What are the characteristics of high aspect ratio wings?

[tuna hp]

If you design a high aspect ratio wing that is going to be efficient at fast cruising at some high altitude, will that wing also be relatively effective during the takeoff, climb, and descent phases of flight done at lower altitudes and at some much lower speed?

For example, a glider has a high aspect ratio and is effective at flying low and slow. A B-52 bomber also has a high aspect ratio and is efficient at flying relatively fast at relatively high altitudes. Since the B-52 is optimized to fly fast only at high altitudes, does it necessarily have good low speed, low altitude performance?

Basically I understand that the tradeoff between a low aspect ratio/ high sweep wing and a high aspect ratio / low sweep wing is in how they incur induced and parasitic drag. Higher aspect ratio is more efficient at countering induced drag which is dominant at lower speeds. Lower aspect ratio reduces parasitic drag which is dominant at higher speeds. I also understand that flying at higher altitudes (in thinner air) "shifts the curve" of induced and parasitic drag so that at some higher altitude at a given speed, induced drag is greater and parasitic drag less, than at the lower altitude at that same speed.

So say you are designed an airplane to cruise high at FL490 (typical of new business jets) and therefore you are giving it relatively high aspect ratio wings to maximize efficiency at that altitude. Does the same wing which maximizes efficiency at very high cruise altitude also give your wing a high lift/drag ratio at the lower and slower speeds typical of the beginning and ending segments of the mission?


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This stems out of my question on why Dassaults have 3 engines. There was no final consensus, but most people argued that 2 engines is in almost every way preferable to 3 and that the main reason that Dassaults have 3 engines is that when they were designed, there were not suitably reliable small turbofan engines for the size of plane that it was building. So Dassault added a third business jet engine to improve redundancy while Gulfstream took the approach of adapting more reliable-but-thirsty commercial airliner engines to the job. And then supposedly the newly designed 7X has 3 engines because of the eccentricities of tradition (or, less likely, because again they didn't like their choice of turbofans in the thrust class that they would have needed to make it a 2 engine plane).

So then the difference in their performance characteristics must be explained elsewhere in the plane. Dassaults have lower runway requirements, competitive climb performance (at least the 7X does), and more fuel efficient cruising. If its not their 3-engine layout giving them this performance, maybe it is the wing. They do have an abnormally high aspect ratio wing which they then cruise at high speeds efficiently on, while having the lowest runway requirements and slowest approaches in their classes.

[lotusexige]

Aspect ratio affects the induced drag. CDi=Cl^2/(Pi*AR).
At lower (indicated) airspeed we have a higher lift coefficient so higher induced drag. The greater the aspect ratio the lower the induced drag at a given lift coefficient. However, the higher aspect ratio wing is always going to be heavier, all other things being equal. Ultimate lift coefficient is probably going to be worse too as it tends to varey with the Reynold number.
So in real life we tend to find really high aspect ratio wings on gliders which need to have a good performace, measured as lift/total drag, at low airspeeds and in long range aircraft, eg Canbera, U2, B52..

[galaxy flyer]

tunahp

May I submit you are looking at this far too simply--every airplane is the result of many, many compromises each decided by the designer's brief as to what performance, size, range characteristics the marketing people think will be attractive to buyers.

The Falcon three-engine decision was originally made in the '70s when the very idea of 2-engine overwater operations were inconceivable. The first airline ETOPS routes weren't extensively used until the mid-80s. Three or four engines were considered essential and Falcon was offering a product that was "airline grade" as the term was then. Additionally, engines tend to drive airframes, not vice versa. The available powerplants, in the thrust range needed drove some of the decisions. In brief, Falcon chose three engines because it was considered "necessary" to market an overwater, airline-grade business jet and the TFE 731 was the only available engine. The didn't plan on or anticipate the problems TFE-731 would have.

On to wings. The GLEX and its competition the G550 have very similar wing spans--the "footprint" on the ramp of the two planes is very close and they have basically the same engines. The GLEX has about 100 square feet less wing area, so has a slightly higher aspect ratio. It has a very sophisticated high-lift slat and flap system, combined with a powerful rudder, the wing allows it to have low Vmcg and Vmca; thus low speed numbers in the take-off and landing regimes--about 15 knots less than the G550 at similar gross weights. Wings are the product of very complex fluid dynamics well beyond aspect ratios, span loading, wing loading, fineness ratios. Yes, high aspect ratios deliver less drag, all else being equal which it never is. Virtually, all planes that operate at relatively high cruise lift coefficients will have high aspect ratios--B777 or a glider. Similarly, longer wings may weigh more for a given area, but cleverly place four engines across the span and that can be overcome--see B747. Boeing is famous for designing wings with efficient span loadings and, hence, good wing bending relief.

The added wing area of the 6,000 pound lighter G550 allows it to come up with similar runway performance, when compared on similar trip lengths. This performance despite a very simple flap system and a Vmcg about 20 knots greater. The lighter weight of the G550 allows it to operate over about 6% greater non-stop legs at the expense of operating about 15 KTAS slower.

The GLEX has a much roomier cabin, about 10 inches wider, flies about M.03-M.05 faster, but the overall fuel burn is a bit greater.

As you can see, the designers of the two planes had very different briefs. The G550 was a derivative of the G IV while the GLEX was a clean sheet design. So the list of compromises were different, they are marketed a bit differently and buyers choose between the two planes based on those different compromises. Some want a big cabin and pay the price in fuel and slightly less range; others want range, the reputation and are happy with other features inherent in buying Gulfstream.

What all this says is that aircraft design is a very difficult and complex process and comparing what are rather simple outward characteristics, like aspect ratios and engine counts, is going to be unrewarding.

[keithskye]

I was about to post my own reply to the question of wing design as well as the issue about 3-engines vs. 2, but Galaxy Flyer summed it all up so accurately that I do not think it is really possible to add anything of any real value. Good job and very well said, GF!!! You definitely know what you are talking about. It is a shame that most "modern" general aviation pilots do not study or learn more about engineering and aerodynamics as part of their education and training. I am always amazed at how much the typical bizjet pilots I have flown with do not know about such subjects. I am actually glad to see Tuna HP start the threads he did because such seemingly simple questions generated a lot of discussion and put a lot of information on the table for the benefit of all here on these forums. I at least was reminded of some things I had forgotten, and I think its good for all of us in this business to get as much "refresher" training or info as we can! Thanks to all here for making this such a great forum.

[galaxy flyer]

Thanks, Keithskye, I appreciate the complement. I agree that a lot of "modern" aviators are brought up with little engineering instruction (that is, nuts and bolts of the mechanical bits) and perhaps even less on the aerodynamics and performance side. We have "dumbed" down a lot of it.

TunaHP

I should have added that the G550 with more wing area and lighter cruise weights probably has a 2000 foot altitude advantage throughout its cruise profile. That is probably worth about 300-350 pounds per hour fuel burn reduction. Doesn't sound like much, but over 11 hours, it adds up.

Why the weight difference? Again, the GLEX was built to a different brief and different FAR 25 standard. The GLEX has a more complicated wing-high lift systems and multi-function spoilers, it has 4 generators rather than 2, 6 hydraulic pumps in three systems rather than 2, and a larger airframe, which means more interior weight and so on.

GF

[keithskye]

of course, with a larger cabin volume available, most GLEX owners stuff more into them than G-V/550 owners.... because they can, and that means higher BOW, etc., etc.

[ap101]

Galaxy Flyer,
Thank you for providing us all with such an interesting and well written reply, I read it with great interest.