Many airliners have LDRs near 20, and it sounds like the electric Eviation Alice may have an LDR of 24 at 240 kts, but many small planes typically have LDRs of only 8-10. It seems like they could easily increase the ratio with e.g. larger wingspans with higher aspect ratios and save a lot of fuel - why don't they do this? What are the tradeoffs? Are there any commercially successful 4-20 PAX planes that have LDRs over 20?
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3 Answers
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There are plenty of smaller power planes that achieve those numbers; motorgliders. And motorgliders with L/Ds in the high teens and low 20s are pretty efficient cruisers.
So the real question is; why aren't all light aircraft made to be like motorgliders?
Well, motorgliders have their disadvantages. The long wing span is a problem fitting in on the ground. The long wings are heavy, which limits payload. The long wings result in slow roll rates and general controlability problems. So if you are going to live with the hassle of long wings, it has to be really worth the trouble.
And probably the biggest one; the long wings stop being an aerodynamic advantage and become an impediment when you want to cruise at significant margins above speed for best L/D. This is the case for aircraft that cruise at low altitudes if you want to cruise at 2-3 times max L/D speed efficiently. In this case, < span = better. WW2 fighters modified to be optimized for low altitude combat had their wing spans reduced.
Airliners are more like the motorglider, and benefit from the L/D benefits of high aspect ratio because of their operating environment. They cruise at very high altitudes where indicated airspeeds are low because of the thin air. At FL400, an airliner might be cruising in the thin air at only 280 kts indicated, maybe only 2X its flaps up stall speed (going 500+ true, but it's the indicated airspeed that matters - if you could stick your hand out the window, you'd feel a 280 kt wind blast, not a 500 kt wind blast).
It's cruising a lot closer to its max L/D speed than the low altitude airplane that flies at 3-4 times its stall speed, and so it benefits from the span and area like a motorglider. To take that to it's most extreme, you have the Lockheed U2, which needs all that span and area to make it up to 75000 ft (where the indicated airspeed is only about 80 kt going, say, 200 kts true or whatever it is).
You do see some aircraft like the Diamond light twins with quite long aspect ratios for what they are. This is done mostly for rate of climb. because in a light twin, single engine climb performance is critical, and the high aspect ratio really helps (climbing close to max L/D speed) and that's factored into the balancing act of deciding just how long the wings need to be for good single engine performance without giving up too much controlability or taking on too much weight (helped there by composites) or too much space on the ramp.
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$\begingroup$ I don't understand why bigger AR is an impediment at speeds much greater than max L/D. Are you thinking of transonic effects? $\endgroup$– JZYLCommented Jan 27, 2020 at 14:40
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1$\begingroup$ I was thinking mostly of the effects of the increased frontal area of a high AR wing relative to a low AR wing of the same total area. $\endgroup$– John KCommented Jan 27, 2020 at 16:23
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1$\begingroup$ I would like to see some reference on this claim. I would think the reason fighters have low AR is for maneuvering and weight. High speed aircraft have transonic effects. $\endgroup$– JZYLCommented Jan 27, 2020 at 17:34
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1$\begingroup$ @JZYL, higher aspect ratio reduces induced drag at cost of some increase in form drag. Induced drag decreases with square of speed, while form drag increases with square of speed. So high aspect ratio give better max L/D, but at lower speed, while at higher speed the total drag increases due to the higher form drag. Still, A320 has AR 10.3 and $V_y$ around 250 KIAS (depending on weight), while C172 has AR of only 7.32 (lower AR means higher $V_y$), but it's $V_y$ is just 78 KIAS. So there is a lot more in play than just AR. $\endgroup$ Commented Jan 30, 2020 at 18:46
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$\begingroup$ As far as the frontal area of the wing itself is concerned, if your high AR wing had the same thickness as the low AP one they should have roughly the same equivalent flat plate area. The problem is you normally can't do that for structural efficiency reasons and are forced to use a thicker section at least at the root, so in practical terms you are stuck with more form drag on the high AR wing compared to a low AR one with the same area because the high AR one has to be more or less almost as thick as the low AR one, at least toward the root. OR, make the whole thing substantially heavier. $\endgroup$– John KCommented Jan 30, 2020 at 18:59
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First of all: Scaling laws. The ratio of inertial to viscous forces in the flow around the aircraft becomes larger with bigger size. This is also helped by the generally higher wing loading of lager aircraft which results in higher flight speeds. This ratio is expressed in the Reynolds number Re: $$Re = \frac{v\cdot l}{\nu}$$ In simpler terms: Friction is relatively lower for larger and faster aircraft.
Next, jet aircraft are more aerodynamically clean than propeller aircraft. Until jets are used for General Aviation aircraft, those will still have piston engines with radiators and propellers. The radiator alone (or the ducting around air cooled engines) can increase drag such that up to 10% of installed power is eaten up by this additional drag. Also, a clever study has identified the additional drag caused by the propeller slipstream on a Luscombe 8b: In that case it was 30% of zero-lift drag!
Last, while airliners are designed to fly at their optimum L/D polar point, GA aircraft are normally flown quite a bit faster which reduces induced drag. In order to enable a high cruising speed, the wing has a lower aspect ratio than that of most airliners. The ever increasing efficiency of turbofan engines has allowed to fit higher aspect ratio wings to airliners such that a Boeing 787 now has an aspect ratio similar to propeller-powered, long-range aircraft like the P-2 Neptune, the Breguet Atlantique or the B-24. Older airliners needed more internal wing volume for fuel storage and their lower aspect ratio also lowered their optimum L/D (which is only 16 in case of the Boeing 747-200, for example).
A well-designed GA aircraft need not be vastly inferior to airliners: While a DHC-2 Beaver amphibian only manages an L/D of 9, a Glasair III achieves a very respectable 19.7.
answered Jan 30, 2020 at 20:15
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$\begingroup$ Lower aspect ratio, given the same surface area, increases Reynolds number and increases induced drag. I don't see why it's more aerodynamically efficient. GA aircraft aren't flying near the Mach regime for subsonic effect to matter. $\endgroup$– JZYLCommented Jan 31, 2020 at 0:28
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$\begingroup$ @JZYL like the way you hang in there, and you're right, for a given area, higher AR is more efficient in cruise. Shame we have confused simply making the wing smaller also makes it shorter, with the same AR (just look at jet powered cruise missiles) and lowering AR, where strength and turning benefits are greater. Notice bomber designs are generally higher AR than fighters, back in the "old days". $\endgroup$ Commented Jan 31, 2020 at 3:45
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1$\begingroup$ @JZYL FAR have mandated in the past a minimum speed of 61 knots for light GA aircraft. This requires some wing area which produces a lot of viscous drag in cruise. Induced drag in cruise is almost negligible, so it is preferred to trade a bit of viscous drag (by increasing chord for the same area) for a small increase in induced drag. Also, the structure becomes lighter with lower AR which again helps to achieve those 61 kts. $\endgroup$ Commented Jan 31, 2020 at 8:06
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Alice is a very promising design featuring these efficiency improvements:
- Vortex cancelling wing tip props
- 2 empennage fins instead of 3
- Aft main propeller mount
- Streamlining of fuselage
- Straight high aspect ratio wing
- Light weight composite construction
Over many miles, as with airliners, these improvements offer significant energy savings which translate into cost savings. But craft such as these are expensive, and must operate for a given amount of time with a given amount of passengers to pay for themselves.
These are different concerns from GA aircraft, which soldier on with their fat Clarke Y or Gottingen airfoils for one very good reason: safety. These wings have very gentle stalling characteristics that, combined with strong lower aspect "Hershey bar" form, and slower flying speeds, provide a much more reliable and easy to fly experience for recreational pilots.
Polar diagrams at Airfoil Tools show the differences in lift to drag and stalling AOA characteristics of many airfoil types, and make good reading to further understand the trade-offs of gliding efficiency and safety.
answered Jan 30, 2020 at 15:23