PPRuNe Forums > Other Aircrew Forums > Flight Testing > Theoretically Maximum L/D Ratios PDA View Full Version : Theoretically Maximum L/D Ratios Jane-DoH25th Mar 2011, 02:48What's the highest theoretical maximum L/D ratio at Mach 0.85, Mach 0.90, and Mach 0.95 for subsonic flight; and what's the highest L/D ratio for Mach 2.7, Mach 3.0, Mach 4.0, and Mach 5.0? TAGBOARD25th Mar 2011, 04:40Given a lack of information on the body you're discussing, I can only speak to generalities. The best glide ratio you can fly in any given condition is dependent on whether you can trim it in all three axes (roll, pitch, yaw) with the affectors, while still allowing room for additional control deflections. For example, if you need to trim roll with 15 deg of aileron, you should have an additional 5 deg remaining for maneuvers. Once this is established, usually by wind tunnel data, you can now talk to the max trimmable glide ratio and max trimmable normal force coefficient (read lift coefficient) across a mach number and altitude (read Reynolds number) range. A typical supersonic body will see it's max trimmable glide ratio in the subsonic regions, then taper off supersonically. This is mostly due to the lack of circulation lift (most efficient type) above subsonic mach numbers. Also, you get a drag buildup due to drag divergence from compressibility. This also factors into the equation of CL/CD. Hope this helps. Pugilistic Animus25th Mar 2011, 19:37indeed it would depend on numerous factors...it's very hard to answer blanket questions concerning aerodynamics because of the numerous factors involved with each type....aspect ratio, sweep angle airfoil designation...etc..etc...and the theoretical computations required, to even approach an answer are well beyond the scope of Pprune...and even then with fancy computers and models...the actual results always come from from flight testing and even then every ship has many other unknowns...:) Jane-DoH27th Mar 2011, 00:49Okay, I'll simplify. 1.) What's the highest L/D ratio anybody here's seen period (I'm pretty sure it will be a glider, but I don't know exactly which model)? 2.) What's the highest L/D ratio anybody has seen on an aircraft that can fly between Mach 0.70 to Mach 0.80 Mach 0.80 to Mach 0.85 Mach 0.85 to Mach 0.90 3.) What's the highest L/D ratio anybody has seen on a supersonic aircraft that can fly between Mach 1.0 to Mach 2.0 Mach 2.0 to Mach 2.5 Mach 2.5 to Mach 3.0 Mach 3.0 to Mach 4.0 Machdiamond27th Mar 2011, 23:48Taken as an individual parameter, L/D does not mean much. What are you going to do with the answer? Jane-DoH28th Mar 2011, 02:20Machdiamond Taken as an individual parameter, L/D does not mean much. Well, it's a measure of aerodynamic efficiency; it's not the only variable even for determining range -- you need fuel fraction, thrust/weight-ratios, specific fuel consumption and so forth. What are you going to do with the answer? I'm fascinated by high performance aircraft (as well as cars and motorcycles for that matter). Brian Abraham28th Mar 2011, 03:08Well, it's a measure of aerodynamic efficiencyAs Machdiamond said Taken as an individual parameter, L/D does not mean muchSome representative L/D ratios. Note not figures for two dimensional aerofoils but for the whole vehicle. The figures range from 70 to 0.368. Ask yourself why did the designer make the choice he/she did if L/D is the measure of efficiency. VehiclemmmmmmmmScenariommmmmmGlide ratio Sailplanemmmmmmmmglidingmmmmmmm45-70 (depending on span) Lockheed U-2mmmmmCruisemmmmmmmm~28 Rutan Voyager mmmm Cruisemmmmmmmmm27 Albatross (bird)mmmmmmmmmmmmmmmm20 Boeing 747mmmmmmmCruisemmmmmmmm17 Hang glidermmmmmmmmmmmmmmmmmm15 Common Tern (bird)mmmmmmmmmmmmmi12 Paraglider (high performance model)mmmmmii11 ConcordemmmmmmM2 Cruisemmmmmmmm7.14 Powered parachute (Rectangular/elliptical)mm3.6/5.6 ConcordemmmmmmApproachmmmmmmmm4.35 Space ShuttlemmmmApproachmmmmmmmm4.5 Wingsuit (Skydiver)mmGlidingmmmmmmmmiii2.5 Northern flying squirrelmGlidingmmmmmmmm1.98 Space ShuttlemmmmHypersonicmmmmmmm1 Apollo CMmmmmmmReentrymmmmmmmm0.368 Jane-DoH28th Mar 2011, 04:08Brian Abraham Some representative L/D ratios. Note not figures for two dimensional aerofoils but for the whole vehicle. The figures range from 70 to 0.368. I didn't know there were planes that had L/D's of 70. I know they went over 50 though. Ask yourself why did the designer make the choice he/she did if L/D is the measure of efficiency. There are a number of reasons, of which here are the ones I can think of -> The wider the range of airspeeds and mach numbers the plane is designed to fly at, the harder it is to design an airfoil that works exceptionally well at one given point on the performance envelope and still perform well at other speeds/mach numbers -> With weight an important factor in an aircraft's design, given the choice of an extremely efficient, but heavy wing, or a very light wing that is still reasonably efficient, sometimes it is better to use the less efficient wing if the weight savings is sufficient. This is one of the reasons the F-15 was designed with a fixed-wing instead of a swing-wing. Boeing 747... 17 Really, I would have thought it would have been more. The B-52 managed a 21.5 L/D ratio. Concorde... Approach... 4.35 I'm guessing the L/D ratio is so low because of the delta-wing's high AoA at low airspeeds? I would assume at Mach 0.95 @ FL250, the L/D ratios would be higher than supersonic? Space Shuttle... Approach... 4.5 I'm guessing this is why they need such a steep approach, without a substantial amount of thrust they'd need a good decent angle to keep the airspeed reasonably good until just before landing, then quickly throw out the gears, flare and land. Apollo CM... Reentry... 0.368 God that's like a brick... Brian Abraham28th Mar 2011, 06:17The 707 had a ratio of 19 and the lower figure for the 747 is said to result from a higher value of ratio of wetted area to wing area. Re the B-52 you do see the figure of 21.5 quoted. The info I have calls a max ratio of 19 for the 52, but this is unattainable because of wave drag when operating above Mach .74, where the ratio drops to 11. First and foremost the 52 was designed for a high altitude capability. mmmmmmmmmmmmmmiiB-52mmmmim747 Typical Cruise Altitudemmm50,000mmmm35,000 Max Altitudemmmmmmmiii55,000mmmm42,000 Aspect Ratiommmmmmmm8.56mmmmmm7.4 Jane-DoH28th Mar 2011, 21:37Brian Abraham The 707 had a ratio of 19 and the lower figure for the 747 is said to result from a higher value of ratio of wetted area to wing area. What's wetted area? First and foremost the 52 was designed for a high altitude capability. So that's why the B-52 has a lighter wing-loading than the 707 or DC-8?