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Jet Aerodynamics

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Active member
Jan 4, 2002
I have been reading about jets and was curious if someone could provide me with an "easily" understood answer to the question "Why do jets have sweptback wings?" I have also come across a term, "balanced field length" if someone could explain this to me also. Thanks.
an easy reply

Jets have swept back wings b/c it improves high-speed aerodynamics. The draw back is poor performance at low speed and altitudes, that way a swept wing a/c will have a higher landing speed then a "straight wing" aircraft. Basically

Balanced Field Length is when accel. stop and accel go are the same.

A better question is what or how take off distances are figured. It is the LONGEST of the following accel. stop, accel go, or two-engine distance to a height of 35' (I think its 35' but what the hey it's 3:30am)
Wing Sweep

All aircraft, jets and props alike, suffer from an increase in total drag during flight as speed increases. I won't address induced drag increases at slow speed here, so let's concentrate on the high speed issue.

This drag increases non-linearly during acceleration from zero to Mach 1 (and above, incidentally) and is especially bad during transonic flight regimes (above .8 Mach) because of the shock wave beginning to form at the nose of the jet. For this discussion, I'll just stay with the shock wave analogy, but keep in mind that at subsonic flight the shock wave is not just a single thin front of energy--it is instead a wide area of compression that maybe be several feet in width, depending on speed and temperature. Interestingly, I've seen this phenomenon first hand during acccelerations at low altitude in humid conditions--it made me say "huh?" when I first saw it, since it caused some distortion in the light coming through it. Nothing like The Right Stuff, but it was interesting.

That shock wave propogates out like an inverted V from the nose and will cause a drag increase as soon as it reaches a part of the aircraft, most usually the wing. The shock wave moves slightly back along the longitudinal axis of the aircraft and becomes more acute (the V gets more narrow) as the speed increases, thus the faster you go, the more likely the shock wave will touch part of the wing. Of course, as soon as you hit Mach 1 most of the shockwave is now behind the aircraft.

In a straight winged aircraft, that shock wave will reach the wing in quick order. Not only did it cause drag issues, but flight controls were an problem as well. The shock wave would interrupt the normal airflow over the ailerons and entire empenage (tail, for non-Frenchies out there) at times. As lots of folks knew before WWII, the further the wing is swept, the longer it takes for this shock wave to reach the wing, imposing a severe drag penalty.

Therefore, swept wings allow this drag penalty to be "delayed" in effect. When the shock wave does intersect the wing, less wing (measured in terms of total chord) is exposed at any given time due to the sweep as well.

The downside? Swept wing aircraft have to fly faster to maintain a given lift coefficient (most often a problem during landing), and thus have either faster approach speeds or increased dependence on high lift devices on the wing. Further, the swept wing requires more engineering to be able to handle the aerodynamic loads--straight wings are easier to design and build. Finally, due to aerodynamics, controlability, especially at higher angles of attack, is often an issue.

Hope this helps. Maybe those years of Aero actually made me learn something...
I don't fly jets but in my understanding a balanced field is when your accelerate-stop distance (the distance required to accelerate to a defined speed, lose an engine at precisely that moment and come to a stop) is less than the runway available. If your runway is 4,000 feet and your accel-stop is 4400 you do not have a balanced field. If it is 3800 feet you do. I believe it is also related to accelerate-go distance which is the distance required to accelerate to the same defined speed and when you lose the engine you continue and climb to clear a 50' obstacle. One or the other must be less than the runway length, I believe.
Balanced Field Length

Balanced Field Length (BFL) is the distance that the aircraft takes to accelerate to V1 lose and engine and stop OR accelerate to V1 lose an engine and continue the take-off and cross the end of the runway at 35ft.

Now obviously these two numbers don't magically always coincide... The engineers adjust V1 to make these numbers equal... When they get these numbers equal your field length is "balanced".

It has nothing to do with the amount of runway you are going to use vs. runway available. Now if your BFL is 6,000 ft and your runway is 5,500 you don't have any business trying to takeoff... Because theoretically if you lose and engine right at V1 and try to stop you are going to overrun by 500 ft. and if you try to go you will be well short of the 35ft crossing the threshold (perhaps even still on the ground, which becomes a high speed overrun). Either way, not good.

Balanced Field Length was developed to avoid having to always calculate your take off distance, accelerate stop and accelrate go distances individually... It is kind of a "catch all".... you know you can lose and engine and stop, lose and engine and go.... and you will be within take off distance requirements.... (Your all engine take off distance will always be less than BFL)

Hope this helps....

Fly Safe....
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Swept wing aircraft are all about increasing the critical mach number, which is where high speed drag starts really picking up. The reason is if you look at the relative wind hitting the leading edge of a swept wing, it doesn't hit the edge perpindicular. It hits the leading edge at an angle equal to the wing sweep. Because the airflow hits the leading edge and wing at an angle, it decreases the apparent velocity over the airfoil. I always thought that the wing is tricked into thinking the air over the airfoil is at a decreased velocity, therefore increasing the critical mach number, which is usually when an shock wave forms on the top wing.

I'm know aerospace engineer but this is the simplest explanation.

In a nutshell, balanced field length is when the distance to take off equals the distance to accelerate to a given point, abort the takeoff, and stop. There are specifics, but the bottom line is that it's about safety. It means either you can complete the takeoff safely, or abort the takeoff and still get stopped safely. It's just as relevant to the high performance turbine pilot as it is to the pilot of a Cessna 152; how much runway do you have, and can you get stopped if you need to? Can you takeoff safely considering density altitude and obstacles?

Eagleflip covered swept wings well; I'd add that swept wings often lead to a "dutch rolling" tendency, which is either designed out by engineering, or eliminated through special automated controls or control surfaces in flight.

Aside from moving the wing back from the advancing shock wave in front of the aircraft (often called a "bow wave"), sweeping the wings has the effect of increasing the aerodynamic chord, while maintaining a high aspect ratio (skinny, front to back, and long) wing. You can see this effect by placing a ruler in front of you. Measure the ruler from front to back, and it's say, an inch. Turn it some thirty degrees from you and measure the distance from the front to back, going in a straight line away from you. You'll find a longer distance, because you're measuring accross the ruler at an angle.

High aspect ratio wings (short front to back, and long wing) provide less drag and more lift, for a given airspeed. They also have other limitations. On a high speed aircraft, the drag increases rapidly in the outboard portions of the wing, where lift is generally greatest, leading to several types of problems. Sweeping the wings eliminates many of these problems due to the delayed effect of the bow wave affecting the wing, as well as increasing the aerodynamic chord. Control effectiveness is enhanced due to the delay in airflow separation, as well as the delay in the shock wave (which also forms on the wing, not just on the nose) moving aft to affect the ailerons.

Sweeping the wings creates spanwise airflow, as well as chordwise airflow, on the wings.

For now, concentrate on learning the aerodynamics that affect the airplanes you fly. These aerodynamic principles will be the foundation for all other aircraft you'll fly in the future. The lessons learned at the student pilot level are some of the most important lessons to be learned in your flying career. Good luck!!
Eagleflip did a nice job of describing airflow at and above the speed of sound, but that mach cone stuff doesn't apply to the speeds that most civil transports fly (Concorde excluded). It is the reason for the high sweep of the fighters, etc.

The other explanation by Cornelius describes it as most textbooks do, without anybody bothering to ask "why?"

The actual explanation is much simpler (if not more verbose). The whole trouble with approaching the speed of sound is that the air has less and less advance warning that the wing is coming. In low speed flow an object moving through the air transmits pressure waves in front of it. These waves move, of course, at the speed of sound, or, perhaps it is more accurate to say that all waves propogating through a medium move at the same speed. These waves start the air moving aside before the wing gets there. If the wing is moving at or near the speed of sound the air has no advance warning of the wing, so it is abruptly moved aside, forcing it to compress. It is literally "shocked", hence the term "shock wave". Shock waves suck in a lot of energy, and that energy comes from somewhere, in this case, the object that is doing the "shocking" has to provide the energy for the shock to exist.

Now, the air over a wing is moving faster than the general flow, and so parts of the wing will see supersonic airflow before the rest of the airplane (this all happening at much slower speeds than is required to get mach cones). The problem is in providing adequate "warning" to the rest of the airflow so it is not "shocked" by the arrival of the wing. The solution is to create a situation where you are creating that "warning". The wave from the wing is not only moving straight ahead but radiates outward in all directions. What sweep back does is to allow the bit of wing inboard of the "area of concern" to provide that "warning" to the air directly in front of the area we are concerned about. You can visually divide it into any increments you want, and in actual fact we divide it to approach infinity, making all of the calculations a reasonably simple calculus problem. Anyway, this makes the wing and airflow behave as if it were flying at a slower speed, as it has the "warning" of lower speed flow. At what speed is the wing acting like? Well, it happens that when you perform the calculations it aproximates the speed of the air if you were to divide it up into vector componets, with one moving along the leading edge axis and the other perpendicular to the wing leading edge. The wing acts about like it's flying the speed of the perpendicular component. Of course, in real life the air doesn't divide itself up, the vector model is just a convenient way to run the calculations.

Now, you can probably also see how all of this might affect the airflow and hence handling/performance at low speeds without further expanation from me.

This is in basic terms, apologize for introducing some slop in the explanation by the use of a more basic explanation.
There have been some very thorough posts here, but I'm gonna take a swing at this and see if I can simplify things a bit...

Chordwise Flow
Remember in basic aerodynamics; the curve on the top of the wing accelerates the air? Well that's great and is the whole point for the shape of the wing, but when you get going too fast it can cause problems.

On a straight wing the movement of the air, for the most part, is from the front of the wing to the back of the wing. This is called chord wise flow, because it is parallel to the chord of the wing.

Spanwise Flow
What a swept wing does is trick the air into thinking it is also moving along the length of the wing, from the root towards the tip. This is called spanwise flow because it is parallel to the span of the wing.

When the air moves from the root to the tip it doesn't encounter any curvature in the wing (camber) and so it doesn't accelerate. This allows aircraft to fly faster without having the air above the wing reach the speed of sound.

Critical Mach Number
When any air around the aircraft reaches the speed of sound a shockwave is formed. The speed at which this happens is called the "Critical Mach Number." Most pilots confuse this term with a lesser known term: "Force Divergence Mach Number."

The force divergence mach number is the speed at which the shockwave is strong enough to cause some real problems with drag and controllability. This is the speed that most people incorrectly refer to as "Critical Mach Number." Don't lose too much sleep over this one 'cause most people don't know it anyway.

Bow Waves -vs- Normal Waves
Unless you fly a supersonic aircraft, the only shockwaves that you will be concerned with are the ones on the surfaces of the wings. These are called normal shockwaves because they are at right angles to the wings' surfaces.

The type of shockwave that forms on the nose of the aircraft (or the front of the wings) only happens when the whole aircraft reaches the speed of sound. These are called bow waves. (To be more correct, this type of pressure wave is actually ahead of you everytime you fly, even in a Cessna, the only difference is that supersonic jets can actually catch up to it and make it stronger creating a shockwave.)

In reality, you don't worry about all of this on a daily basis when you fly a jet. You just make sure you don't fly too fast or too slow or too high for your weight and temperature. If you're in an advanced jet, the computers figure this all out for you. If you're flying an older airplane, you have to look at some charts once in a while.

Swept Wing Problems
There are really only a few downsides to sweeping the wings. One is a tendency to dutch roll, which is usually taken care of by a yaw damper, which is basically an autopilot for the rudder that keeps the plane from yawing around and making everyone sick.

Another is that swept wings don't provide as much lift at slower speeds so the flaps tend to be more elaborate and most jets also have "slats" that are sort of like flaps on the front of the wings. They both help to create lift at slower speeds so the jets can take off and land in a reasonable distance.

One more is that airplanes with swept wings have really bad stall characteristics. Because the tips tend to stall first, the center of lift moves forward and the airplane actually pitches up; just the opposite of what you want. Most swept wing airplanes have "stick shakers" that cause the control yoke to vibrate, simulating a stall buffet so that you can recover early. Some airplanes even have "stick pushers" for pilots who don't clue in quickly enough. A pusher will actually push the yoke forward for you so that you can't enter an unrecoverable deep stall. (I won't even get into Airbus fly-by-wire control laws.)

About as clear as mud? Ah, just keep the shiny side up and everyone's happy. Hope this helped. :)
Thanks Falcon Capt,

for the clarification. Your explanation makes perfect sense. I know I read what I used to think in more than one book or notes so I wasn't the only prop guy confused.

Another thing I seem to remember is that it only legally applies to part 25 certificated aircraft, not part 23. The principle applies to any twin but if you fly a part 25 aircraft you are legally not allowed to make a bad judgement call.

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