What does "energy retention" *really* mean?

[Neil Walker]

Not sure if the power fliers will have heard the term, but anybody who flies slope gliders should be familiar with it. Slope heads typically talk about a glider having "good energy retention" when describing a ship that doesn't scrub off too much speed during the turns, and holds speed well coming out of a dive (or pulling up on a vertical line).

It really seems to be a slope head euphemism for "Goes really fast and keeps going fast" but from what I can tell it it's just a synonym for "low drag". Am I missing something? Is there anything other than low drag that would lead to describing a glider as having good energy retention?

There also seems to be a generally-held belief that heavier gliders have better energy retention. Ignoring for the moment that I haven't come up with an exact definition of energy retention yet ;) why would this be the case? Certainly a heavier glider will be able to penetrate in high wind better, but why would it help it "hold energy" any better than the same glider at a lighter weight?

Just trying to sort out exactly what the term means, or if it even has a concrete definition...

[Flyingwingbat1]

Everything I've read on the subject leads me to say it means that a glider (or any plane) retains speed throughout maneuvers; i.e. low induced drag at high g's with deflected control surfaces. This is a huge consideration in fighter aircraft design; the (extended) swing wings on the F-14 allowed it to retain energy quite well in dogfighting maneuvers (lower induced drag), one reason for its success, not to mention that it just looks awesome!

[nauga]

Just trying to sort out exactly what the term means, or if it even has a concrete definition...
At the level we're talking about, total energy (TE) is the sum of kinetic energy (KE), or energy due to speed; and potential energy (PE), the potential for energy due to acceleration (through gravity).
In general, TE = KE + PE, and
KE = 1/2*mass*velocity*velocity
PE = mass * net acceleration due to external forces (drag and gravity) * height

All other things being equal, a glider with higher mass will have both higher KE and PE, and a glider with lower drag will hold speed better and decelerate less (or accelerate more).

For a given TE, KE and PE are essentially interchangeable, i.e. you can be fast and low or high and slow. In glider terms, higher mass and lower drag will result in higher total energy *if* you can get to speed, and due to reduced deceleration will tend to lose less total energy while exchanging PE and KE. This means higher pumps and faster speeds on the flat.

Helpful?

Nauga,
doin' the Rutowski shuffle

[raptor22]

Well, thats just about it...it means that it holds its speed well through turns and maneuvers.

One key thing is for it to have a good lift/drag ratio, which allows it to provide the lift for a turn with minimal drag.

[Ollie]

There are two kinds of energy, dynamic energy and potential energy. With no drag (no friction) the total energy is not lost. The total energy is the sum of dynamic energy and potential energy. The aircraft can change the two kinds of energy within total energy.

In slope flying, the wind adds lift by adding aircraft enough energy that it overcomes drag so that it flies forever (till the weak lift goes away). So the lift has a three dimensional map of lift strength, depending on wind three dimensonal direction and speed. Where the wind velocity changes abruptly, such as turbulent flow, makes the lift map complex.

The dynamic energy is half the mass times the squared velocity and potentional energy is mass times altitude.

Drag has two kinds, pressure drag and induced drag. The induced drag depends on the square of the coefficient of lift. So for sharp turns and high speed the lift is high and the induced drag is high.

[Neil Walker]

At the level we're talking about, total energy (TE) is the sum of kinetic energy (KE), or energy due to speed; and potential energy (PE), the potential for energy due to acceleration (through gravity).
In general, TE = KE + PE, and
KE = 1/2*mass*velocity*velocity
PE = mass * net acceleration due to external forces (drag and gravity) * height

All other things being equal, a glider with higher mass will have both higher KE and PE, and a glider with lower drag will hold speed better and decelerate less (or accelerate more).

For a given TE, KE and PE are essentially interchangeable, i.e. you can be fast and low or high and slow. In glider terms, higher mass and lower drag will result in higher total energy *if* you can get to speed, and due to reduced deceleration will tend to lose less total energy while exchanging PE and KE. This means higher pumps and faster speeds on the flat.

Helpful?

Nauga,
doin' the Rutowski shuffle


Thanks Nauga - that helps, but you did spell out the confusion I have with the idea that mass helps energy retention, specifically when talking about higher pumps. Since both PE and KE are proportional to mass, you should be able to ignore it in the conversion, i.e. if you calculate the KE of a given glider at a given speed, the height that you should get out of it when converting all of that kinetic energy to potential energy should be independent of mass.

So yes higher mass means higher total energy, but does it make any difference in energy retention? I can't see how it would. Seems to me like it's all in the drag, and as someone mentioned earlier drag at high G's with deflected surfaces.

[yoyoML]

Good energy retention means low drag. It can be stated under certain conditions, like "good energy retention when pulling 3G," which simply means "low drag when pulling 3G."

There's only one force that reduces a plane's energy, and it is drag. This is by definition, because drag is the force parallel to flight path but in the opposite direction.

[nauga]

So yes higher mass means higher total energy, but does it make any difference in energy retention? I can't see how it would. Seems to me like it's all in the drag...
Gotcha. (high) mass and (low) drag will allow you to get to an energy state, (low) drag alone allows you to maintain (or retain) that state. I suspect off the cuff that induced drag in the pulls at the bottom of a pump is a far bigger contributor to drag increase than the basic drag of deflected surfaces. And don't forget that you need lift in the turns, so like Raptor22 says, high L/D on condition means low(er) drag in the pull or turn.

Nauga,
whose energy retention is usually ground-limited

[Troy]

To use Tic's terminology of "Flying by the seat of your pants" testing: Energy Retention in the contex of model airplanes means that you are doing 'something' and then change 'something' retaining some amount of (in this case) energy. With a slope racer or even a powered pylon racer, you go into a turn with energy (mass and velocity), then yank, then exit the turn at a slower speed having lost some energy in the turn. How much you slow down is a combo of weight, surface loading, wing design... overall airframe design (drag). The better airframes are a balance of straight line speed with turning ability. With a given power system or wind speed/lift at the slope, the airplane who comes out of a turn faster has better energy retention.

[Neil Walker]

I suspect off the cuff that induced drag in the pulls at the bottom of a pump is a far bigger contributor to drag increase than the basic drag of deflected surfaces.

Hmmm, so maybe increased mass would have an effect on energy retention - but negatively. Increased mass would mean a higher AoA is required for the same flight profile, meaning more induced drag, no?

Neil,
who is inducing a headache

[nauga]

Increased mass would mean a higher AoA is required for the same flight profile, meaning more induced drag, no?
Yes, but (again, all other things being equal) the speed(s) for L/D max at any given g load will be higher, so you're more efficient at higher speeds than a light loaded glider - assuming mass is the only difference between them.

Nauga,
the mission profiler

[Airboatflyingshp]

Low drag/ good penetration...........it needs to be slippy enough to be able to carry whatever KE it has at any point through the air and series of manoeuvres without loosing too much height; speed or forward motion in that order.

[Thomas B]

Good energy retention means low drag. It can be stated under certain conditions, like "good energy retention when pulling 3G," which simply means "low drag when pulling 3G."

There's only one force that reduces a plane's energy, and it is drag. This is by definition, because drag is the force parallel to flight path but in the opposite direction.


Drag is a lot of the issue, but not all of it. There is some happy compromise of mass vs drag where energy retention is the best.

It is like the ballistic coefficent for a model rocket. Given two identical models with identical finishes, a heavy one will not go very high on a given motor, and neither will a really light one on the same motor, as the drag overcomes the KE very quickly.
Somewhere in between there is a given weight that gives the best altitude.

A very very light glider will not have the energy retention of a heaver version of the same thing. It has so little KE due to the light weight, it bleeds off a higher percentage of the KE from drag lossess than a heavier model does.

At typical R/C glider weights and sizes, drag is the major issue, but mass and KE also have their part to play. Otherwise, no one would ever ballast sailplanes........

[vintage1]

I think weight to drag is what you are all searching for.

A good armor piercing sabot round of depleted uranium has good energy retention.


A feather does not. Great on sink rate tho :D

With models, drag tends to go up with weight tho, as induced drag increases..

so there is a happy medium..a thinnish wing and a very slippery model. Heavy enough but not so heavy that the wing has to operate at an angle of attack that destroys its slipperiness.

[yoyoML]

I think weight to drag is what you are all searching for.

A good armor piercing sabot round of depleted uranium has good energy retention.


A feather does not. Great on sink rate tho :D

With models, drag tends to go up with weight tho, as induced drag increases..

so there is a happy medium..a thinnish wing and a very slippery model. Heavy enough but not so heavy that the wing has to operate at an angle of attack that destroys its slipperiness.

Ahh... right. So it's drag/weight, or simply acceleration due to drag. It's like how you judge by thrust/weight, which is really the acceleration due to thrust... Or how many G's a plane can pull, which is acceleration due to lift.

Of course for planes of the same mass one simply compares the forces, but with planes of different masses one should compare the accelerations instead.

Sounds like a cute little unified theory.