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Josh-D. S. Davis

Xaminmo / Omnimax / Max Omni / Mad Scientist / Midnight Shadow / Radiation Master

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Airplane lift, drag, rambles
Josh 201604 KWP
joshdavis
Flying is all about energy management. Two forms of energy, one conversion process between them, and an inefficiency factor. A Frisbee is an excellent example of a simple aircraft. Thrust is you throwing it. It has Lift from a pocket of pressure under it and a vacuum above it due to the shape, which causes it to defy Gravity so long as it's speed is fast enough and so long as it stays horizontal (more spin helps). Eventually, wind resistance, known as Drag, will slow it down, and it will begin to fall, until it lands.

So, the four basic forces in flight, with their counterparts, are:
* Thrust counters Drag
* Lift counters Gravity

Thrust is the energy input. For a frisbee, it would be the throw. For a glider, the tow rope or plane. For an airplane, it would be the engine. Thrust is a conversion of chemical energy (fuel) into kinetic energy (movement * mass). You use that kinetic energy either for forward movement, or conversion into Lift, which both keeps you safely away from obstructions, and provides a potential energy reserve due to Gravity. Thrust can be provided by one impulse (explosion, throw) or multiple (engine with multiple explosions per second). Thrust can also be provided by the wind. If it's at your nose, it slows you down. If it's at your tail, it speeds you up. But the wind can go in any direction, including up. Catching a ride on an updraft gives you vertical speed, which increases your potential energy. The key summary point is that Thrust Energy can be converted into airspeed.

Gravity is how hard the Earth pulls it down. Gravity must be overcome for flight. You overcome gravity by spending energy. If you're converting kinetic energy (movement) into potential energy (altitude) by way of Lift. The higher your altitude above the ground, the more potential energy you have. You can convert this altitude into kinetic energy. Effectively, you control your fall to be at an efficient speed which keeps you moving forward until you find a safe place to land. You can counteract this by application of more thrust, and conversion of that thrust into lift. You can also counteract this by flying into an air mass that is moving upwards faster than you are falling downwards (riding a thermal). That's free energy. The key summary point is that you can regain some airspeed by trading altitude into the gravity bucket.

Lift is how much your aircraft climbs. Lift is created by trading kinetic energy into a change in direction, usually perpendicular to the kinetic energy converted. If your kinetic energy is forward, and your lift is up, then this trade creates potential energy (altitude). If the lift is sideways, it changes your direction.

The catalyst for this conversion is a fluid, in our case, air. As the air moves over the wing, it moves faster, so it creates a vacuum (venturi effect). It might help to think of it as also trying to pull away from the wing at the thickest part, which also creates a vacuum. Because of the weight of the wing, and air flowing under, you'll also have pressure build-up under the wing, but even for a flat wing, there will be a pressure differential. The difference between these pressures is lift. The more air over the wing, the more difference in pressure. More air can be gotten by flying faster, or in thicker air (colder, or lower altitude).

The angle of the wing also affects the pressure differential (more pressure under), which causes more lift.

Lift is not a 100% efficient conversion. Some energy is lost to drag. The increase in drag from a higher "Angle of Attack" or higher airspeed will cause you to slow down unless you apply more engine power. Because of this, the saying is "Pitch controlls Airspeed".

If you apply more thrust while maintaining your angle (pitch), then you don't speed up. You might have a tiny burst, maybe 5% or less, but then the lift increases, and your plane climbs, which increases drag, which keeps you from accelerationg. Because of this, the saying is "Power controlls altitude".

Drag is effectively wind resistance. Any energy expended to overcome drag is 100% wasted. Once energy is dumped into drag, you cannot get it back. There are two major categories of Drag.

"Induced Drag" is a byproduct of lift. This is where the energy is put into churning the air which doesn't provide lift. It can be in the form of wake, wingtip vortices, friction heat, vibration, etc. The faster you go, the lower the effects of Induced Drag.

"Form Drag" is inefficiency of design, such as a wrinkle in the skin, a rivet sticking out, landing gear hanging low, etc. As you go faster, the exponentially higher the effects of Form Drag.

All types of drag are based on the design of the airplane, and the indicated airspeed. I say Indicated because in thinner air (hotter, or higher up), you have less mass pushing against the plane, so to get the same effective force at say 40,000 feet, you would have to fly at twice the True Airspeed to get the same effect on the wings, etc. You fly up there to get better True Airspeed) for the fuel used. The Indicated Airspeed is the effective speed for the basic forces, but there's a catch.

The Mach Limit (speed of sound) is based on the true airspeed, and the speed of sound decreases as the air gets colder (aka the higher you go). This is an issue because if you exceed the "critical mach" of a wing (commonly 0.7x to 0.9x on commercial airliners), then a bubble forms on the wing, destroying lift. Without lift, the plane noses down, and speeds up. You can't "pull up" to stop the descent, because the wing doesn't have enough lift. Maybe if you idled the engines and had speedbrakes which wouldn't tear off at that speed... but really, you'd put so much force on the plane that it would come apart, either from the air pressure of accelerating down, or from the ground pressure of suddently decelerating.

Anyway, in measuring Drag, at any given indicated airspeed, for any given airplane, the sum of Induced Drag and Form Drag is known as Total Drag, or simply Drag. Total Drag starts out fairly high at low speeds, then drops as ID goes down. However, as you speed up, FD increases. Remember, the more Drag you have, the more of your energy is wasted. There are 2 ways to decrease Drag. You can change the shape of the plane (expensive and a lot of paperwork), or you can change your airspeed. Changing your airspeed changes the amount of drag for any given lift. This is known as the L/D ratio, "lift to drag ratio". If your L/D ratio is higher, then you waste less energy to Drag in your creation of Lift. Your best LD ratio is where your ID and FD meet. This point is known as your "Best Glide". That's what you fly at when you have no more energy inputs (engine fails, not enough wingspan to ride up a thermal) and you're not on final approach to a runway.

Drag slows you down and gives you nothing in return. To keep flying, you have to have a minimal amount of air passing over the wings and control surfaces. This minimal speed is known as...

Stall Speed, or "Vs". There's also Vso, Vs1, Vs2... The o (sometimes shown as 0) indicates it's in landing configuration. Vs1 and any other mean any other configuration. Vs with no number means clean configuration (least drag). You can decrease the stall speed to the landing configuration, called Vso, but you increase drag (waste energy) substantially to do that. Until you are sure you have too much energy for your needs, you need to be as efficient as possible. That means flying at best glide (usually around Vx, higher than Vs).

The reason you need to "keep flying" after an engine failure is because flying means countering gravity with lift. If you don't do that, you fall. This is called a "Stall". An airplane Stall means there is insufficient lift being generated to control the plane. The relative wind is no longer in-line with your control surfaces, so you can't control the plane. Your drag goes way up which causes a loss of energy, but you know, gravity always wins.

Without control authority, you can no longer keep the muddy side down and the shiny side up. You end up accelerating towards the ground. If you're close to the ground, then the sudden deceleration at contact would cause you and the plane to come apart in unrepairable ways. Stall/Spin accidents have the highest death rates of any other types of aircraft accidents. You want to avoid them.

If you're far enough up, and slow enough, you can suggest the plane aim into the relative wind, and after a bit, you'll be flying again, and regain control authority until you run out of energy. Your wings won't have control authority, but if your center of gravity is far enough forward (ie, within the certified flight envelope), the control authority of the tail is increased. Why? The tail is a big lever that swings the plane around on its center of gravity. A longer lever means less force (airspeed) is required to swing it around. The tail swings the plane left and right with your feet (rudder pedals) and up and down with the elevator (forward and backwards on the stick or wheel). When stalled, keep the ailerons level (no left or right) because they work by increasing lift on one wing and decreasing lift on the other. This can cause one wing to become MORE stalled than the other, and will impart a spinning motion into the plane. Spins take more time to recover, and as said above, stall/spin accidents are killers. This is especially problematic because we're used to using our arms, or a wheel to 'turn' things, like a car. You don't do that in a plane. You BANK with the wheel, and you TURN with your feet.

Also, these controls aren't for relation to the Earth. It's in relation to the aircraft. If you're banked 60 degrees, most of your elevator is functioning like a rudder, and most of your rudder is functioning like an elevator if you consider in relation to the ground. Much of your lift is going into the turn. If you're stalled, you can't use your ailerons to get you out of the bank, and pulling back won't raise the nose. You use the rudder to rotate the plane, which increases lift on the stalled wing, which will rotate you back upright.

There's one other possibility here. If you don't recover control authority, the plane continues to accelerate until....

Velocity Never to be Exceeded, or Vne. This ties into drag. In addition to wasting energy, Drag places forces against the structure of the plane. If you put too much force on any part of the plane, it will deform. That leads to flutter (think of a flat jackhammer), loss of control (no, I'm aiming THERE but the controls are pointing HERE), and in-flight break-up (The wing snapped off!). The speed at which a plane is certified to withstand the forces of level flight is known as "Vne".

Now, a plane isn't guaranteed to hold together at Vne always. If it's turbulent, then that speed is Vno, which we remember as "Velocity Normal Operatin" but which really means "Velocity for maximum structural cruise". This is the speed where if you get hit by gusts of 20inch/second, the structure will hold up. If you're fighting the controls, then the certified speed is "Va" or which is "Maneuvering Speed" but you can remember it as "Accelerated Stall". Va is the speed where if you yank back on the stick with full power, you'll stall the plane before you break the plane.

Now, while nothing is 100% efficient, much effort is put into reducing drag. How much depends on the missions the aircraft will perform. All things considered, if everyone were willing to fly around at 90mph, efficiency would be MUCH higher, but there are other needs. Some planes need to be able to fly at 40mph so they can take off and land on very very short fields. Other planes need to be able to fly at 500mph to keep from having to store as much food for the passengers, or to deliver supplies or mail.

Remember, the mass of air flowing over your wings determines the amount of lift. If you had a big wing, with high lift, and high thrust, you would keep climbing until you hit mach tuck, then you'd come back down and probably break up in the process. As such, If your plane goes fast, it will have small wings designed for relatively low lift. This also means less induced drag. Also, for these planes, it's worth the money to design them for lower form drag.

If the plane is designed to fly slowly, then form drag isn't as important because you'll never get fast enough for it to be a huge issue. The wings will have higher lift, so the plane can stay in the air at the lower speeds. This might be bigger wings, or thicker wings (more bubble on top means more venturi effect, more lift, but also more drag). You might still need the same sized engine for the same sized plane, but one might be able to land in the outback, and one might only land on a 13,000 foot runway.

Usually, more advanced planes will have wings that can change shape during flight. This allows them to have high lift at low speeds so they can get off the ground and begin climbing (remember, altitude is your energy source after engine failure). But, once you're at cruise altitude, or at least safely away from the grouns, you convert the wings into a lower lift, but lower drag shape that allows you to fly faster using less energy.

Among the lowest L/D ratios are 2.5:1 in the squirrel suit some people use to fly their bodies... Best glide is around 250mph in those. Thruse is from the tow plane that pulls you up, and then you convert altitude into airspeed until you are close enough to land. Since the human body can't land at 250mph, usually there is a point where a parachute is deployed, which slows them down to 30mph.

On the opposite side of the spectrum are gliders. These range up to 60:1 for the best technology, but good, human piloted gliders will be around 28:1 or so. These can stay aloft for hours and climb thermals and mountain waves up to 40,000 feet from a simple 2000' tow.

For a common GA plane, L/D is from 7:1 to 13:1. That means with a dead engine, you can go forward in still air 7 miles for every 1 mile of descent. This is affected by winds. If your best glide is 90mph, and the wind is 45mph, then you can cover thee times the ground going downwind than upwind. This is because going upwind, your groundspeed is 90-45=45mph, but going downwind it's 90+45=135mph. Take this into account (and terrain) when planning an emergency landing.

Now, your best glide speed and your stall speed do vary with the weight of the plane, but in a small plane, the difference from minimum to maximum weights might be 82mph to 90mph. You can't really calculate this on the fly when you're freaking out because of the loss of engine power, so just keep it at best glide as published in the book (should be pretty close to the attitude for maintaining Vx).

I've been writing on this for hours. I finally have all of the info I need in here, and it all flows properly together, but I don't have a closing. Tough patooties. I'm tired of writing. This was more for thought organization than anything. So, I'll give you a summary:

* Flying is about energy management. Speed and altitude are your two primary sources of energy. Thrust is secondary.
* If you lose engine power, save energy by flying at the best glide speed for your plane. Have it memorized.
* Flying too slowly can stall the plane, whether you have engine power or not.
* Stalling near the ground has a high mortality rate.
* Turning decreases vertical lift and should be avoided during take-off engine failures.
* If your plane is stalled, do not turn the wheel.
* An aft CG will negatively affect your stall recovery.
* Steer with your feet. Push forward to increase airspeed. Increase engine power to increase lift.
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