What Are You Lifting?

Mention aerodynamic efficiency to most pilots and designers and you’ll probably hear the phrase “lift to drag ratio” within a sentence or two. Common aerowisdom dictates that for maximum efficiency, a high lift to drag ratio is required of an airframe. Of course, like most truths it is true and false at the same time. I’ll attempt to explain without going into a thesis level discussion as to why a high lift to drag ratio (L/D) is not the be-all-end-all of aerodynamic efficiency.

Long thin wing of a 757-200. Design is great for reducing drag on transcons, plus blended winglets provide ample space to advertise.

Now I’m going to I make designers start ripping out their hair and turning over their drafting boards…wait, it’s almost 2012…turning over their CAD mainframes. This is not exactly a law but it should be, and it will help many budding aircraft designers achieve their intended goals:

Any lift to drag ratio without a measure of what is being lifted is not operationally useful. 

(and in plain English)

Lift to drag is useless unless you’re lifting something useful.

Sorry, but I had to say it. Obsession over a high L/D is great if you want your airplane to look like a U-2. It’s not great if you want to be able to land it without pogo sticks under the wingtips. The first question you should be asking is “What do I want to lift?” More accurately, “What payload do I want to lift?”

The wing does not care how you divide up weight. If you build a 10,000lb airplane that’s creating 500lbs of drag at high altitude, you’ve got a 20:1 L/D ratio. That’s an excellent ratio, BUT…what are you lifting? If your aircraft empty weighs 7,000lbs and your useful load is 3,000lbs, it means you are experiencing a 6:1 load to drag ratio. If on the other hand your aircraft weighs 5,000lbs empty and your load is also 5,000lbs, your load to drag ratio goes up to 10:1 (designer Barnaby Wainfan describes this as an aircraft’s “transport efficiency” in his excellent article on low aspect ratio wings in the February 1997 issue of Flight Journal). Losing a bit of total L/D may be worth it to increase load to drag. Thus begins the tradeoff between light structure and increased L/D. Longer wings are heavier wings, with the attendant bending moments, possible zero-fuel restrictions and clearance problems for taxiways and hangars.

Yes, this high aspect ratio glider is efficient, but only at low relative speeds.

The other missing aspect is airspeed. While discussing the D-21 triple-sonic drone in his book “Aurora”, Bill Sweetman calls attention to its relative efficiency compared with a B-52. Using a modified version of the Breguet Range Equation, he multiplies the L/D of each aircraft by its cruise Mach. The B-52 with a 19:1 L/D and a Mach 0.85 cruise came out at 16:1. The D-21 with a 6:1 L/D and Mach 3.5 comes out to 21:1.

While this number (Mach to drag?) doesn’t do much since it ignores the D-21’s limited fuel supply compared with the B-52’s massive reserves and therefore does not give any useful information on actual range, it is interesting nonetheless. Personally, I’d use it as an early benchmark in the design process. If your intent is to go fast, a lower L/D is not as damning as one might expect, particularly since you’re “wasting” fuel for a shorter interval of time to cover a similar distance as a slower aircraft. This of course assumes you are not travelling supersonic where thrust specific fuel consumption begins to rise, further eating away at your endurance (but if your drag is low enough, you may still be able to recover range by the increase in speed…whew!). One way or the other, you’re putting either the slow penny or the fast nickel out your exhaust pipe.

I tried and probably failed miserably to get the point across. The point being that L/D is just one part of the equation when it comes to efficient design. If I had a choice between an airplane that had an L/D of 8:1 and a load to drag of 14:1 versus an airplane with an L/D of 18:1 and a load to drag of 5:1, I’d pick the 8:1 L/D aircraft 50% of the time…maybe.

Why? Because what I intend to do with the aircraft is what should drive its design. I’m not going to lie and say that high aspect ratio or low aspect ratio is better than the other because they’re not. I’m not going to say ignore L/D completely because that’s just stupid. But I do recommend that you look at your range, speed and payload requirements before committing to a design. Simply copying what comes out of Renton or Wichita doesn’t make much sense if you’re going to be doing something different than what they intended with their designs.

In summary, what did we learn?

• Low aspect ratio aircraft tend to have lower empty weights and can be very strong structurally.
• High aspect ratio aircraft tend to have higher empty weights and can require extra strengthening in extreme cases.
• Lift to drag ratios are not fully useable until you know what fraction of the weight is payload/useful load.
• An airplane with a high L/D and high aircraft empty weight has less transport efficiency (thanks Mr. Wainfan!) than an airplane with a lower L/D and lower empty weight.

Unfortunately, like most things in aviation, what I brought up here leaves a lot more to be addressed, like the role of wing loading, or integration of wings and fuselages. Or even why all my designs look like guitar picks (seriously, not all of them). We’ll continue soon with more on wing loading and how reducing it can improve high speed performance.

Dyke Delta N18DW at Oshkosh. Low aspect ratio double delta wing design very resistant to stalls and adds at least 200mph to every pass, at least in the spectator’s mind.