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In the domain of aircraft, and more specifically powered flight, is there any case in which you wouldn't want the airfoil used to generate as much lift (and as little drag) as possible at a null angle of attack?

I'm writing a genetic algorithm that picks the best airfoil for the required environment. I tried to visualize the output, asking myself this question. Since the lift coefficient is known at the time of creation of the airfoil (I'm using NACA's 4-digit asymmetric airfoil equation), or at the very least can be stored in a table, simulating the performance of each airfoil encountered in the algorithm might be a waste, and you could simply query the table for the best airfoil given certain variables.

I considered that perhaps having too much lift (drag) at an angle of attack of 0° might rip the air-frame apart, or at least make it very unstable and create unwanted vibrations (or rip the wings off), but I'd like to know if there are other reasons.

Thanks in advance.

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There are two aspects, the first of which you alluded to. If you have much more lift than you need, than what you designed has been over-engineered. And over-engineering something means wasted money. For instance, bigger wings generate more lift but weigh more and require more structure to hold together. This costs materials, which costs money. So you don't want to just make something as big as possible -- you want it as big as it needs to be to fulfill the role you have in mind.

The other issue is a physics one. Lift-induced drag is the drag that occurs due to the lift generated by a body. As you get more lift, you also create more drag by design. This means you need bigger engines. Bigger engines weigh more, cause more drag, and cost more to run. All of this is a negative if you don't need that much lift.

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  • $\begingroup$ Just wanted to add that induced drag is a 3-D effect, it may not be considered if the OP is only interested in doing 2-D airfoil analysis which assumes infinite length wing (though it is very important practically). $\endgroup$ – Yandle Nov 13 '14 at 22:26
  • $\begingroup$ @Yandle I took the question to be about real, 3D problems since the only possible danger listed is "ripping the airframe apart" which would not be a concern if only 2D is considered. I could be wrong though, just wanted to add some food for thought! $\endgroup$ – tpg2114 Nov 14 '14 at 0:07
  • $\begingroup$ Although I'm simulating 2D airfoils, I was asking in a general case. I just wanted to verify that there was a purpose in writing "the best algorithm" for finding the best airfoil. I don't have 15 points yet, but this answer satisfied my question. I now know how to parameterize my algorithm. Thanks. $\endgroup$ – Mischa Alff Nov 14 '14 at 12:31
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One example I can think of:

Stunt aircraft want a symmetrical airfoil so they can fly upside down equally well. Their lift is entirely due to angle of attack.

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You don't have to worry about ripping the wings off. That can happen with any aircraft, with sufficient angle of attack and/or sufficient speed. The way aircraft fly is by adjusting the angle of attack so as to get the amount of lift they desire.

In straight and level flight, they only lift 1G (by definition). In violent aerobatic maneuvering, they may pull up to 12G, but in a simple plane like a Cessna 172, pulling more that 4G will "void the warantee".

The advantage of maximizing the L/D ratio is more efficiency. For example, the P-51 Mustang was designed with laminar flow wings (see Wikipedia) to get longer range.

But L/D isn't everything. The P-51 was known for having sudden stall, because of the laminar flow wing. That is a safety issue at low altitude.

Please read this site. It is easy to read and accurate.

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I think it is more a matter of control rather than $L/D$ ratio. For example you don't want to have to "trim down" in order to cruise at normal speeds. You want a plane to fly itself as much as possible, unless it is a military aircraft when you want the opposite. You want the plane to be unflyable in order to make maneuvering faster.

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Too much lift can be bad if your aircraft do not have the structural ability to support it. I once had a glider I built snap in half down the middle of the body because of this.

In general, you always want to maximize your lift to drag ratio since this is a measure of efficiency. Off the top of my head, if your aircraft does not require very high lift, and your propulsion system cannot overcome the extra drag to operate at certain speed, you might be inclined to use an airfoil with a lower Cl/Cd if that airfoil gives you a lower value of Cd. I'm sure there are other reasons someone more familiar with this topic can add to.

However, you always want to simulate the performance of an airfoil for each new design because they need to be analyzed on a case by case basis based on the design requirements. Some airfoils are optimal for gliders and others for powered aircrafts. There are other considerations (controls, performance, flow separation etc.) that might dictate which airfoil you choose. Some airfoils are designed for low Re and others for high Re, some are good for a certain range of Re. The Cl/Cd is also a strong function of the angle of attack. Your wing design might dictate which airfoil is more appropriate etc. etc. There's also 3-D and practical considerations for a real design.

Here is a nice database I've used of different airfoils if you are interested in them: http://m-selig.ae.illinois.edu/ads/coord_database.html

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