Twin Wasp

What you've described is parasite drag. Think of it this way, you have to move very few air molecules out of the way to push a bullet through the air. Many more molecules have to be pushed aside to move a house through the air. Parasite drag is broken down into some parts, friction drag caused by the air flowing over the skin of the aircraft , interference drag caused by air flow meeting as it passes over different parts of the aircraft (say over the top of the wing and along the side of the fuselage and form drag, just pushing the shape through the air.

Yeah, they talk about induced drag being caused by the vortices wrapping around the wing tip. What you need to know is that induced drag is caused by creating lift and increases as the aircraft slows down. Parasite drag is caused by moving through the air and increases as you speed up. Where the two lines cross is your best L over D.

And you wonder why people say you should get your CFI.

Cubdriver

Funny someone mentioned water, because Hydrodynamic theory (theory of water flows) served as the basis of aerodynamic theory, which is the physics of water flowing around the hulls of ships. It was motivated by the need for faster warships many centuries ago. Basic low speed aerodynamics is really just hydrodynamics up to an including theory pertaining to circulation. Slow-speed air is considered to be incompressible as water is incompressible. Hydrodynamic theory is and was a good fit to aerodynamic theory of low speeds flows, and early theory on the subject was derivative. High speed aerodynamics is really not something most people care much about because there are a lot more variables owing to the entropy losses and thermodynamic effects, and accounting for those losses requires some fancy math. All textbooks therefore start with low speed flow, which should be understood before moving on to high speed flow.

Ok, I have a minute before going back out into the wild blue yonders of America so let me take swag at it.

Inviscid means the flow does not have much thickness to it, for example it has no shear-stress properties like say, motor oil. Inviscid means the flow has easily-flowing properties much like a noble gas. If you stick your hand into a flow of a fast (but not too fast) moving inviscid fluid, say helium, you would not feel very much force on your hand. Inviscid theory does not include the existence of drag because it takes some viscosity to create skin friction drag. We call this skin friction drag or parasite drag. For example, if we drive a truck through pure helium there will be no drag of this type, but if we were to measure the drag driving it down a normal highway there would be many pounds of drag from skin friction or parasite drag, and form drag as well. There would be no form drag for the inviscid case because according to theory the flow would perfectly slow back down to its prior speed when it got to the back side of the truck. In real life we know this is not true, because it take many pounds of force to overcome form and skin friction drag, but as long as the fluid is inviscid the theory is correct. Waxing the truck might help a little bit on the skin friction drag. Reducing pressure gradients by shaping the truck a bullet would reduce the form drag part. Inviscid theory is usually introduced first because it is true, simple, and Bernoulli's relation can be used to determine the pressure-velocity field around an object.


Reynold's number is the ratio of the inertial to viscous forces for a fluid and serves to help predict how flows will behave to a certain extent. For low Reynold's numbers there is a lot of skin friction drag. A paper airplane has a very low Reynold's number and experiences a lot of viscous effects, while a 747 does not experience these effects very strongly. Going back to our truck example, if the air speeding around it encounters a high enough gradient (change) in pressure as it works its way around the back, it may exceed the allowable shear stress of the air itself and on the back side of the truck the flow will separate or burble. No lift is involved in this example, but when there is lift the Reynold's number largely determines whether the air coming down the top side of a lift producing airfoil will stay put or decide to burble off the top of the wing.

Does this help any? I may be able to drone on some more if you like, but you'll have to be patient because I am going back on the road here in a bit.

slipped

OK the hand out the window WOULD work but really your hand is not a wing and is not going to make much lift. You got the idea though, for sure.

detpilot

Correct me if I'm wrong, but I think total lift acts perpendicular to the chord line, while the vertical (relative to the airplane) component of lift was perpendicular to the relative wind.

Chord line tilts up (increased angle of attack), the total lift vector angles backwards (higher drag component).

ryan1234

The lift is always perpendicular to the local relative wind. The trailing vortex sheet's downwash changes... actually inclines the relative wind and an aft angle to the freestream. The angle at which the vorticity does this is the induced angle of attack. The component of lift that is parallel to the free stream is induced drag.

The total lift is not actually perpendicular to the chord line. The local angle of attack is the difference between the chordline and the local relative wind. The angle from the freestream to the local relative wind is the induced angle of attack. The local angle of attack plus the induced angle of attack equals the total angle of attack.

In my first post in this thread, I was trying to a little too simple - and left out some key differences. Local Relative wind is not quite the same as what we think of as 'relative wind' (which is the 'freestream' in this case). Hopefully this post is a little more specific.

leardriver

Clear as mud.

Wow you guys are smart!

I was taught many moons ago. Induced drag is the downwash off the back of the wings. It is a by product of producing lift. The more lift (higher aoa) the more induced drag.

What does all this mean to a regular pilot type like me? Well i learned as a lear pilot. cruising in choopy air helps save gas. Ride in the turbs and you will have to pull your throttles back a bit. The turbulences releases some of the downwash from the wing lowering your total drag. It is way more obvious in the large aircraft such as the 747 classic i flew.

Now thats some aerodynamics soup an average joe can use!

How much fuel are we going to land with?

atpcliff

Hi!

I am a bit bummed, because this thread is so technical, and all.
It thought it would be a humorous thread about male pilots who were talked into flying while wearing women's clothing!

cliff
NBO

IFlyRandom

I could give you a very technical explanation, but it seems you have already read it anyways.

Facts: Air is a fluid. Lift is all about pressure differentials on a wing. You have to have velocity to have lift. You can't avoid drag if you have lift.

In your car, if you get to above 40 mph (to clearly experience the effect)
and stick your hand out the window (forearm PARALLEL the car for better experience) your hand and arm will be pushed UP and BACK. The BACK part of pushing is the induced drag. As simple as that.

The angle between oncoming wind (relative wind) and your arm is the AOA. The more your rotate your arm upward, the greater BACKWARD pressure will become.

It is impossible to have AOA and have all of the actual pressure force act perpendicular (straight up) to the relative wind. The actual pressure force will ALWAYS act at a rearward angle (UP and BACK). The upward part is LIFT that we want. The backward part is INDUCED DRAG that we dont want, but have to have as inevitable by-product of lift generation.

AOA + motion = > pressure increase on the bottom surface of (arm, hand, wing, ironing board, whatever), pressure decrease on the top surface of the same

Pressure increase on bottom/decrease on top => Pressure differential

Pressure differential = > Actual pressure force acting upward and backward

Pressure force => Lift (upward) + Induced Drag (backward)

More AOA => More Lift => More backward acting force => More Induced drag


Keep digging at it, just not while in an airplane!

IFlyRandom

Sorry, but I respectfully disagree. Your hand, given enough forward velocity, at a given AOA, will generate nearly the same amount of Lift in PSI(of PSF if you prefer) as ANY wing at the same velocity, air density, and AOA. Your hand is not as clean or efficient as a wing but most of the lift WILL result simply from AOA.

Stick your hand out the window, give it some AOA and accelerate to any large/heavy a/c rotation speed, you will NOT be able to keep your hand down! Even at 70 MPH (approximate rotation speed of many small a/c) on a highway try keeping your hand at a low fixed AOA, you will find it very tiresome very quickly as your hand just can't wait to fly away.

It will produce tons of lift, in relative terms, in PSI/PSF. The only thing that stops your car from lifting off is that your hand (your simulated "wing") is of insufficient square area. It is generating sufficient lift per square area all right, but the square area is too small.

If you made your hand sufficiently large and rigid, your car would lift off the ground and fly away (at first, anyhow ) the moment total lift would equal the weight of your car.

Cubdriver

Well, not quite. There are many forms of drag and induced drag is only one type. As an easy experiment, the hand-out-the-window technique is unable to distinguish between them. Form, parasite, and induced drag are all there in the mix, a big part of it in this experiment is form drag. I do not know any simple experiments that will show only induced drag isolated from the others, because any lifting surface will carry along with it some of the others. It is true that if you vary the angle of your hand while driving, you can discern a big difference in the pull backwards, which is coming from the induced drag. But the hand-in-the-wind experiment has a lot of form drag, because a hand is bluntly shaped. A piece of painted balsa wood would have less form drag.