ryan1234

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...

ryan1234

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....

Cubdriver

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:



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.

Cubdriver

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.

2StgTurbine

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.

ryan1234

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

UAL T38 Phlyer

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.

gestrich19

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.

Cubdriver

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.

NoyGonnaDoIt

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?