What is "lift"?

[RatherBeFlying]

Etkin's Dynamics of Atmospheric Flight is a rigorous treatment of aerodynamics for those who want to get a real understanding. The math is heavy.

In the Vehicle Frame of Reference (one of several), there are forces along the x, y and z axes and moments around those three axes.

Surfaces and bodies moving through the air produce forces and moments. Thrust too produces forces and moments along and around these axes.

g never fails. In a glider the thrust is m*g*sin(alpha) in "level" flight.

[photofly]

I need to hear about gliders with thrust. Tell me more.

Is there a diagram in Dynamics of Atmospheric Flight that illustrates the balance of Lift, Drag, Thrust and Weight for specifically for a glider? If so, could you post a scan or picture of it?

[goldeneagle]

I need to hear about gliders with thrust. Tell me more.

This one is my favorite...

[photofly]

I enjoy the perspective in that picture... looks like the aircraft is 1000x bigger than the car just underneath it....

[RatherBeFlying]

Screenshot_20200723-125102.png (313.83 KiB) Viewed 1240 times

[photofly]

And where does it say any of those diagrammed quantities is Thrust?

I don't see any forces in either diagram. In 4.6, g is an acceleration, V the direction of flight. It's unclear just from the diagram what physical quantity Czv is, but it's a scalar quantity so can't be a force (which should be a vector). And there's no suggestion of any kind of equilibrium balance there. 4.7 is all about moments.

Is there a diagram that shows the balance of Lift, Drag, Thrust, and Weight, for a glider?

I'm just really curious about this statement: " In a glider the thrust is m*g*sin(alpha) in "level" flight." because it sounds off, to me.

The reason it sounds "off", is because a usual treatment of the force balance of a glider in steady flight is "Lift + Drag + Weight = 0". There is no thrust, because thrust is the force provided by the engine(s), and a glider has no engine. But if Professor Etkin has a different force balance for a glider, or a different usage of the word "Thrust" I should like to read about it.

EDIT: Through the magic of Google, one can search the index of Prof. Etkin's tome, and I see it doesn't have an entry for "glider" - so I don't think he's going to be able to help us cut through to the meaning of " In a glider the thrust is m*g*sin(alpha) in "level" flight."

Can you help me out?

[rookiepilot]

viewtopic.php?f=118&p=1122693&sid=94577 ... 9#p1122693

Not enough Lift.......

https://www.youtube.com/watch?v=7V2XtrC ... e=youtu.be

Just barely enough.......

[RatherBeFlying]

We can define thrust as the force along the vehicle x axis. That force can come from a prop, gases from exhaust including turbines and gravity.

Etkin does reserve the T term for engine thrust, but in a glider the T term is zero and the "thrust" is m•g•sin(alpha) which in a steady state = drag.

Going back to the original question in this thread:

Screenshot_20200723-163329.png (392.29 KiB) Viewed 1169 times

I do not pretend to totally understand these equations, but the ground school textbook authors don't seem to have either.

[photofly]

We can define thrust as the force along the vehicle x axis. That force can come from a prop, gases from exhaust including turbines and gravity.

Well, no. Nobody defines thrust like that. Not Etkin, not Von Mises, not anyone. Thrust is the force provided by the engine, or engines. And that's it. It's not the "total force along the x-axis of the vehicle", and never includes gravity. There's no need to change the definition or invent a new one. It doesn't help anyone to do so.

Etkin does reserve the T term for engine thrust

Of course he does. So does everyone who has cause to examine the subject.

but in a glider the T term is zero and the "thrust" is m•g•sin(alpha) which in a steady state = drag.

Why do you want to say Thrust = 0 (which it is) and then invent a new thing called "thrust" (in quotes) that isn't zero? How does that help us understand the subject? In a powered plane thrust is rarely equal to drag, and never, for a glider, since Thrust is always zero, and drag is never zero.

Going back to the original question in this thread:
Screenshot_20200723-163329.png
I do not pretend to totally understand these equations,

Then why did you post them? How do they help?

['97 Tercel]

I understand those equations...

the answer is 34.

[Gannet167]

42.
Final answer.

[tsgarp]

Read the Advanced Pilots Manual by William Kershner

[youhavecontrol]

Read the Advanced Pilots Manual by William Kershner

+1 Love that book and his random diagram notes