View Full Version : Maximum glide ratio

22nd May 2006, 14:29
It seems that the glide ratio L/D of airliners has not changed since 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?

22nd May 2006, 14:41
Perhaps the fact that L/D ratio is only one of innumerible performance factors which need to be incorporated in a design :rolleyes:

John Farley
22nd May 2006, 16:03
You say that "It seems that the glide ratio L/D of airliners has not changed since 1930-s."

I would be interested to know the evidence for that assertion.

22nd May 2006, 18:16
High wing aspect ratio is a key element of high L/D design (all your cited examples have it to a degree unknown in commercial aircraft).

The impact on civil airfield infrastructure and capacity is one significant barrier. Although folding wings are a potential means to alleviate that point, the next try will have to overcome bad memories of the reaction to and eventual complete demise of Boeing's 777 effort.

23rd May 2006, 11:43
High wing aspect ratio is a key element of high L/D design (all your cited examples have it to a degree unknown in commercial aircraft).
Not quite all.
U-2 has a wingspan of 31 m and wing area of 93 square metres. This gives mean chord of about 3 m and aspect ratio of about 10. Which is not unknown for airliners.

The impact on civil airfield infrastructure and capacity is one significant barrier.
Perhaps. Though something interesting seems to have happened there.

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...

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...

24th May 2006, 11:38
A little more on Wizofoz's post (and apologies in advance for gross plagiarism from Kermode's Mechanics of Flight and other excellent P of F texts):

If you want long range, you'll have a good cruise CL/CD ratio.
If you want long endurance, you'll have a good maximum CL/CD ratio.
If you want to get there fast, you'll have a low minimum CD.
If you want to take-off and land slowly (read short runways), you'll have a high maximum CL.

If you want to fly fast, you'll use low-bypass turbojets with highly swept wings.
If you want to fly efficiently over a wide range of speeds, you'll use high-bypass turbofans with mildly swept wings.
If you want to fly at very high speed at 80,000'+ over Communist SAM operators, you'll have maybe 70° wing sweep, huge zero bypass turbojets, and a fleet of air-to-air tankers. (Couldn't resist that one.)
If you want to take-off and land slowly, you'll have really big heavy high lift devices.
If you want to cruise as efficiently as possible, you'll have the cleanest lightest wing possible with very simple high lift devices.

If you want your passengers to feel comfortable, you'll have a big cabin with big windows.
If you want to save fuel, you'll have a slender cabin with small windows.

No doubt you can imagine some more "ifs". I'm sure John Farley can add lots, and probably correct some of mine too!

And lastly, if you want to have fun, you'll fly a two seat aerobatic trainer. But that's off the topic. :ok:

More seriously chornedsnorkack, which of the above do you want? All of them? Then you'll understand the dilemma of a vice president at Boeing or Airbus.


(PS - one last one. If you want to actually sell your aeroplane, you'll avoid radical solutions because airlines don't like them, you'll avoid "bleeding edge" technology because no-one understands the implications, and you'll take a variety of trade-offs between efficiency and cost. Hopefully you'll sell slightly more aeroplanes than your competitor, at a slightly lower price.)

24th May 2006, 14:35
My 1968 built glider has an L/D of 44 with just under 18m wing span and average chord of about 0.5m. A more modern 18m design will have L/D around 50.

To make this work, it has only one seat (i.e. pilot only) with just enough room to wriggle in flight, and no sticking out bits like engines to spoil the aerodynamics.

Scale this up to take a few hundred passengers, and you can see why it's not a commercial proposition.

Plus, the trade off for the high L/D is that high speeds become impossible because the tips start to produce down forces (amongst other complex engineering and aerodynamic problems).

BTW, ETA is not really a powered aircraft but a self-launching glider with a fold away engine. The claimed L/D of 72 is only achieved in glider configuration. Only 2 seats.

25th May 2006, 09:50
I think the glide performance of airliners has been getting better. The 777 which I fly now is noticeably more slippery than the 737 I used to. A 777 will accelerate on a 3deg slope under most conditions, meaning the glide angle is > 20:1. OK, the engines are still running at idle but this is only going part-way to counteracting the drag of the nacelle and the windmilling front fan. If you took the engines off completely, I wouldn't be surprised to see 25:1 or better, although it would be flying a little fast to thermal efficiently. ;)

I also fly a self-launching sailplane. It has the fuselage of the ETA but slightly less span (28m vs. 31m), a sort of ETA-lite. They don't quote any performance figures (adequate, sir!) but it seems to have very long legs...:)

John Farley
25th May 2006, 10:23

I agree with you that L/D max has been getting better since the 1930s. That was why I asked chornedsnorkack for the evidence behind his assertion that it was unchanged. To my mind having flown the DC3 and the DC6 I would think the odd Boeing or Airbus from today would be a rather better bet as a glider!!


25th May 2006, 12:58

Figures I have for 777 are L/D_max ~=18.

For a 733 its more like ~=14.

Out of interest, a S360 has a L/D_max ~=19.

Surprised John?


25th May 2006, 13:30

Well, I suppose it depends on what your definition of glide ratio is for a powered aircraft. Engine(s) windmilling, seized, dropped off...? Generators/hydraulics on/off...? RAT out...?

I prefer to think what it would be if there was no thrust/drag from the engine installation, so you get an idea of what the airframe is capable of. In this respect, I still think modern jets are way ahead of their predecessors. :)

John Farley
25th May 2006, 14:00
Not really...Mr H-D strikes again!



25th May 2006, 14:03
Which end is the front?

P.S. I had a bit of a double-take there as it looked like it had been used for target practice, then I realised they were tell-tales! (Well, I think that's what they are...)

John Farley
25th May 2006, 15:03
Yep they are tufts...I used to work on the basis that the seat was usually at the front and installed so that it made you face the way it would go. More seriously was the matter of which side was the top because if you stalled it (CL max of 7) when one side let go it rolled faster than a Red Arrow. Can't recall the max L/D.


On Glide
25th May 2006, 16:05
In comparing maximum L/D ratios don't forget to consider the Mach number. It's one thing to do 20:1 at Mach 0.6 and quite an achievement to do the same L/D at say Mach 0.85 with all the shocks dancing around the wings, bouncing off pylons and winglets etc.

On Glide

25th May 2006, 17:30
You are right, checking numbers there seems to have been a slight improvement after 1930-s:

14,7 for DC-3, and in the range 16...19 now. Wonder where DC-4 and Boeing 307 fall?

But at least after 1960-s... B707-320 has 19,4 and everything else has lower L/D. A320 has 17.

John Farley
25th May 2006, 18:49
Thank you. An interesting site that I notice lists the Wright Flyer and the DC3 in a table entitled:

Table 2: L/D of Subsonic Jet Aircraft

Perhaps they need a proof reader


Proof Reader
26th May 2006, 00:46
O.K. John
They say that one volunteer is better than ten pressed men don't they?