How's lift&Drag characteristics on altitude and speed?

[Reusables]

It'll be really helpful to know the Lift&Drag characteristics on altitude and speed, to fly a proplane. Anyone knows something about this?

[bewing]

For stock parts, lift is directly proportional to drag, assuming a constant AoA.

At a constant altitude, drag goes approximately as velocity squared. (There is a slight increase above that as you pass through the speeds just below Mach 1.)

At a constant speed, drag is directly proportional to atmospheric density, which decreases as a rather complicated function of altitude.

But propellers are not stock, and may not follow the same rules. Since IRL they rotate much faster than the plane actually flies, and do not have a simple AoA relationship with the airflow -- IRL they certainly do not obey the rules that I wrote.

 

 

[Reusables]

1 hour ago, bewing said:

But propellers are not stock, and may not follow the same rules. Since IRL they rotate much faster than the plane actually flies, and do not have a simple AoA relationship with the airflow -- IRL they certainly do not obey the rules that I wrote.

Well there are stock propellers which works based on stock lift. It's what I'm using.

By the way, how about lift characteristics? Is it proportional to speed? What about altitude?

[diomedea]

7 hours ago, Abastro said:

Well there are stock propellers which works based on stock lift. It's what I'm using.

By the way, how about lift characteristics? Is it proportional to speed? What about altitude?

You can't be using stock propellers because the base game has none. What you may be using are propellers built out of stock parts. This is essential to understanding what follows, because some propellers have been modelled (as add-on parts) which work very differently, in particular they directly provide thrust as stock jet engines do based on altitude and speed, but don't show any torque nor lift/drag of the blades.

Using stock lifting (or control) surfaces as propeller blades instead works based on lift. Now, please refer to lift here, as KSP since version 1.0 models aerodynamic effects based on such laws (Lift = 1/2 ρ v² A Cl; Drag = 1/2 ρ v² A Cd). Both coefficients Cl and Cd actually exhibit a complex behaviour, as they depend on shape and speed (in Mach or Reynolds number). Here a pic showing how Cd changes with Reynolds, therefore speed: KSP bases Cd and Cl on a similar curve. Of course both Cd and Cl are function of angle of attack, Cl in particular (roughly proportional to sin(AoA) up to a maximum).  Another important factor with both lift and drag is "ρ", air density, that changes with altitude following a curve made to mimic the barometric formula. Therefore, the lower the altitude, the higher air density and both lift and drag.

As you may have noted, lifting surfaces (both in reality and in KSP) can't provide lift without also having drag (and the two are always correlated). All blades of a propeller show drag, so also in KSP your propeller built from stock parts will show torque (opposite to the rotation of the propeller) that increases with rotation speed and angle of attack of the blades.

[Reusables]

10 minutes ago, diomedea said:

You can't be using stock propellers because the base game has none. What you may be using are propellers built out of stock parts. This is essential to understanding what follows, because some propellers have been modelled (as add-on parts) which work very differently, in particular they directly provide thrust as stock jet engines do based on altitude and speed, but don't show any torque nor lift/drag of the blades.

Using stock lifting (or control) surfaces as propeller blades instead works based on lift. Now, please refer to lift here, as KSP since version 1.0 models aerodynamic effects based on such laws (Lift = 1/2 ρ v² A Cl; Drag = 1/2 ρ v² A Cd). Both coefficients Cl and Cd actually exhibit a complex behaviour, as they depend on shape and speed (in Mach or Reynolds number). Here a pic showing how Cd changes with Reynolds, therefore speed: KSP bases Cd and Cl on a similar curve. Of course both Cd and Cl are function of angle of attack, Cl in particular (roughly proportional to sin(AoA) up to a maximum).  Another important factor with both lift and drag is "ρ", air density, that changes with altitude following a curve made to mimic the barometric formula. Therefore, the lower the altitude, the higher air density and both lift and drag.

As you may have noted, lifting surfaces (both in reality and in KSP) can't provide lift without also having drag (and the two are always correlated). All blades of a propeller show drag, so also in KSP your propeller built from stock parts will show torque (opposite to the rotation of the propeller) that increases with rotation speed and angle of attack of the blades.

That's what I meant by stock propellers. I got that thrust of single propeller engine is only affected by AoA and craft speed.

I didn't know that L/D is constant, but it seems now that it's the case, so I know that there's certain limit of thrust for specific engine.

Anyway, thanks for the explanation!

[diomedea]

1 minute ago, Abastro said:

That's what I meant by stock propellers. I got that thrust of single propeller engine is only affected by AoA and craft speed.

I didn't know that L/D is constant, but it seems now that it's the case, so I know that there's certain limit of thrust for specific engine.

Anyway, thanks for the explanation!

Actually no, L/D isn't constant. Both change with angle of attack, but their dependency isn't the same. Lift is roughly proportional to sin(AoA), at least for small AoA; Drag goes with the cross-section presented to the airflow, it generally increase with AoA but is never zero, and its function on AoA is distinctively different for different shapes.

[Reusables]

14 minutes ago, diomedea said:

Actually no, L/D isn't constant. Both change with angle of attack, but their dependency isn't the same. Lift is roughly proportional to sin(AoA), at least for small AoA; Drag goes with the cross-section presented to the airflow, it generally increase with AoA but is never zero, and its function on AoA is distinctively different for different shapes.

I meant exactly that; it's constant for certain AoA.

[diomedea]

In reality L/D depends also on speed, but believe here is where KSP lacks accuracy. For a more accurate aerodynamic model, you may wish to use FAR. Factors like turbulent effects and detachment of the airflow with Reynolds (which affect lift too, not only drag) aren't adequately modelled in KSP, neither is Bernoulli's principle that plays a major role with real lifting surfaces.