Some force diagrams to clear up misconceptions

[Flyingwingbat1]

Here's something I belatedly made up for another thread, but I thought it might be useful here. It deals with forces acting on a plane for various flight conditions. All diagrams are for constant velocity flight. The lengths of the lines indicate the relative magnitudes of the forces. Note the relationships between thrust/drag and weight/lift, especially. I hope this helps some folks.

[Brandano]

Your constant rate of descent does give the impression that the drag is actually higher than in level flight or constant rate of climb... maybe you should make such a diagram but assuming the same plane in all situations. The only place where the drag should have such a massive change is in high AOA level flight
[edit] Oh, yes, you should also add speed to the equation. Drag and lift increase with speed for the same AOA.

[Flyingwingbat1]

Thanks for the crits and comments. I will do a more accurate diagram later on, as time allows. As for the drag being higher in descent vs climb or level flight; for a given aircraft and a given thrust level, it WILL be higher, though not as much as my diagrams indicated. Think of it this way; you know a plane will achieve a higher top speed in a dive vs. straight-and level. What maintains the plane's higher speed in a dive? Thrust plus Gravity, or at least that portion of gravity that's directed ALONG the plane's flight path. The ultimate would be a 90 deg power dive straight down. ALL the thrust plus ALL the plane's weight will be balanced out by drag at terminal velocity. Drag can be less than thrust too; the ultimate example would be a 3D plane in a slow, pure vertical climb; most of the the thrust is used in overcoming the plane's weight in that case.

[MarkusN]

Good stuff. One nitpick, though: Lift in the "High AOA" example should be vertical, not perpendicular to the wing.

[Brandano]

Ok, but that won't be a constant rate of descent, will it? Unless you mean that, by going faster, the drag increases so much that excess thrust won't be able to accelerate the plane further. If instead we assume comparable speed for both constant rate of climb and constant rate of descent, we should see the same amount of drag, but a large difference in thrust.

[Flyingwingbat1]

Brandano:

As long as the arrows form a closed loop, tip-to-tail, then the aircraft will be at constant velocity. You're right about comparable speed.

MarkusN: thanks for the correction, the diagram will be fixed shortly.

[kcaldwel]

One thing to keep in mind is, by definition, lift is perpendicular to the flow, and drag is parallel to it. These directions are rarely also perpendicular and parallel to the aircraft as shown in your first four diagrams.

There are no such constraints on thrust, which will rarely be parallel to drag, or perpendicular to lift. These effects make big differences to the actual force diagrams.

Kevin

[vintage1]

well a vector is a vector.

Splitting it up into lift and drag is merely a convenience. It means less than people suppose.

[MarkusN]

well a vector is a vector.

Splitting it up into lift and drag is merely a convenience. It means less than people suppose.
True. But having convetions about use of terms heps tremendously in discussions of the subject. And wing section data IS presented in the form of components.

[kcaldwel]

es, if you are trying to clear up misconseption, I think it would be best to follow the conventions aeronautical science does. They use them for good reasons.

Your diagrams are inconsistent - the last one at high alpha level flight shows the lift and drag with respect to the air flow, not the aircraft axis as the other ones.

Kevin

[Accu157]

I don't see what the misconception is. :confused: I get the idea behind the diagrams, when in constant velocity flight, the net force on the aircraft is zero. Newton's first law. If following aeronautical conventions to their entirety, isn't it possible that would just introduce more confusion? Or can it be reduced enough to make things clear? Otherwise, I'd say these diagrams are close enough.

[Flyingwingbat1]

Accu157:

Kcaldwel is correct; there are some inconsistencies. The thrust vector does NOT necessarily have to be parallel to the flight path, as shown in the last diagram. The other diagrams were meant to depict high-speed flight, in which case the AOA will be low, and the fuselage centerline and thrust vector, for all intents and purposes, would be parallel (or nearly so) to the flight path. Thrust lines differ depending on the aircraft, obviously.

Here's some more accurate diagrams. There's more to do.

[Neil Stainton]

Thanks for these diagrams, I found them useful.

They prompted me to think about calculating the optimum climb angle of an electric glider, but rather than post an off topic question here I have started this thread (http://www.rcgroups.com/forums/showthread.php?t=763585)

Regards,

Neil.

[kcaldwel]

This is an example where these diagrams are deceiving, and definitely won't clear up misconceptions. The thrust line will not be perpendicular to the lift vector as shown in the top left diagram.

The reality will be part way between the top left diagram, and the lower left diagram. And the distinctions are necessary to calculate the best climb angle.

Kevin

[Flyingwingbat1]

The original diagrams are gone now. The newer ones are better, and they show the thrust vector at its own angle, not always perpendicular to the airflow (and by logical extension, not always perpendicular to the lift, or parallel to the drag vector)

[kcaldwel]

Looks great Taillessflyingmammal1!

Kevin