Maximum glide ratio

I fully agree with 25 deg south about the human factors influence on the Global Flyer;- As with pretty much all A/C the final design always ends up being optimised to meet the whole end user requirement set and hence each single design element is compromised to some extent.

Eta – Although yes it does have an engine and can take of, its much more glider than powered aircraft. Put an average power pilot in an S10 and they will feel at home (whilst keeping the engine on most of the time) and pretty uniquely the same applies to an average glider pilot in an S10 (keeping the engine off most of the time). With Eta however, because of the very slow climb rates and other interesting handling issues I think an average power pilot in Eta would “roll it into a ball” (or refuse to ever fly it again if they did get down in one piece);- Eta is for experienced glider pilots only and is designed to spend most of flying time engine off…….i.e. A futher example of the above paragraph.

Global Flyer of course was a single seater and had to move pretty quickly to get round the earth before the pilot conked out through fatigue. In addition the aircraft had to be flyable solo for what was still an extended period . Consideration of these aspects alone would have driven the concept away from that of Voyager for example. The jet configuration arrived at of course pushes the optimum operating altitude way up and then one starts approaching all the Mach/IAS/TAS “compressibility corner” constraints ( N.B. The B-47 as an early example). Very high aspect ratio wings ,apart from presenting many structural problems regarding issues such as aeroelasticity,also involve an increased pilot work load ( e.g. handling an extremeA/B ratio) – undesirable on long missions , especially in the descent and landing phases,witness the U-2.
So the logical statement from chorned.etc. is probably correct for an optimum L/D solution aerodynamically. I would consider that the final design compromise for Global Flyer would however , in addition to engineering constraints, have of necessity included a high consideration for the Human Factors aspects of the overall project ,thus pushing the final aircraft design well away from an aerodynamic ideal.

Both Reynolds number and Mach number effect the degree to which laminar flow is possible, with Mach number probably being the more significant.

So with Global Flyer being jet powered and hence with its need to operate at altitude to get the best engine presure ratio, this probably suppressed its LD, but this is not the whole story. Several other factors apply;-
– The best L/D has an optimum speed which is generally quite low and above this the LD slowly decays. For example the S10 although it has a top speed of 270kph, its LD at this speed is about 20:1;- its best LD of 50:1 occurs at about 110 kph. I suspect Global Flyer again being jet powered achives its optimal propulsive efficiency at a speed higher than its best LD.
– Higher wing loading generally suppresses LD (and laminar flow?)
– Laminar flow breaks down with the slightest provocation (ice, dead flies etc). For a really long flight it may be necessary to select a wing section which is a little more bug/water drop tolerant, but again this is at the expense of really optimising the LD. (Competition Gliders often use “bug wipers” to clean the laminar flow portion of the aerofoil I.e. thin plastic strips drawn along the leading edges by fine wires whilst the glider is in flight )

For the highest LD ever for a manned aircraft try the glider called “ETA”.

Ah yes. L/D 72 – and that is a powered aircraft no less.

It was mentioned that the “Open” class gliders are restricted to 850 kg total weight – and that this causes design compromises.

So, logically, propeller craft staying at low TAS and Mach should have better L/D than the Global Flyer… especially if they also use wing wipers…

Both Reynolds number and Mach number effect the degree to which laminar flow is possible, with Mach number probably being the more significant.

So with Global Flyer being jet powered and hence with its need to operate at altitude to get the best engine presure ratio, this probably suppressed its LD, but this is not the whole story. Several other factors apply;-
– The best L/D has an optimum speed which is generally quite low and above this the LD slowly decays. For example the S10 although it has a top speed of 270kph, its LD at this speed is about 20:1;- its best LD of 50:1 occurs at about 110 kph. I suspect Global Flyer again being jet powered achives its optimal propulsive efficiency at a speed higher than its best LD.
– Higher wing loading generally suppresses LD (and laminar flow?)
– Laminar flow breaks down with the slightest provocation (ice, dead flies etc). For a really long flight it may be necessary to select a wing section which is a little more bug/water drop tolerant, but again this is at the expense of really optimising the LD. (Competition Gliders often use “bug wipers” to clean the laminar flow portion of the aerofoil I.e. thin plastic strips drawn along the leading edges by fine wires whilst the glider is in flight )

For the highest LD ever for a manned aircraft try the glider called “ETA”.

Helios 270 km/hr. . I.A.S. or T.A.S.? It seems quite high for I.A.S. IMHO. Just wondering if we are comparing like with like in some of these figures.

It’s actually an issue regarding Laminar flow (and improvement to materail allowing higher aspect ratio’s).

The Laminar flow sections that were first introduced in the early 40’s were a head and shoulders better than any wing section used up until then but where aimed at quite a wide speed range. However as you get faster the area of the wing subjected to laminar flow reduces quite rapidly. Above 0.7 Mach natural laminar flow is pretty much impossible to achieve. Remember when comparing really old aircrafts LD with modern aircraft the key difference is the cruising speed. However the observation that modern airliners have an LD not that much different to the early generation is quite valid. (first generation Comet, 707 had an LD of about 17-20:1 and the latest B7/A3something have LD’s of around 25-26:1……. Interestingly Vulcan has an LD of 23-24:1, stunningly good for it’s time I think!)

Why have Gliders improved so much more in the same time period well, the first really practical low speed optimised laminar flow wing sections were introduced by Prof Wartman in the late 60’s. As gliders only operate at relatively low speeds, the introduction of these sections resulted performance jumping from a max of about 38:1 (BS 1 – Eppler – glassfibre – c1965) to 60:1 (Nimbus 3 – Wortman – glassfibre c1975) in a very short space of time.

If the speed is low enough to allow extensive laminar flow (and the aspect ratio is high enough) then LD’s of 50+ are possible from a self lauching powered aircraft indeed this has already been done – Check out the Stemme S10

Is laminar flow dependent on Mach or Reynolds number?

Stemme S10 has a wingspan of 23 m, speed goes to 270 km/h, wing area 18,7 square metres, MTOW 850 kg, OEW 660 kg. So, wing loading in the region of 40 kg per square metre.

Global Flyer has a wingspan of 34 m or so, speed much faster (over 460 km/h), wing area 40 square metres, MTOW 10 tons, OEW 1600 kg. So, wing loading in the region of 250 kg per square metre on takeoff.

Helios has a wingspan of 75 metres, speed up to 270 km/h, wing area 186 square metres, MTOW 930 kg, OEW 600 kg. Wing loading in the region of 5 kg per square metre.

Did the Global Flyer have trouble maintaining laminar flow because of its higher speeds, larger size and therefore higher Reynolds numbers than Stemme S10?

And would a powered glider the size of Helios also have trouble with high Reynolds numbers causing loss of laminar flow?

LD quoted above are at cruise equilibrium (lift = mass & thrust = drag)
Source – Prof A Allen paper given to the Royal Aeronautical Society on LD progress over the last 50 years

It’s actually an issue regarding Laminar flow.
(first generation Comet, 707 had an LD of about 17-20:1 and the latest B7/A3something have LD’s of around 25-26:1……. Interestingly Vulcan has an LD of 23-24:1, stunningly good for it’s time I think!)

Vega ECM – an interesting & informative post 🙂

Are the airliner L/D’s measured with engines at zero thrust, or off/windmilling ? Could be quite a big difference there.

Totally agree about the Vulcan. Awesome.

It’s actually an issue regarding Laminar flow (and improvement to materail allowing higher aspect ratio’s).

The Laminar flow sections that were first introduced in the early 40’s were a head and shoulders better than any wing section used up until then but where aimed at quite a wide speed range. However as you get faster the area of the wing subjected to laminar flow reduces quite rapidly. Above 0.7 Mach natural laminar flow is pretty much impossible to achieve. Remember when comparing really old aircrafts LD with modern aircraft the key difference is the cruising speed. However the observation that modern airliners have an LD not that much different to the early generation is quite valid. (first generation Comet, 707 had an LD of about 17-20:1 and the latest B7/A3something have LD’s of around 25-26:1……. Interestingly Vulcan has an LD of 23-24:1, stunningly good for it’s time I think!)

Why have Gliders improved so much more in the same time period well, the first really practical low speed optimised laminar flow wing sections were introduced by Prof Wartman in the late 60’s. As gliders only operate at relatively low speeds, the introduction of these sections resulted performance jumping from a max of about 38:1 (BS 1 – Eppler – glassfibre – c1965) to 60:1 (Nimbus 3 – Wortman – glassfibre c1975) in a very short space of time.

If the speed is low enough to allow extensive laminar flow (and the aspect ratio is high enough) then LD’s of 50+ are possible from a self lauching powered aircraft indeed this has already been done – Check out the Stemme S10

I think 37 is an extremely good L/D – indeed it’s only really the top sailplanes that get over 50.

17+ metre wing span in other words.

I think 37 is an extremely good L/D – indeed it’s only really the top sailplanes that get over 50.
Off the top of my head, I’d also be supprised if most airliners exceed the 10-12 range. The trend to large diameter (hence drag area) fan engines would be a major issue to L/D.