I flew 65 hours in Stearman Primary Trainers with fabric covered wings. The top fabric on the wing doesn not bulge up from lowered pressure on top and the bottom fabric doesn’t bulge into the wing from higher bottom surface pressure either either.
The wing redirects the airflow downward relative to the wing which results in upward lift. As our aeodynamics instructor in ground school said, “The plane flies by throwing the air downward behind it.”
I’m going to stay out of this discussion except to say that was because the fabric covering was done properly. Back in A&P school, we were shown inflight photos of improperly covered aircraft that had the fabric on top of the wing bulging up between the ribs. Big no-no.
I’ll second that.
I used to fly ragwing ultralights, where the fabric covering is somewhat looser than on an airplane like a Stearman. In flight, the top fabric does… well, it’s not quite enough to say “bulge” but there is an effect, and the underwing fabric is pushed in slightly between the ribs.
Yes, there is a higher pressure on the bottom than on the top and there is a slight effect on the fabric. However it doesn’t seem consistent with the amount of force/sq. in. on a material that has essentially zero resistance to bending.
The lowered pressure on the top can be seen when taking off from an airport when the temperature of the air is right at the dew point. As soon as a plane with tricycle gear rotates so that the wing begins to develop a lot of lift, condensation occurs and a cloud of mist appears from the change in temperature on the top as the pressure suddenly drops. I’ve seen it at Boston and Los Angeles airports several times. They are both at approximate sea level and tend to be cold and damp in the morning.
If you incline a flat plate at an angle of incidence to the airstream the airsteam will be diverted downward over the flat plate and two high pressure areas will develop, one on top near the trailing edge and one on the bottom near the leading edge. If the cross section of the flat plate is modified to an airfoil shape the bottom high pressure area stays in appoximately the same place but the top one moves backward off the wing.
This airfoil shape doesn’t have to have the top surface longer than the bottom surface in the direction of the airflow. The North American P-51, Douglas A-26 (later called B-26), the Vultee BT-13 basic trainer and several others all have symetrical top and bottom curvature.
Yeah, I used to fly a Pitts Special which had a symmetrical aerofoil section. This means it flies rather poorly (approach speed of 90 kts on a very small light biplane!) both right way up and up side down.
What I’d like to know is, what do I tell the (possible) interviewer who asks for an explanation of lift when I’m going for a (possible) job. What is the accepted industry explanation?
I think there are still some misconceptions floating around about why the wing is shaped as it is.
There are two components to lift on a wing at a positive angle of attack. The first is ‘flat plate lift’. This is essentially air molecules banging against the bottom surface of the wing and being deflected downwards. In paper airplanes and paper plates, this is the sum total of lift.
The second source of lift is also acceleration of air downwards, but it occurs over the top surface of the wing. The shape of the wing is such that at a ‘flying’ angle of attack, the air will follow the top surface of the wing, and the air stays attached to to the top surface of the wing long enough that by the time it leaves it is accelerating downwards. This air, when it leaves the back of the wing, will have a net downwards vector. The result is lift.
The Bernoulli effect might be considered more effect than cause of lift. When the air gets pulled out of the free stream and deflected downwards, the accceleration of the air creates a low pressure zone on the top of the wing. Many confuse the low pressure with a suction that ‘lifts’ the airplane, but in fact it’s just the result of the air molecules spreading out as they travel across the curved surface of the wing and accelerate downwards. The real work is done by the mass of air being left with a net downwards vector as the aircraft passes through it.
A symmetrical airfoil needs a positive angle of attack to fly. When you put a symmetrical airfoil at a positive AOA, it has the effect of increasing the curvature of the top, and descreasing it on the bottom, giving you the shapes needed for lift. The advantages of a symmetrical airfoil are, 1) The AOA can be reduced at higher speeds, maintaining efficiency. An airfoil like a thick Clark-Y reaches a limit because of its shape that creates a lot of drag at high speeds. 2) A symmetrical airfoil cares not about whether it’s upside down. That doesn’t mean the plane will fly as well upside down, because the angle of incidence of the wing to the fuselage means that upside down the plane may be at an odd attitude and may have more drag from the fuselage, but the wing itself doesn’t care, 3) A symmetrical airfoil can be ‘unloaded’ by lowering the AOA to 0, giving the aircraft more manoeverability.
At least, that’s my understanding.
I probably wasn’t clear. I understand the principles behind a symmetrical aerofoil. My first and second paragraphs are not related to each other.
What is the accepted industry explanation for how a wing works. What is the correct exam answer (understanding that it may not be the actual correct answer). When I was learning to fly, the bernoulli thingo seemed to be the standard answer. Has this changed in the aviation industry or is it still being taught in schools far and wide?
I’m not in the business but I don’t think there is any question that lift is produced by deflecting a stream of air downwards and I think aerodynamicists have known this for a long time. As I said, our aerodynamics instructor said, in 1944, that an airplane flies by “throwing the air downwards behind it” and he didn’t think this up by himself. I’m sure he put it that way to make it sensible to us (dumbjohn cadets).
I think Sam Stone made a key point about the lowered pressure on the top being an artifact of the change in direction. If you take a slab of air and deflect it downward the top portion of the slab will be spread out relative to its undeflected condition. Sort of like when you bend an iron bar downward the top fibers are in tension, i.e. pulled apart. The airfoil is the device that deflects the slab of air downward and is shaped so as to do that with minimum disturbance and loss of energy from drag.
My impression is that the Bernoulli explanation is still widely taught, but Newton has been gaining considerable ground.
My explanation to students usually includes mention of a hovering helicopter. Most people have seen film of one over water or vegetation, and are aware that a huge amount of air is being driven downward. Most are also aware that the rotor blades are wings moving in a circular path. If you were able to measure pressure above and below the blades, you’d certainly find it lower on top. But which is the better explanation: that a helicopter stays aloft by flinging large amounts of air downward, or by some obscure process whereby pressure on top of the rotor blades is lowered?
As anyone who works with helicopters knows, the real reason a helicopter can fly is that it is so ugly the ground rejects it. Ha! Another good one is: A helicopter doesn’t fly, it beats the air into submission.
You were asking about the accepted explanations, so I will give them to you in order of acceptance:
Air on the top surface has to go faster to meet air on the bottom surface at the trailing edge. This will usually result in anger and a tirade about the quality of this nation’s public school system.
Momentum deflection. This is how “throwing air down behind it” is usually refered to. This is what we tell non-aerospace people to give them a physical understanding of what is happening. If you give this explanation in an aerodynamics class the professor will think it is cute.
Bernoulli’s effect. This is what you will learn in your first aerodynamics course. If you take away the requirement of the air meeting up at the trailing edge you get something workable.
Circulation theory. Oh, so you actually want to be able to predict something? Then circulation theory is for you. In its simplest form it states that the air passing around an airfoil is a combination of the freestream air motion and a vortex centered around the airfoil. When you add them up it looks like the real flow over an airfoil. The lift is proportional to the strength of the vortex. This is what most computer models will use and is the work horse for aerodynamics.
Navier-Stokes equations. So you have realized that all those other methods are just ad-hoc ways to describe a physical effect, but you want to know WHY it happens. These are the equations that model the motions of all fluids and are insanely complex. For any real life situation they CANNOT BE SOLVED (at least with our level of technology). The are used for hardcore ananysis and the prediction. Any use of them requires mega-computer time after you have simplified the analysis and made a lot of assumptions so the worst terms drop out of the equation. The problem is that they do not give you any good physical intuition as to what is happening as they deal with very small elements of the fluid.
So, the problem with understanding how an airplane flies is that you want a macroscopic explanation for something that is really a some of many microscopic events. Methods 2-4 are all correct as far as a description of what is happening, but they are not really very good explanations of the root meaning behind it. Flight is a very complex thing and has always been more of an engineering concept based on trial and eror than a scientific one, though we have caught up recently.
Oh, and if you think all this talk of lift is difficult, it absolutley pales when compared to the nightmarish problem of trying to calculate drag.
argh, chang “some of” to “sum of”
I hope the prof. will also think it’s correct (albeit, as you note, a bit crude and not necessarily the best way to start a detailed analysis).
Yeah, it is fine as an explanation, but useless as a tool. We don’t have any way of quantifying the momentum imparted to the air unless you already know the lift on the wing.
In the real world, most airfoils are asymetrical. The reason is efficiency: they are rarely called on to fly upside-down, and an asymetrical shape is thus more efficient (less drag for a given amount of lift). Such an airfoil can fly upside down, but when it does so will be significantly less efficient.
This is the case with the frisbee: can fly upside-down, but with noticeably lower L/D.
Hmm. The Douglas A-26 had a symetrical wing and was about 20 mph faster than the Martin B-26, which didn’t, with the same engines at the same manifold pressure and rpm.
Without knowing the details of these aircraft, I’ll note that it has never been difficult to design inefficient airfoils. Mere asymmetry is no guarantee of efficiency.
And the drag of the wings is only part (and not necessarily the largest part) of what controls the maximum speed of an aircraft.
Asymmetrical airfoils are not necessarily more efficient. They will generally create more lift at a given airspeed, but at the cost of increased drag. That’s great for airplanes that fly slowly or need to lift a lot, but not so good for aircraft that have a very wide speed range. High-speed aircraft will generally have thinner wings with a more symmetrical airfoil, trading off low-speed performance for less drag and a higher top speed.
That’s also why big jets have deployable leading-edge slats. They can increase the camber of the airfoil at low speeds, and decrease it at high speeds.
Agreed - they must be properly designed for their application if this is to be true.
I question this. Pretty much the highest L/D achieved today (around 70) is seen in Open-class sailplanes such as this, which use airfoils that are definitely non-symmetrical.
If you’d say “more nearly symmetrical”, I’d agree. Here’s a link to the 757’s “supercritical” airfoil - not wildly far from symmetrical, but far enough that its inverted performance is going to be very different from rightside-up.