Maximum glide ratio

chornedsnorkack · Post by **chornedsnorkack** » 24 May 2006, 12:09

It seems that the glide ratio L/D of airliners has not changed since 1950-s or even 1930-s. The numbers are something like 16...18.

Gliders are said to have achieved glide ratio of over 60 already in 1970-s. The current record L/D of a whole, manned airframe is held by a powered plane named ETA, and is 72.

Global Flyer is said to enjoy a glide ratio of 37. It has long, narrow wings: wingspan about 35 m, wing area 40 square metres.

Global Flyer is constrained in design by the huge fuel load (10 tons MTOW, 1600 kg ZFW... very limited structural mass available, and needs to stay efficient over a wide range of weights). Also, as a jet it has relatively high Mach numbers and also is constrained by the time the solo pilot can fly without sleeping.

U-2 is said to have glide ratio of 28. And it has wingspan of something like 31 m, wing area of 95 square metres... 1950-s design (back at the time when the L/D of gliders was in 40-s or less, not past 60), a jet plane, flying at high subsonic Mach numbers... and achieved L/D that large.

What is the main technical issue preventing more extensive use of planes having L/D in the 20...25 range?

teddybAIR · Post by **teddybAIR** » 24 May 2006, 12:46

That's an interesting question you raise there!

But for the less technically educated among us, could you please explain what L/D means and what the scale is? Not that I will be able to help you out, but just out of interest.

Kind regards,
bAIR

chornedsnorkack · Post by **chornedsnorkack** » 24 May 2006, 13:03

teddybAIR wrote:But for the less technically educated among us, could you please explain what L/D means and what the scale is? Not that I will be able to help you out, but just out of interest.

L/D or glide ratio is the ratio of lift to drag.

It has no unit, because it is a ratio of two forces - lift and drag.

Avro · Post by **Avro** » 26 May 2006, 00:29

jan_olieslagers wrote:

Of course all these parameters have their interaction / but glide ratio is the decrease in altitude compared to the distance covered. So no direct relation with lift or drag.

Well for an unpowered aircraft where you don't have any thrust the Lift to drag ratio (L/D) is equal to the glide ratio (Delta_H/Delta_d).

However I prefer to call it in any situation the lift to drag ratio in order to avoid any confusion.

What is the main technical issue preventing more extensive use of planes having L/D in the 20...25 range?

Very good question !

I wonder why a commercial plane should have a wing with a L/D ratio of let's say 60 ? Yes for a glider you need a lot of lift with minimum drag otherwise you go nowhere, but for an airliner you don't need that much lift. I'm not talking about approach and T/O which are different situations. I'm only looking at cruise conditions where all high lift devices are retracted. Through time the lift coefficient has increased on commercial wings which enabled to reduce the span of the wings (in proportion to size of plane). But I don't see a further evolution in that direction as you would suggest.

I don't know the answer, however personally I don't see the advantage of building a wing with L/D = 60 for a commercial jet. This would namely mean that you'll build a "much" smaller wing. The advantage you'll gain from it is less drag but I see a very big disadvantage and that's the lost space. You will lose a lot of space to store fuel and mechanisms.. This means that by developing such a wing you'll have some rather marginal benefits in the drag production but quite some penalties in fuel storage.

I might be wrong though

It's a very interesting subject and would be interested to hear other opinions

Chris

chornedsnorkack · Post by **chornedsnorkack** » 26 May 2006, 09:10

Boeing 747 used to be the biggest civil plane, with wingspan of 59,6 m. All other widebodies were much smaller - DC-10 and Tristar had wingspan of 47,3 m, A300 and A310 had around 44 m, Il-86 had 48 m, B-767 had 47,6 m, even MD-11 had just 51,7 m and 767-400 has 51,9 m.

But Airbus 330 has a wingspan 60,3 m. Much slimmer than the 747 wing: a 747 wing spans 59,6 m and has area of 511 square metres, while a 330 wing spans 60,3 m and has area of just about 360 square metres. That means mean chord about 6 m and aspect ratio of about 10, like U-2...

Was there any effect on L/D, comparing, say, A330 with A300 or A310?

The other widebodies seem to have followed suit - B777 wingspan is 60,9 m and Il-96 has 60 m wingspan as well. B787 is supposed to have 60,1 m.

How do airports manage with that?

Also... Tristar and DC-10 are out of production, so are MD-11 and Il-86, A300 and A310 are no longer offered... there is talk of replacing B767 with B787.

This would mean that no aircraft with wingspan between 40 m and 60 m would be produced... what happens to airports built for such kinds of planes? Too small for A330, B777 and B787, while the narrowbodies in production - B737 and A320 - have wingspan of around 35 m... much space wasted in airports built around MD-11 or B767...

earthman · Post by **earthman** » 26 May 2006, 12:12

chornedsnorkack wrote: This would mean that no aircraft with wingspan between 40 m and 60 m would be produced... what happens to airports built for such kinds of planes? Too small for A330, B777 and B787, while the narrowbodies in production - B737 and A320 - have wingspan of around 35 m... much space wasted in airports built around MD-11 or B767...

First of all, there is still a large number of these planes flying around. Otherwise, it's nothing a few sticks of dynamite, some concrete, steel and glass can't fix.

chornedsnorkack · Post by **chornedsnorkack** » 26 May 2006, 12:42

earthman wrote: First of all, there is still a large number of these planes flying around. Otherwise, it's nothing a few sticks of dynamite, some concrete, steel and glass can't fix.

Nobody bothered to fix the abundance of leftover military landplane runways after Second World War. Instead, people built civil landplanes to fill the runways - and they are still around.

Knight255 · Post by **Knight255** » 29 May 2006, 23:02

Okay, to my understanding of aerodynamics, L/D max is more or less a speed at which there is the least amount of parasite drag and the least amount of induced drag. Induced drag is the rearward component of lift and is most pronounced at lower airspeeds where your AOA is higher. Parasite drag is caused by protruding appendages (wings, gear, antennae, etc..) and is most damaging at higher airspeeds. Obviously, L/D max occurs at a speed where a compromise between the two types of drags are reached.

Regarding your question about why airliners can't reach a glide ratio of 20-25 to 1 is complicated. First of all, airliners aren't meant to glide, in fact, you'd be hard pressed to find a published L/D max speed or glide ratio for most airliners. Another thing to consider is that if you generate too much lift, you could over-flex the wings and they will break. The wings have to support the weight of the airplane PLUS be able to flex upward while generating lift. That is an incredible amount of stress being placed on the main wing spars and I honestly don't think they could handle that high of a lift ratio. (BTW, this second part is just a shot in the dark, I really haven't a clue why they can't reach a glide ratio of 20-1.)

chornedsnorkack · Post by **chornedsnorkack** » 30 May 2006, 08:55

Knight255 wrote:
Regarding your question about why airliners can't reach a glide ratio of 20-25 to 1 is complicated. First of all, airliners aren't meant to glide, in fact, you'd be hard pressed to find a published L/D max speed or glide ratio for most airliners.

However, it is the drag which the engines have to overcome. Obviously, an airplane has to create lift to match its weight... the less drag, the less cruise thrust needed.

Knight255 · Post by **Knight255** » 30 May 2006, 16:41

However, it is the drag which the engines have to overcome. Obviously, an airplane has to create lift to match its weight... the less drag, the less cruise thrust needed.

Very true. Any aeronautical engineers out there that can help us out????

earthman · Post by **earthman** » 30 May 2006, 18:57

I suppose that whatever makes a plane a good glider, will make it create a lot of drag at higher speeds.

The U2 achieves high speeds but keep in mind that it flies at ridiculous altitudes, where the air density is low and thus there is less drag.

Lyulka · Post by **Lyulka** » 30 May 2006, 20:40

So in fact the question is why it is that airliners don't fly higher.

earthman · Post by **earthman** » 31 May 2006, 01:09

Because at higher altitude in turn the engines produce less thrust I guess.

The U2 had to fly high to avoid SAMs.

The Concorde had to fly high to avoid overheating as a result of drag at high speeds.