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Best Glide Ratio. Best Glide Speed.

Mon May 06, 2002 11:40 pm

I have read somewhere in a topic that a heavy jet aircraft can glide further (in calm air) then the same jet at light weight. I believe that this is not true. I think that they they will be able to glide over the same distance (glide ratio will not change). The best glide speed for the heavy aircraft will be higher though then the best glide speed for the light aircraft. (best glide speed will increase when the weight increases)

What do you think?
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RE: Best Glide Ratio. Best Glide Speed.

Tue May 07, 2002 1:02 am

You are correct in that what you heard is completely incorrect.

Two of the same aircraft (whether they are both 757s, 767s or PA28s), both losing all engines at the same time in the same place will all hit the ground in exactly the same place (providing they do not change course). In other words their glide distance / ratio is identical.

The heavier aircraft will simply get there sooner. (Due higher best glide speed @ higher weights).

Some people just don't believe this for some reason, but I can assure you it's true.
I used to love the smell of Jet-A in the morning...
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RE: Best Glide Ratio. Best Glide Speed.

Tue May 07, 2002 1:53 am

That is if their glide ratio is the same, there are some major diffrences between the Jet and a prop plane.

A Jet has a tremedous amout of drag when the engines are out, the engines are still spinning and windmilling, creating drag. Compared to a prop plane where the prop can be adjusted to a postion of little to almost no drag.

But if their engine out glider ratios are the same the heavier aircraft will get there sooner. Look at racing gliders, they want heavier for speed so they can get their faster, but for distance where time doesn't matter you want a lighter glider so you can thermal better.

In most cases the L/D is almost the exact same, its just the weight that they choose either gets them there faster or slower.

One man in fact sells rides in his glider for is practice (and I heard real) record breaking flights. Thats how he is able to continuely pay for them.
At worst, you screw up and die.

RE: Best Glide Ratio. Best Glide Speed.

Tue May 07, 2002 11:52 am

Excuse me while I put on my CFIG hat...

You're right, the best L/D remains essencially constant over the weight range of the aircraft. (It is actually an "angle of attack".) The the speed required to achieve the best L/D angle of attack is a function of aircraft weight, the heaver the weight the higher speed required. (This is one of the main reasons many competition gliders carry water ballast - so that they can fly faster and have better penetration. It also gives the pilot the option of dumping the ballast when conditions weaken.)

As far as jets have a lot of drag in an engine out situation. That's wrong. They actually have very little drag and some of the jets actually have pretty good glide ratios. I've goofed around in a Citation and actually got it to climb in ridge lift with both engines pulled back to flight idle.

In the case of a propellor-driven airplane, a wind-milling propellor has essentially the same drag as a flat plate of the same diameter. That's why it's importent to get the prop stopped after an engine failure - either by feathering it if you're in a twin, or pulling the nose up to get it to stop on a compression stroke it you're in a single. If you've got a constant-speed propellor and you can't get it to stop, you might also try pulling the propellor control back as far as it will go. This will put the prop in "coarse pitch" and will make a significant difference in the amount of drag, but it's not as good as feathering it.

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RE: Best Glide Ratio. Best Glide Speed.

Tue May 07, 2002 12:52 pm

The initial poster has been told something that is based on a misunderstanding. What is true is that for the constant speed descents most airlines use you will need more distance to descend when you are heavy as opposed to light. IOW, if you are heavy you might need 10 more miles to get down than if you're light.

The reason is actually fairly basic, and you should be able to sort it out with a bit of thought. I'll leave it to you for now.
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RE: Best Glide Ratio. Best Glide Speed.

Tue May 07, 2002 1:08 pm

A citation is vastly diffrent from a 767, look at the fan size. The amount of frontal plate area on the large underwing jets is huge. That is an enormous amount of drag. And you can't feather them.

As far as climbing a jet in ridges, it can be done. Personally on a particularly good thermal day (I did the work up, it predicted about 10kts) I was able to catch some thermal lift with a nole, but centering it was almost as bad as when I had first started soaring.


BTW where did you get your rating? East coast or West Coast?

Right now I am personally planning a trip next year to get over by Mindin to learn about wave and ridge soaring, since I have only done thermal soaring.

Do you hold any badges (beyond the ABC's)?
At worst, you screw up and die.

RE: Best Glide Ratio. Best Glide Speed.

Tue May 07, 2002 11:52 pm

Perhaps I should have been more specific, but my point still stands - generally speaking, turbojet aircraft have comparatively little drag from windmilling engines. Certainly, the new generation high-bypass engines will have more drag than older engines, but everything is realitive. You only find the really big engines on really big airplanes... Having never flown a 757, 767, or 777, I can only guess what the power off glide ratio would be. I'm sure that there are some pilots here who would have that information, but I'm sure that it would be somewhere in the hight teens or 20's or maybe even higher - pretty amazing when you consider that some very successful glider designs are in the low 20's.

You hit the nail on the head when you made the comment about trying to center in the lift, that's why I could only get the Citation to climb along the ridge - no turning required.

I got my commercial add-on in Boulder City, NV in a 2-33 about 25 years ago and my CFI in a Blanick about 20 years ago. It was a lot of fun. Over the years I have owned a 2-33 and a 1-23. I did most of my instructing in 2-33s and Blanicks. I also served my time as a tow pilot, using a Bellanca Scout, a Super Cub, and a 150 HP Cessna 150.

As far as badges go, I've never really got into getting them. In our operation it was always such a pain in the rear to make all of the necessary arrangements "just in case" and it just never happened. I did manage several 5+ hour flights along a 50+ mile ridge. I've also had several flights to 17,000' (my 1-23 had an oxygen bottle) but ATC was never too keen about opening the window for us on short notice so I just kept below 18,000. (By the way, for years, a 1-23 held the altitude record of 47,000 some odd feet.) It wouldn't have been too difficult to just keep going as several of the guys in our soaring group did. One bit of advise, when you go to wave camp, get a good pair of electric socks and dress warm. It gets pretty frosty up there and you can't move around very much to keep the blood circulating.

I am currently gliderless, but one of these days I'm going to figure out a way to get some sort of motorglider. There are a couple of new ultralight designs, that if sucessful, could really be fun to own.

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RE: Best Glide Ratio. Best Glide Speed.

Wed May 08, 2002 2:04 am

I just remember about what the Captain said about the Gimili glider, he said it creates alot of drag. His descent rate was something like 2000 FPM.

Eck I would hate to be coming down that fast.

Thanks for the advice, I was told a similar thing. He said, "You will be sweating on the ground and shivering at altitude."

Yeah, myself I'm considering buying an old L-23 or something like that. I mostly just take the cheap rental up, but it is a challange to soar in those things.

Probally looking to rent a Grob 102, when I go for the wave. So I am thinking about getting some time here in Florida before I head out there.
At worst, you screw up and die.
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Joined: Thu Feb 07, 2002 9:51 pm

RE: Best Glide Ratio. Best Glide Speed.

Wed May 08, 2002 5:41 am

you can be coming down at 2000 fpm even with a glide ratio of 40. You just have to fly fast enough...  Smile

I thought I was doing good trying to avoid those airport hotels... and look at me now.
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RE: Best Glide Ratio. Best Glide Speed.

Fri Jul 07, 2006 9:25 pm

Hi All.

Can anyone suggest why, in the 737 checklist for "Loss of Thrust on Both Engines", it suggests that after failing to start either, a speed of 220 knots is used, regardless of weight? On other transports, are weight related glide speeds provided?

Is the Best Endurance glide speed different from the Best Distance glide speed?

It would be a simple task to find best L/D speed from the FMC Hold or One Engine Out data pages (if the FMC's still powered), so wouldn't this be more relevant than 220?

Is 220 knots simply a compromise because finding the optimum speed is too much work in a busy situation? Surely a simple table in the QRH could provide the correct speed at a glance.

Regards - Musang
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Joined: Thu Apr 27, 2006 11:12 pm

RE: Best Glide Ratio. Best Glide Speed.

Sat Jul 08, 2006 7:39 am

Quoting Musang (Reply 9):
Is the Best Endurance glide speed different from the Best Distance glide speed?

No help on the big jets questions, but for light aircraft (and I'm betting the big ones too) there are two different speeds for distance and endurance.

Best distance is a balance between the rate of descent and the forward airspeed, and can be found at the best lift/drag ratio. Best duration is a slower speed where you descend at a lower rate - but going slower doesn't get you as far...

In a previous discussion, it was mentioned that big jets are primarily interested in range so as to reach a suitable airport. In light planes, a reasonably lengthy field is good enough (given that you can't make an airport), so time may be more important - giving you a chance to communicate your location, and giving rescue more time to respond, etc.

In a Piper Archer (and likely other planes too), you'll find that best glide matches best rate of climb, and best duration matches best angle...

"Simplicate and add lightness." - Ed Heinemann

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