Author Topic: P38 question (Read 308 times)

AKcurly · « **on:** October 07, 2001, 03:22:00 AM »

Based on some channel 1 conversation, my interest was piqued about the P38 and its airspeed in a steep dive.

First, choose *ANY* American fighter, take it to 25k and nose it over. Keep the power on until you see an airspeed of 600. Reduce throttle, apply a little elevator trim and you gradually pull out.

Now, take a P38 to 25k, set the dive flaps and nose it over. Keep the power on and guess what?

You never get past 500 a/s. Even more surprising, it pulls right out of the dive after reducing power. I didn't even have to trim the elevator.

Take the p38 back up to 25k and fly level with wep. Stay after it. I never could past 405 airspeed.

Definitely some changes were made to the 38 flight model, but were they correct?

No criticism implied, I'm just curious.

AKcurly

Tails · « **Reply #1 on:** October 07, 2001, 04:47:00 AM »

If it hits critical velocity (wont dive any faster) at 500 with dive flaps, then maybe they atleast made the dive flaps do something usefull. Also, I remeber hearing someone say that the '38's dive flaps do cause the plane to nose up.

Tac · « **Reply #2 on:** October 07, 2001, 08:59:00 AM »

Been wondering the same thing for the past 3 tours.

Another interesting tidbit, it wont "break up" due to speed... prolly because it barely goes above 500 mph (on 90 degree dive with wep).

Tac · « **Reply #3 on:** October 07, 2001, 12:08:00 PM »

Tails, 38 dive flaps, just like its damage model, nose guns and flap induced spin are flaws.

Sent pyro 5 megs of stuff on the dive flaps and other stuff, never heard if he got it. I know it wasnt used though.

38 dive flaps should pull nose up significantly (3g's at 420mph+) if deployed after the dive, if deployed before the dive it prevent the "tuck under" effect.

D.Flps now barely prevent the tuck under effect.

tshred · « **Reply #4 on:** October 08, 2001, 06:07:00 AM »

Shoot, they can't even model it correctly visually. They have the inboard flaps visually the split type, and the outboard fowler type. Both should be modeled as fowler type. Wonder if that's how the flaps are modeled in the FM as well.......

ts

tshred · « **Reply #5 on:** October 10, 2001, 01:00:00 AM »

<punt>

Tac · « **Reply #6 on:** October 10, 2001, 12:27:00 PM »

curious tidbit: Get a 38 to 33k, nose down 90 degrees, shift-X it, WEP on. (no dive flaps).

The 38 will go at 590 mph. But if you start the dive at 25k or so it will not get past 510mph. WHY?

I used the elevator to pull nose up (since the dive flaps did NOT pull nose up by themselves, something they SHOULD do) at 5k and pulled nose to horizon at 400ft.

Lephturn · « **Reply #7 on:** October 10, 2001, 02:12:00 PM »

Seems to me the changing density of the air due to air pressure means that compression will happen at different speeds at different altitudes. Therefore the higher you are, the faster you can go before you hit compression, right?

tshred · « **Reply #8 on:** October 10, 2001, 07:19:00 PM »

Ooooh, just went and checked again (shoulda before last post doh!) they finally fixed the inbd flaps and they are visually fowler now. Not sure when you guys did it, but thanks HTC!

ts

PS Sure is damn hard to find close up pictures of the P38's flaps deployed, been researching for an R/C model I'm building.

Seeker · « **Reply #9 on:** October 10, 2001, 09:42:00 PM »

Back to front, Leph...

Compression's linked to the speed of sound....it's a non sonic airframe getting as close as it can to the sound barrier, it's "critical mach number". It's given that way instead of a fixed speed because the speed of sound changes with air density.

500 MPH IAS may well be near the planes mach limit at 25K, and you'll compress. As you descend into lower, denser air, the speed of sound rises, thus 500 MPH is no longer so close to the mach limit, and you start to regain control authority.

Lower is better..

Tac · « **Reply #10 on:** October 10, 2001, 10:16:00 PM »

no, no, you guys not getting the point. On a 90 degree dive with wep I dont care if it compresses, the whole point of the thing was to see if it would go past 510'ish mph.

It doesnt if you start dive below the 25k or so alt mark. Why doesnt it go all the way to 590 like it does from 33k?

batdog · « **Reply #11 on:** October 11, 2001, 08:18:00 AM »

Because it has anthor 8k to get more speed?

xBAT

Seeker · « **Reply #12 on:** October 11, 2001, 08:35:00 AM »

It's still linked to compressability and critical mach numbers, Tac. Critical mach is in effect the plane's sound barrier, it's a shock wave holding the plane back. At higher, less dense alts you can get more vertical TAS even though the IAS is unchanged.

Here's the numbers, which will give you an idea of the *sustained* dive rankings.

Me262 M0.86
Spitfire M0.85
P-51 M0.77
Fw190 M0.75
F4U M0.74
Bf109 M0.70
Mosquito M0.70
Ki-84 M0.70
P-38 M0.67 M0.70 with dive brakes

a Mach number of 1 would mean you're diving at the speed of sound, what ever that speed may be at the given alt. The P38 may have been an efficiant airframe, but it wasn't a "clean" one. It was draggy, and you can see that "dive brakes" is the wrong term, as they actualy raise the mach number. They don't slow you down to keep you under mach, they raise mach so that it affects you less.

N.B. I blatently stole the numbers from Badz, so it's his fault the P38 won't dive.