz uses the same analogy I use: if your best L/D = the headwind, you aren't going anywhere...and generally, you want best glide to actually make some useful distance (such as not ditching when you have 4 gallons of fuel remaining...)

The important point: while L/D max is a certain Angle of Attack, and while one must generally increase the speed into a headwind, it will never be as good as could have been at no-wind L/D max.

But it is better than having a groundspeed of zero.

This works for optimum cruise speed in low-drag airplanes, as well (although I have not found this to be true in airliners). In the T-38, I came up with an empirical formula nearly a decade ago to increase cruise speed in a headwind; slow down with a tailwind. We got a "divert profile" software mode a few years back, and it validates my formula within about 3-5 knots.

There is a point of diminishing returns, of course. In the T-38, that limit is about 350 kts: the drag rise starts to negate any gains above this speed.

In a light airplane, power-off, trying to glide 150 kts probably wouldn't work either. Point is, the change in airspeed is a function and fraction of the wind affecting your progress.

You are correct here. The performance is relative to the air mass the wing is sensing, not any fixed point like the ground. To get the best performance to something relatively motionless to the air mass, L/D max would work best. However, since the air and ground are diverting with increasing time, time in the air becomes a factor as well. By spending less time in a headwind, the distance the moving air mass traveled away from the ground is reduced.

While out-speeding a headwind definitely moves an airplane over more ground in a given time, and if the time is limited then it would get the vehicle farther before it hits the deck, it is definitely at the expense of time aloft and horizontal air distance over using the L/D max speed. So, we don't teach that in engine-out situations. Perhaps the reason gliders use it is they have no engine to worry about and therefore less of a need to stay airborne and to get the most air distance possible. In that case they are not using a "best glide speed" in the conventional sense of the term, they are using a "best ground distance speed", and there's a difference obviously.

[edit] I got my glider handbook out. It says the term for this kind of glide speed is "the speed to fly" (as Oldveedubs pointed out also), and it is not technically a best glide speed.

I'm not sure how much you can limit the ball park. Yes, at a given IAS, you will have a lower ground speed (which means you will cover less ground/minute) with a headwind than with a tailwind.

So take a typical piston single with a glide ratio of about 1.7 miles/1000', aside from the realization (as in ryan's example) that the landing site you can get blown to with a tailwind is "closer" than the one you get to by plowing through a headwind, how much of a practical advantage is an adjustment of your IAS really going to get you?

He said, "distance over the ground" while you are discussing movement through the airmass. He has it right. Though I would swap some of his words for clarification: best glide angle -> furthest distance achieved & best L/D -> L/D max.

Again, angle relative to the ground, he is right. Relative to the air, I would swap terms as mentioned above just for clarification.

Again, in terms of ground location as his post suggested before, he is right.


Unless you have a variometer equipped, speed to fly is irrelevant. A glider pilots speed to fly accounts for lift, sink, headwind, and tailwind. Interestingly enough, lift/tailwind result in flying closer to, or at, min sink, a slower speed. Conversely, a sink/headwind results in increasing your speed to fly above Vbg. The reason was already given by ryan; reducing or increasing the effects of the moving airmass, both up/down and forward/backward.


Ryan: I wouldn't have any issues with the engine one as engines build, to my very limited knowledge, is fairly linear. Increase piston size x% increases fuel flow y% and power z%. Drag curves are not linear, leading me to believe, as UAC seemed to confirm, there is a limit to the rule of thumbs usefulness.

I do introduce it, though I only gloss over it. I much rather focus on judgement required to pick a landing site and skills to safely fly at or near best glide until the flare. If pilots did that, I suspect 90 percent of the engine failures encountered would result in a safe landing.

In response to your real world experience, you had a particularly unusual situation. For the most part, we are talking about what speed to fly when total power is lost. Your situation would have been best if flown at min sink with the engine as lean as possible, I assume you did this.

I don't mean to beat the dead horse, but I am really against this sort of thinking for the airplane pilot. Here's why:

1. You don't know the magnitude or direction of the headwinds or tailwinds in most cases unless you have a wind vector on board. You are guessing about all this at a time when there are far better things to be doing given your engine just quit. And if you do happen to have a wind vector on the cockpit panel, now you have to watch the thing as you turn the airplane in the wind which is a heads-down activity you could really do without.

2. As discussed, not flying the Vg speed robs you of precious time that you could be using to address the dead-stick situation.

3. Why do you need more distance over the ground anyway? The chances of not being near a suitable landing site concurrent with being in headwind or tailwind conditions where you happen to know what those winds are and where they are coming from is unlikely. Furthermore, these winds may change as you descend, and in fact are likely to do so. Why not just use Vg, the speed at which you have the most time and as far as you know, distance to land?

4. This speed we are talking about is not technically a best glide speed. It is a ground-based best distance speed under known wind conditions, a "speed to fly" as the glider crowd puts it. I do not ding them for using something that works for their purposes, but I point out they are not very commonly our purposes as airplane pilots. The whole discussion is perhaps an interesting engineering topic, but it is confusing a more valuable issue- that of using the best glide speed as defined universally by aircraft engineering texts and pilot training manuals for use in emergencies.

I rest my case, Your Honor.

*bangs gavel*

I'll give you my thought process during that situation (after thinking about what was wrong):

1) The Atlantic is a pretty big place
2) Ditching is going to suck:
a) it's starting to get dark (my current course is making it worse) -it's going to double suck being in the ocean at night
b) high-wing airplane
c) how am I'm going to get this 40lb life raft out of the airplane and then inflate it - it'll probably sink

3) Wow the Atlantic is big... I can see how people get lost out here and never found (note: avoid ditching)
4) Thank God for true airspeed (vs. indicated)
5) No one is ever going to believe me that the fuel valve stuck to one side!

Did you declare? If not, did you at least use pan, pan, pan? If you don't know what that is/does/provides (anyone else reading this, not directed at ryan) you should research it!

Sounds like your thought process was:

oh ****...
no one will believe...
oh ****...

My only question is, why didn't you think of the sharks man! :D

Actually every center was a little out of range (on the longest leg over water - about 400nm) - A Jet Blue flight out of PR relayed my request to contact San Juan Center. After a few freq changes and finally being able to receive them (and visa versa)... I told them I had a fuel valve problem, only feeding one tank now - Turks are too far to turn back, there looked like a significant weather build-up over PR, so diverting to Punta Cana (my alternate) was really an easy decision...

I didn't want to blow things out of proportion, and I'd been flying since about 2am that day (it was about 5pm then), but I thought about it some more... I was diverting from my eAPIS (so I asked him to inform US Customs - which never happened), the Atlantic is a big place... giving them my present lat/long and fuel left/persons on board/course would not be a bad idea if worse came to worse.

How accurate are the 172 fuel gauges?

The controller wished me luck, chatted a little bit, and then switched over to Punta Cana ...So I actually declared with them. That airport is pretty busy, when I passed over the coast my fuel gauge was sitting on about 1 gal - turned base for 9 and it started coughing, shut off on the apron.

Didn't speak much Spanish, so trying to get AVGAS was a bit of a challenge... finally did - had to go to the tower to file an official IFR paper flight plan (which was in Spanish:D), because it was night time (night time in the Islands means IFR). The plane needed to be in PR that night, so I figured with one good tank the jump from shouldn't be too hard. In hind sight I should have probably waited until someone checked out the airplane, but that's the kind of decisions no sleep will get you...plus the logistics of getting an A&P at that airport, well I wasn't too sure about anyways. Of course they didn't have any low-enroute charts for Dom Rep... so I had to use a high-alt to mark my way points. It's only a little over an hour.... so no big deal. Landed at Aguadilla (Borinquen), US Customs AIAs surrounded my plane... told them about what happened, they were pretty cool about it all (after a brief search).

The A&P in PR said that the "ball" in the fuel valve disconnected and got stuch in the right tank, so only the left would feed.