Aerodynamics discussion question

I was reading up on AOA and came across a great question, posted by a student engaged (evidently) in a epic battle of wits with his instructor.

Here's the question.

What's the best way to make the jet go up? By increasing AOA (pulling back on the stick) or by increasing thrust (shoving throttle forward)?

I know just barely enough to see it's a clever question.

I was reading up on AOA and came across a great question, posted by a student engaged (evidently) in a epic battle of wits with his instructor.

 

Here's the question.

 

What's the best way to make the jet go up? By increasing AOA (pulling back on the stick) or by increasing thrust (shoving throttle forward)?

 

I know just barely enough to see it's a clever question.

Depends. :D

Both. Engines wont push you "up" much without increased AoA. You wont go "up" much with increase AoA and no increase in power.

Both. Engines wont push you "up" much without increased AoA. You wont go "up" much with increase AoA and no increase in power.

Exactly. You want to fall from the sky like a rock, increase the AOA without enough thrust until you hear the stall warning.

Always more power unless you're in a really specific situation.

Since you want to talk about aerodynamics, try this:

Load up any aircraft in dcs (or any modern-ish sim) and get yourself set up to land. Get on a glide slope that is too steep. Cut the power and push your stick forward to get on the right glide slope. Take note of where you touch down on the runway.

Then set up the aircraft in the same situation, cut the throttle, but this time pull back on the stick (but keep control of course). Take note of where you land.

Interesting!

Not only does changing your direction "upword" cause you to slow down, but the increased aoa causes more drag as well.

I read online a student pilot was on final, his instructor put his throttle to idle and said "oops, engine went out in final, what do you do?"

The student pilot thought a moment and then dived down toward the ground, gained a bunch of speed, and easily made the runway.

I've also seen on youtube an instructor show how to get a cessna that is too high in a glide slope in the groove: He decreased the throttle a little and pulled up on the stick a little for about 5 seconds. When we went back to his glide path he was right on.

I have a lot of fun flying my DCS aircraft as SLOW as they can go. It really helps my understanding of how to manipulate aircraft during takeoff and landing.

The question is actually very broad and it depends on what you’re trying to achieve ?

L = 1/2pv2CLS

L= Total Lift

P = density of Air

CL = coefficient of Lift

S= Wing surface area

Using these variables, you can create many situations to reflect on.

Easiest answer that's taught to USAF pilot trainees (and thus probably dumbed down a bit to be used as a rule of thumb) is that best angle of climb is obtained at the highest thrust/drag ratio (so around best glide speed because that's best lift over drag), and best rate of climb is obtained at highest power/drag ratio (around best cruise speed.) Flight manuals should give you best speed/AoA for both though, so go with that.

For F-18 purposes though, remember that you set and trim AoA and adjust glide path with the throttle.

Interesting reading:

https://www.av8n.com/how/

With your question addressed a bit there:

https://www.av8n.com/how/htm/energy.html#sec-change-throttle

https://www.av8n.com/how/htm/energy.html#sec-pull-yoke

The question is kind of a trick. So let’s look at it and break it down. What’s the best way to make a jet go up. What do we mean by go up? if were talking about a purely mathematical perspective of increasing the total lift. Than increasing the velocity of the craft is the most efficient means. As velocity is squared in lift equation. That is Lift force = Coefficent of Lift * Area * .5 * Density * Velocity Squared. So lets look a quick example and see that lift increases linearly as our Cl doubles, while Cl increases exponentially when velocity is doubled.

.5 = 1 * 1 * .5 * 1 * 1^2 Our Base Figure

1 = 2 * 1 * .5 * 1 * 1^2 Doubling our Cl, e.g. pulling back on the stick

2 = 1* 1 * .5 * 1 *2^2 Doubling our Velocity, Pushing the throttles up.

However I consider this a bit of trick because it ignores some of the more practical aspects of aerodynamics and some edge cases.

So what do we mean by best? If we’re talking about the shortest amount of time to reach a desired rate of climb, then pulling the stick back is the best means. It takes way more time and work for an aircraft to double it’s velocity compared to doubling it’s angle of attack by pulling the stick back. Therefore pulling the stick back is the most efficient means to change our rate of climb.

The edge case where this breaks down is at the stall. Stall is an increase in the angle of attack which results in a decrease in the lift coefficient. Thus at max Cl the only way to increase your lift is to increase your speed. If you’re low on your approach and pull back to fix you can make the problem worse by stalling the craft. In this case the best way to make the plane go up is to increase the throttle. So the question really does have multiple right answers.

I read online a student pilot was on final, his instructor put his throttle to idle and said "oops, engine went out in final, what do you do?"

I was told to always keep one hand on the throttle so the instructor/examiner can't push in said throttle like they always do when your hand is off the throttle ;)

In flight sims at least, I've always gone by the rule "increase power first, then increase AoA."

Depends. :D

That was my answer.

Above minimum drag speed, I figure you're balls to the wall anyway, you're working the stick. We tend to talk about optimum climb speed but I guess it's still an AOA question.

Below minimum drag speed, you need power, and a lot of it. Especially if you're indexing AOA, which is the point I guess. To keep the AOA at a certain value. And to not fall out of the sky, of course.

It's a trivial (but important) question. People try to complicate it with fluff. The relationship of climb performance to speed or lift is of lesser importance than excess power/thrust available.

If you had even an impossibly efficient wing capable of producing enormous amounts of lift per unit of weight or drag (take your pick,) would it fly without something imparting energy to it? No. Speed is fun, and you can trade kinetic energy for potential energy, but you had to get that kinetic energy from somewhere. In maximum performance climb profiles for jets, there are situations where calculated transitions from one speed to another commensurate with specific combinations of drag and thrust production will provide superior climb rate, but that is all achieved through excess thrust. Lift is great, yes, but it does not make an airplane climb on its own. Gliders do not climb without being imparted energy in the form of lifted air like thermals. Without excess power or thrust, nobody climbs.

Law of conservation of energy.

Jetsun's source is excellent, read that!