Directional drag

[Jarin]

So, in my continuing attempts to push the envelope of airbreathing flight, I've run across something very odd. My supercruise craft that can hit 1750+m/s flying east is catching more drag or less lift going west, and tops out at 1730, in addition to burning significantly more fuel since it's having to run at max throttle to maintain that lesser speed. Anyone know anything about this?

Edit: That's an even weirder anomaly. I'm legitimately burning fuel twice as fast, even factoring throttle into it. It's not just drag. Westbound at full throttle I have 15 minutes at cruise altitude, estimated by KER (and tested at empty halfway around the world). Eastbound it's 30. Something seriously screwy going on here.

Edited January 25, 2017 by Jarin

[Blaarkies]

12 minutes ago, Jarin said:

So, in my continuing attempts to push the envelope of airbreathing flight, I've run across something very odd. My supercruise craft that can hit 1750+m/s flying east is catching more drag or less lift going west, and tops out at 1730, in addition to burning significantly more fuel since it's having to run at max throttle to maintain that lesser speed. Anyone know anything about this?

Wait at the landed at the launchpad/runway, look at your surface speed. it shows: 0m/s...good(obvious I know)
Now, click on "Surface" and it will switch to orbital velocity. This will show +- 174m/s in the east direction(because kerbin spins relative to the stars)

When you fly east you are already 174m/s closer to being in orbit. Even worse when flying west, you are not just standing still but actually flying against that bonus(so going west you have 348m/s disadvantage)

My point is, check your orbital velocity when doing those east west tests. You will see that east has a greater orbital velocity, and therefore less gravity drag(which results in less wing lift needed, less angle of attack and better flight profile...more speed thus)

Edit: Jet airbreathing engines burn more or less fuel depending on the altitude AND speed they are currently going at. You have to meticulously check that those are equal in both tests

Edited January 25, 2017 by Blaarkies

[bewing]

You should look at your orbital velocity in both directions. Going east, you have an additional 150 m/s from Kerbin's rotational speed. That reduces the weight of your airplane by maybe 20%. Which reduces the amount of lift you need. Which reduces the AoA you need to maintain.

Going in the other direction it's the opposite. You have 150 m/s less, your plane weighs 20% more, you need more lift, so you have a bigger AoA and generate more drag.

Try going north or south -- it should be halfway in between.

 

[Jarin]

Right, I'm flying high and fast enough that planetary curve is important. Forgot that. Still doesn't explain the doubled fuel burn though.

[Jarin]

Apparently my fuel efficiency doubles when maxing out at 1748m/s vs maxing out at 1732? Still not making sense...

Spot the westbound flight:

Half the thrust, half the fuel flow. Both are at max throttle, speed 0.07 mach difference.

 

Edited January 25, 2017 by Jarin

[Blaarkies]

4 hours ago, Jarin said:

Apparently my fuel efficiency doubles when maxing out at 1748m/s vs maxing out at 1732? Still not making sense...

Spot the westbound flight:

Half the thrust, half the fuel flow. Both are at max throttle, speed 0.07 mach difference.

 

Check your drag. I dont know if KER shows it(or calculates it), but FAR certainly does. You will have more atmospheric drag going west bound because of the poorer flight profile(need more lift to fight gravity going west bound).

So going east bound, you dont need to pull up as much to stay at altitude(because you fight less gravity going east bound). Pulling up less causes less atmospheric drag, causes better efficiency(even though the thrust to fuel flow ratio is consistent)

EDIT: Just saw the "A.Cd: 0.07" text. Thats your drag coefficient iirc. More Cd means more drag(it describes how UN-aerodynamic the aircraft is, at the current angle of attack)
So suddenly pulling up should increase that number, but point perfectly at prograde should minimise that number

Edited January 25, 2017 by Blaarkies