Fighter Acceleration Results: 18'000 ft

[dtango]

Fork: - former member of the 412th?

Regarding the FM-2 / F4F-4 differences at 18,000 ft vs. near SL.

(1) Already pointed out by Funked and others, the critical alts of the engines between the FM-2 and F4F-4 are different - FM2 being lower vs the F4F-4.

(2) The acceleration tests being analyzed are average acceleration figures from 175mph to 275mph and not instantaneous acceleration figures at a specific velocity.

(3) The velocities being analyzed (175 mph TAS and above) are above velocity for best rate of climb of the a/c.  In these conditions the a/c with the higher max level speed (F4F-4) should have better average acceleration because of  the effect of parasite drag increasing by square of velocity becoming more pronounced.

Tango, XO
412th FS Braunco Mustangs

[HoHun]

Hi Dtango,

>Already pointed out by Funked and others, the critical alts of the engines between the FM-2 and F4F-4 are different - FM2 being lower vs the F4F-4.

The FM-2 is rated at 1000 HP at 17800 ft, the F4F-4 at 1040 HP at 18400 ft - that's nothing to write home about.

>The velocities being analyzed (175 mph TAS and above) are above velocity for best rate of climb of the a/c.

Actually, best climb speed of the FM-2 is 125 KIAS, which is 196 mph TAS at 18000 ft.

>In these conditions the a/c with the higher max level speed (F4F-4) should have better average acceleration because of the effect of parasite drag increasing by square of velocity becoming more pronounced.

The higher maximum level speed of the F4F should be mainly due to its 40 extra HP, which seems too little to make such a difference at low to medium speeds.

In principle, you have a valid point however - the Me 109 has both the advantage of a much better power-to-weight ratio and and a superior top speed should really out-accelerate both Grummans with ease.

Regards,

Henning (HoHun)

[Karnak]

Originally posted by frank3
I hate to see the spit ouperformes the P-47 and P51...

Why not?  The Spitfire Mk IX should out accelerate the P-47 and P-51 at those speed ranges at pretty much all altitudes.

You do realize that the Spitfire Mk IX is a high altitude tuned fighter, don't you?


Part of the problem that Mister Fork's tests have is that they are only testing acceleration at low speeds.  If you were testing the acceleration of the Spitfire Mk IX, P-51D and P-47D-30 from 350 to 410 at 25,000ft the Spitfire IX would get toasted because it is at the top end of its speed whereas the P-51 and P-47 still have a ways to go.  Add in the Ta152H-1 and Spitfire Mk XIV and the P-47D-30 and P-51D get beat because those two aircraft have higher absolute speeds.

The low speed tests that Mister Fork is doing highly favor the light aircraft.  Those same light aircraft that frequently have realtively low top end speeds.

[Mister Fork]

Not a valid point Karnak. I'm measuring accleration rates, not a measure to top speed.  The acceleration rates of aircraft at where Vf and Vo were not affected by top speeds or drag limits.  We all know that a P-51D is a speed demon comparied to a Spit V/IX but the Spit IX is a fast accelerator.

The problem is that if I increased Vo to 250, then other factors will play a role like top speed limits.  Most planes accelerate at that factor up to their drag limits when the last 10% of their top speed is a slow crawl.

It's why Car and Driver and 99% of other automotive acceleration tests measures 0-60Mph, not 30-90, 100-160 etc.

I will point out that at 25'000 feet, the test results are different again. The Spit IX takes 67 seconds from 175-275, while the Mustang takes 39.