Hi ive been reading through martin simons basic of model aerodynamics and so far its been really helpful but i still go a couple things that a dont understand:
1) It says the lift is equal to weight and drag is equal to thrust. I understand the second part but about the first part i dont get. If a plane is going 250mph its giving more lift than 50mph right but the weight will still remain the same? So why is the weight always equal to the lift.
2) I haven't read this in the book but theoratically on a wing (and only the wing) is lift = drag. Cause if i increase camber, AOA, thickness,speed, air density they all increase both lift and drag? (this true)
3) Vescocity is drag, right?
4) For some reason i still cant get the P factor if a prop turns clockwise from the cockpit view shouldn't the P factor shift the plane right not left?

Thats all the dumb questions i can think of now probably haev some more later.

Thanks alot,

Papa

1) The faster you go the smaller the angle of attack, hence the same lift at a higher speed. The smaller the angle of attack (at a given speed) the smaller the lift. So taking speed, wieght and AoA as an equation, it's always constant for level flight. I tried to simplify the explanation, so I hope it still makes sense.

2) For the most part you get drag whenever you generate lift. There are other sources of drag, however.

I'll let the others answer the rest.

-Q.

Papa,

1) If lift is greater than weight, for level flight, then the plane will be moving and/or accelerating upward. When speed increases, something has to change to keep the lift the same, if you want to fly without climbing. Most likely, what changes is the angle of attack, as a088008 says. You don't think about it this way when you fly, you just use the elevator as needed to keep flying at the same altitude. The result is that you reduce the angle of attack until the lift is again equal to the weight. If your plane were doing a tight turn, or climbing, it would not be true that lift=weight.

2) There is no general rule that lift=drag, even for the wing. In fact, a wing for which that is true would be performing very badly indeed. There is a component of drag, called induced drag, which is a direct consequence of the fact that the wing is generating lift, but this drag component is much smaller than the amount of lift being generated. It is true that increasing any of the values you list increases both lift and drag ( although increasing AoA increases lift only until stall occurs ), but that doesn't mean that lift and drag are the same, it just means that they tend to increase and decrease together.

3) Viscosity is a property of air, which you can think of as its 'thickness', in the sense that oil is thicker than water, and honey is thicker than oil. Air is 'thinner' than all these, or less viscous, but it still has some viscosity. Viscosity causes drag in several ways, some of them rather indirect. One component of drag, the skin friction drag, is quite directly caused by viscosity, and is sometimes referred to as viscous drag.

4) P factor is a little tricky to visualize. The best way is to get a 2-blade propeller in your hand ( fortunately, conventional R/C props have the rotation you describe in your question ), and look at what happens to the angle of attack of each blade when the prop is horizontal and the aircraft is at a positive angle of attack. In this situation, you will see that the right blade has a higher angle of attack, relative to the oncoming air ( recall that the aircraft is at a positive AoA, so that oncoming air is coming up at the prop a little bit ). Thus, unless the blade stalls, the right blade makes more lift than the left one, which will tend to turn the plane to the left. Don't worry too much though, the P factor is a small influence on the plane, and can usually be safely ignored.

There aren't any dumb questions.

banktoturn

Regarding the P factor.

I read an article once where Mustang fighters were used as an expample of the P factor.

With the brakes on there is no P factor as the prop on both sides sees the same air. But as the plane starts it's takeoff roll the air is coming into the angled back disc at an angle. Because of this airflow and the angle of the prop disc with the tailwheel on the ground the upward traveling blade is seeing a much smaller angle of attack than the downward traveling blade. (If you think up and down blades then it doesn't matter which way the prop is turning) So the downward moving blade gets a bigger bite and pulls harder causing the plane to want to veer away from the down moving blade. Once the tail comes up and the angle of the blade disc to the airflow is reduced then the P factor is reduced along with it.

Good Morning,

The lift = weight and thrust = drag comes from the physics that effect the motion of things. Remember the old F=ma (force = mass times acceleration). If something is sitting still or moving in a constant straight line at a constant velocity then all the forces on it are equalized so that there is zero net force and no acceleration. To change position or direction you have to unbalance the forces and create some acceleration.

If you are flying level at a constant speed (a steady state condition) then lift = weight and thrust = drag. All of the forces are equal and the airplane tends to keep flying straight and level.

If you increase the power setting or angle of attack you unbalance the forces. Then lift no longer equals weight and thrust no longer equals drag. The airplane speeds up or down and goes up or down (what ever the change in conditions and its aerodynamics allow it to do) until the forces on it are again all equal.

When a new steady state condition is achieved then again lift = weight and thrust = drag. That is also a boring way to fly unless you are in a passenger airplane. Most aerobatic airplanes are never in a steady state condition and lift never equals weight. When looping for instance lift is greater than weight so the airplane accelerates up in a smooth way and the result is a change in the position of the airplane so that a loop is the result.

On the lift = drag question. If we measure it in a wind tunnel to get the answer (the most accurate way) for a particular airplane we find it is a curve relationship depending on the characteristics of the airplane in question but is generally shaped like the little picture I have attached. As the lift goes up the drag goes up much more. The wing itself and even the airfoild has the same kind of shape. The particulars of the shape depend on the thickness of the airfoil, camber, and most everything else that is related to the geometry of the airfoil, wing and airplane.

This kind of stuff is interesting isn't it?

Good book, by the way.

There has always been arguments over whether the pull to the left on a plane is torque, P-factor or a spiral prop blast. You can create experiments that show each to be the culprit so I would assume all of them have some effect.

On smaller RC planes you get very little P-factor or torque effect and, as has been mentioned, you can for the most part ignore it. As planes and props get larger, it begine to show up. Even the smaller giants, 80" spans with engines turning an 18 inch prop will exhibit a left pull on a pull up. The quicker the pull up, the more the left movement. It is definitely noticeable on squares. With the big planes, you learn to use the rudder and the throttle.

Another comment on lift and drag.

It should be obvious that in level flight lift equalst weight, otherwise the plane would rise. As you go faster, the wing generates more lift, causing the pilot to trim the nose downwards, neutralizing the increased lift from the speed.

You may notice some overpowered trainers at your field. Check the elevators at neutral stick. They are probably trimmed down to compensate for the higher cruising speed.

In a full scale, the wing incidence is set for the most economical flight at cruise speed. In other words, at the cruise speed, the wing and fuselage will be level with the air flow for the least drag. If you fly faster than normal cruise, you will need to trim the nose down. Now the fuselage is presenting a larger cross section to the airflow, creating more drag.

The ideal solution to this is a swing wing airplane. Many years ago (in another lifetime) I flew the F-111. The normal cruise was .9 Mach. With a 26 degree wing angle, which was the nominal cruise wing position, the AOA was fairly large negative, I forget the actual amount. The fuel flow was right at 6000 lb/hr. Several of us started playing with the wing settings, finally arriving at close to a 50 deg. sweep. At this wing sweep, the AOA was close to neutral. You could hold .9 Mach at 4200 lb/hr fuel flow, a huge savings. This was, of course due to the drag reduction from the airframe meeting the relative wind at a more efficient angle.

Ed - was it due to the fuselage changing it's angle of attack to a lower drag condition or just the lower drag at that mach number due to the increased wing sweep angle. There is a big drag difference between 26 and 50 degrees sweep especially at .9 mach number. I could see 26 degrees at M=.6 or so but not at .9.

Although we never made a wing sweep airplane at McDonnell Douglas we studied them relentlessly. Lost the F-14 contract with one. Won the F-15 without one. Finally gave up on them.

If the CG of the airplane is located so that at a low sweep angle the airplane had good flying qualities (true there is a lot of electronic wizardry going on in the control system too) it would give an angle of attack of the fuse and a trim setting on the tail.

Move the wing to a high sweep and the neutral point moves aft making the airplane more stable requiring more trim from the horizontal, the CLalpha of the wing decreases so the angle of attack of the airplane goes up to make up for it and indeed the fuselage is at a different AOA.

The induce drag from lift is the same in each case, the trim drag probably increases, the wing sweep drag is significantly lower and the fuselage drag may decrease.

Looking at them I doubt the fuse drag decrease alone is enough to show the fuel flow difference you mention.

It is something to do to pass the boring hours though isn't it. You do have to wonder why the flight manual had you doing it that way.

The AOA change was 10 degrees or so and I always felt exposing more of the big broad fuselage, especially the top added a lot to the drag. I am sure that it is a combination of both.

We here at Eglin AFB, where munitions are done, hated to see the 'Vark go, it was one of the best bombers we ever had.

It is interesting to me that the B-727 flies more efficently in cruise at mach .80 with a somewhat aft CG. I think this has to do with unloading the tail surfaces.

Question: Isn't some of the "P" factor due to the prop turning right causes the fuselage to twist to the left?

Thanks alot for all the answers,
New questions:
1) In renalds number its density/viscosity * length * velocity. What the difference between density and viscosity?
2) Is ther a formula that tells us the stall angle of the AoA of a wing for a plane (bad english)?

Thanks alot,

Papa

PaPa,

Density of a fluid is the mass divided by the volume. Some fluids are denser than others; for example, water is much denser than air, since one cubic centimeter of water has much more mass than one cubic centimeter of air. Since air is compressible, air at higher pressure is also denser than air at lower pressure. Viscosity is an entirely separate property of a fluid. It is more complicated to define and describe viscosity entirely accurately. Roughly speaking, viscosity is a measure of how hard you have to push a fluid to get it to flow. For example, it is much harder to push honey through a small hole than it is to push air through the same hole, because the viscosity of honey is much higher than that of air.

Unfortunately, there is not a simple formula to calculate stall angle.

banktoturn