Best Glide and Approaches

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noahcarroll
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Best Glide and Approaches

Postby noahcarroll » Wed May 13, 2009 2:09 am

In the manual for the Cessna 172S it shows a normal landing speed range of 65-75 KIAS with the flaps up. I always use the low end (65 KIAS) in that situation because I prefer to keep my runway usage to a minimum. Why do they give a range of speeds? Why do they not recommend the best glide speed for a landing with the flaps up?
Noah Carroll
CFI CFII MEI EMB145 MD80

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champguy
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Re: Best Glide and Approaches

Postby champguy » Wed May 13, 2009 11:31 am

The "best" speed varies with weight, wind conditions, and density altitude. Since we use air speed rather than angle of attack there is no way to publish an exact number.
That is why good safe planes like Cessnas have such a wide envelope rather than being optimised for speed like some of the new fast glass wonders out there.
Now if you want a really wide envelope, talk to me about Aeroncas, and remember the good old wisdom, "time to spare, go by air"
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noahcarroll
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Re: Best Glide and Approaches

Postby noahcarroll » Wed May 13, 2009 11:49 am

I agree with you that the best glide is really an angle of attack and not an airspeed, and since we do not have an AoA indicator they provide a speed for unaccelerated 1G flight. The speed provided for the C172S is 68KIAS. I always understood that the number published for best glide was meant for max gross weight and that the best glide speed for lower weights is the square root of the percentage of max gross weight times the max best glide speed. So if you decrease your weight 10% to 90% max gross then you use 94.8% (square root of 0.9 is 0.948) of the original best glide speed which works out to be 64.5 KIAS.

So it seems that the range provided should have a cap at the best glide and have a lower end number, something like 58 - 68 KIAS.

Also, I was just looking at the POH for a DA40 and it provides 3 approach speeds for three different weights. Then I noticed it also has three best glide speeds for those same three weights. The approach speeds are 2 or 3 KIAS lower in all cases. Again the question, why is the approach speed lower (and not a range in the DA40) than best glide?
Noah Carroll
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Dave Higdon
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Re: Best Glide and Approaches

Postby Dave Higdon » Wed May 13, 2009 3:06 pm

Noah -- think this is right: landing speed is lower to reduce the touchdown speed with the final few hundred feet at a lower vertical speed than the best-glide...

touching down at best glide means you touch down faster, roll longer and need more braking on a short runway...in a low wing, in particular, you may experience considerable ground effect getting to within a wingspan at best glide...

In actual flight performance, best glide takes you farther on every foot of altitude lost; approach gives you a lower vertical speed component at the expense of a steeper descent angle --- a lower glide.

You also get a lower glide ratio and a higher sink rate at speeds above best glide -- but you'll get farther for every minute you stay aloft...

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Re: Best Glide and Approaches

Postby PropFan » Wed May 13, 2009 4:09 pm

Good question, and good answer. I had been wondering about this myself.

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Re: Best Glide and Approaches

Postby noahcarroll » Sun May 17, 2009 12:00 am

Glider pilots use what is called a polar curve to help decide the best speed to fly in a given situation. On these curves you learn something that is not taught to most powered airplane pilots. These curves help you find the speed called the minimum sink speed. The minimum sink speed is a speed that is lower than the best glide speed and has a lower sink rate than does the best glide speed (although the angle of descent is steeper). The best glide speed provides the most forward distance for altitude and the minimum sink speed provides the most time for the altitude. In other words, if we want to get farther we use best glide, and if we want more time and are not concerned with distance we use the minimum sink speed. (When gliders are thermaling they often use the minimum sink speed to get the best climb rate.)

When it comes time to land, approaching at a speed a bit lower than the best glide speed does a couple of things for us. If we slow down from best glide to a speed closer to minimum sink not only does our forward speed decrease, our rate of descent also decreases. The decreased rate of descent makes it easier to arrest our rate of descent and round out for the landing.

The other thing the lower speed provides is what I consider to be probably the more important of the two. As we slow down below best glide any decrease in speed causes an increase in drag (as angle of attack increases the induced drag increases). That increase in drag is important. As we raise the nose for landing and increase the angle of attack the drag is constantly increasing which helps to quickly lose speed and use as little runway as possible.

The problem with approaching at a speed above best glide is that as you round out and flare for a landing, the drag decreases momentarily as you slow from a speed a few knots above best glide down to the best glide speed. So at the time when you are trying to slow the airplane down to land you experience a decrease in drag and the airplane seems to refuse to slow down and you just float down the runway wasting precious runway on which to land.

It is best to approach at a speed below best glide so that when you round out, you increase the drag and the airplane quickly slows and you can land in a short distance without floating needlessly over precious runway.
Noah Carroll
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Re: Best Glide and Approaches

Postby jackhodgson » Mon May 18, 2009 8:07 am

Thanks Noah, that's pretty fascinating.

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joemurffy
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Re: Best Glide and Approaches

Postby joemurffy » Tue May 26, 2009 5:29 pm

As has been pointed out in this thread, the best glide speed is dependent on multiple factors, including weight. If two identical planes (and pilots), one at minimum allowable weight, and one at max gross weight (talking about the planes, not the pilots), were flying side-by-side in the same airmass, and they both lost power at the same time and were trimmed for their best glide speeds, which one would go farther?
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Re: Best Glide and Approaches

Postby PilotBillFromTexas » Tue May 26, 2009 7:39 pm

I'm going to guess the heavier one just because I know that some long distance sailplanes carry water as ballast for that reason.

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champguy
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Re: Best Glide and Approaches

Postby champguy » Tue May 26, 2009 10:09 pm

I want a second oppinion on this. I can imagine the heavier plane going faster, but not farther, unless the extra weight is an aft CG, and that is what is making the difference.
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