Drag index and CRUS
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Two questions:
1. How much drag do pylons and ejector racks have? i.e. after I drop my weapons how much does my DI reduce by (so that I can calculate my best climb speed for the journey home)?
(edit: I suppose I can probably calculate this to a certain extent by seeing how the drag values differ for 0, 1, 2, and 3 bombs per station).
2. Why does Crus often suggest 34k ft or so? Feels like climbing as high as I can go uses less fuel. Is it something to do with running out of stratosphere, or perhaps does the climb use too much fuel?
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1. Pylons, adapters, launchers, tanks, etc. add small amounts of DI each, single to small double digits DI. A fairly slick post-attack DI might be as low as 50 but often 100 or a touch more. I’d have to run a few but 50% of the loaded DI is probably a good rule of thumb. 380-400KCAS up to M0.82-0.84 should do. Home profile should understand your DI based on SMS and reinforce a particular speed.
2. CRUS page shows the optimum alt for current load conditions which is not including any allowances for climb or dive at all. It says if you are at this altitude you are burning the least fuel per distance forward. Best specific fuel consumption is not a trivial bit of math and no higher isn’t always better. 36kft sounds about right for a medium-light loading. Home profile considers the actual distance to go to reduce the climb leg for shorter transits.
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1. Pylons, adapters, launchers, tanks, etc. add small amounts of DI each, single to small double digits DI. A fairly slick post-attack DI might be as low as 50 but often 100 or a touch more. I’d have to run a few but 50% of the loaded DI is probably a good rule of thumb. 380-400KCAS up to M0.82-0.84 should do.
Thanks, I’ve been assuming 100, so sounds like I’m in the right ball park at least.
Home profile should understand your DI based on SMS and reinforce a particular speed.
Ah - I didn’t consider that.
2. which is not including any allowances for climb or dive at all.
Good to know, I did wonder why it would tell me to decrease my altitude if I’m high.
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For the ranges considered in F4, you never get near the normal values for best specific range or max range, simply due to the short legs. Mission planning data is optimized for legs of 250 nm or more. Shorter range missions than that, the climb and descent end up using more fuel than you would otherwise save by climbing to the most fuel efficient altitude.
Graphic calculation of specific range from the tables is a trivial piece of maths, but factor in leg range, climb and descent, and you end up relying on multiple iterations, looking for a min value of total fuel consumed. Its easy to say “this is the most fuel efficient altitude you can fly at for this GW and DI”. Its a little harder to say “this cruise altitude for this mission will result in minimum fuel consumed”. Unless the leg is more than 250 miles, in which case, its easy.