What is "lift"?

[RedAndWhiteBaron]

I fly a plane where the approved spin recovery procedure includes pro-spin aileron, to recover from “difficult” spins.

I would actually like to hear the aerodynamic explanation of this. To increase drag on the outside wing and therefore reduce the rate of spin, perhaps? It goes against all spin training I've ever been taught.

I must be fair to my instructor - he taught differently. He taught spin recovery by the PoH. It goes against all my training previous to the aircraft I am training in now, but this is not contrary to what my current instructor is teaching me.

[Bede]

However, what would be the effect of pro-spin aileron?

I fly a plane where the approved spin recovery procedure includes pro-spin aileron, to recover from “difficult” spins.

Interesting. I have never heard of that before. What type is that?

[AuxBatOn]

Bede: any fighter aircraft (large fuselage mass vs wing mass).

[digits_]

I fly a plane where the approved spin recovery procedure includes pro-spin aileron, to recover from “difficult” spins.

I would actually like to hear the aerodynamic explanation of this. To increase drag on the outside wing and therefore reduce the rate of spin, perhaps? It goes against all spin training I've ever been taught.

The following explanation is based on a Pitts, but I'm sure there are other aircraft with similar characteristics, and probably a few with different ones.
Let's say you enter a vanilla spin. Once you are established in that spin, usually after one rotation, you can do 2 things with your ailerons: turn them into the direction you are spinning, or turn them to the opposite direction.
If you turn them opposite, you'll flatten out the spin. A bit like the flat spin in Top Gun that killed *spoiler alert* Goose.
If you turn them into the spin, you'll start to turn faster and lose more altitude. I have the impression the nose is a bit lower in this scenario.

If you are turning in a flat spin, there is a lot of sideways force on the rudder, not much forward force. Like a car out of control on the highway. This makes the rudder les effective. The rudder could be, sort of, stalled.

If you turn into the spin with the lower nose and more altitude loss, there is a bit more airflow over the tail, reducing the angle of attack on the rudder. I assume this makes the rudder more effective.

Generally, if you are trying to recover from a flat spin, you want to turn it into a "normal" spin first.

Note that there is a big difference between the POH spin recovery, and what aerobatic airplanes do. The POH spin recovery is designed to demonstrate certification requirement compliance. -now I'm on pilotDAR territory so I'll proceed with caution-
It's unlikely a plane would get certified if the spin recovery depended on wether you are in a flat spin or a normal spin. The manufacturer will decide what the most bullet proof way is to recover from any spin, in particular a spin you didn't plan to enter.

You don't necessarily need power idle to recover from a spin, you don't necessarily need full rudder, you don't always need ailerons neutral. But if you find yourself spinning out of control outside of your control, the POH spin recovery procedure is the most likely to get you out of it.

And for the record, I still disagree with the commonly accepted definition of lift. All parts of the airplane can generate lift - including the propeller (and, for that matter, the horizontal stabilizer, which in stable flight generates negative lift). But I do recognize that I am arguing over a definition, which is of limited value.

Lift is the sum of all those little forces. To make it easy to visualize, people draw the lift vector on the wings in drawings and schematics. But what you are really interested in, is the center of lift for lift, and the center of gravity for gravity.


If you want to annoy your instructor a bit more, ask him if the tail on a plane generate an upwards or downwards force

[photofly]

... for that matter, the horizontal stabilizer, which in stable flight generates negative lift

If you want to annoy your instructor a bit more, ask him if the tail on a plane generate an upwards or downwards force

It's been a few years since we last kicked that one around the patch.

[RedAndWhiteBaron]

If you turn them opposite, you'll flatten out the spin. A bit like the flat spin in Top Gun that killed *spoiler alert* Goose.

The spin didn't kill him. An ejection malfunction did.

As for the tail, I sure hope that if the centre of gravity is foreward of the centre of lift that the tail generates a downwards force to counter that turning moment, but I don't know if the CoG can ever be aft of the CoL.

If you want to annoy your instructor a bit more, ask him if the tail on a plane generate an upwards or downwards force

I don't need to. I flew my first solo yesterday, so I know everything now.

[digits_]

If you turn them opposite, you'll flatten out the spin. A bit like the flat spin in Top Gun that killed *spoiler alert* Goose.

The spin didn't kill him. An ejection malfunction did.

As for the tail, I sure hope that if the centre of gravity is foreward of the centre of lift that the tail generates a downwards force to counter that turning moment, but I don't know if the CoG can ever be aft of the CoL.

If you want to annoy your instructor a bit more, ask him if the tail on a plane generate an upwards or downwards force

I don't need to. I flew my first solo yesterday, so I know everything now.

Start a new topic about that question and we'll have some fun.


Oh, and congrats!

[tbayav8er]

Sorry, I didn't read every reply here, so my apologies if this has already been mentioned. It also depends on aircraft type. A multi engine propeller aircraft definitely has a lower angle of attack with climb power while maintaining the same airspeed as was maintained while level before starting the climb. For example a Q400 has about a 13' propeller diameter, and the prop hub is pretty well in line with the leading edge of the wing. So there's 26' of accelerated airflow going over the wings in the prop wash. If you go from 40% torque in level flight at 200 kts to 90% torque in a 200 kt climb, there's a massive increase in lift just from the increase in airflow over the wings from the props. I know you're not flying a large turboprop yet, but you will some day, and it's some theory worth taking into consideration.

[RedAndWhiteBaron]

If you turn them opposite, you'll flatten out the spin. A bit like the flat spin in Top Gun that killed *spoiler alert* Goose.

The spin didn't kill him. An ejection malfunction did.

As for the tail, I sure hope that if the centre of gravity is foreward of the centre of lift that the tail generates a downwards force to counter that turning moment, but I don't know if the CoG can ever be aft of the CoL.

If you want to annoy your instructor a bit more, ask him if the tail on a plane generate an upwards or downwards force

I don't need to. I flew my first solo yesterday, so I know everything now.

Start a new topic about that question and we'll have some fun.


Oh, and congrats!

There is no point in debating. Goose died because Ice pointed his jetwash at him. It was Ice's fault.

I'd love to have the tail lift argument (I really do love a good argument), but I'll have to take a raincheck. I'm off on a family camping trip for a few days, then back for a few days, and then off to Armstrong and Wabakimi for a wilderness expedition. You can expect my argument around the middle of August, if I don't drown in a rapid.

Another topic I'd actually like to ask the hive mind - if all else fails, in a piston single, would full power produce additional empennage authority in a flat spin?

I know you're not flying a large turboprop yet, but you will some day

I like the cut of your jib, sir.

[Zaibatsu]

Most lift discussions are academic. There’s absolutely no way to tell through empirical data what it’s actually doing, and for the most part it’s a complete waste for piloting an airplane.

I could teach lift completely wrong and it would have no net effect on the safety of a pilot provided these points are made:

Maintain your airspeed, coordinate your aircraft or cross-control in a safe manner (top rudder, raise dead engine), and respect stall warnings.

[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 2184 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 1958 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