P38 Performance

[Brooke]

Note the problem in this case is separation on the top wing changes the down wash angle.  The bottom flap counters that by producing its own separation.

Downwash is produced by lift.  Compressibility results in stall of the wing, hence reduction in downwash.  Add some lift by the dive-recovery flap, and you get downwash re-established.

So, there are three positive things the dive-recovery flaps do:  add lift, add drag, add downwash.

[FLS]

Stark is the designer of the flap. 


"Stack's idea was to use the flap to maintain lift in the face of the compressibility burble, hence eliminating the change in the downwash angle, and therefore allowing the horizontal tail to function properly."

Note the problem in this case is separation on the top wing changes the down wash angle.  The bottom flap counters that by producing its own separation.

The compression problem isn't the airflow over the tail. The problem is the center of pressure moving back on the wing and pitching the nose down.

[cactuskooler]

[FLS]

Thanks for posting that. One of my first posts here 12 years ago was about the P-38 dive flaps modeling. It's good to see more info available now.   

[Brooke]

Awesome post! 

[Randy1]

Cactuskooler your reply should be a Sticky.

I am glad we all kept digging to get to the technical explanation of the dive flap's function.  Between this and the document FLS found.  It nails it down.

[GScholz]

Nails what down, exactly?

[save]

nice post on the p38.

P38s still look funny trying to pull up at 580mph 

[Randy1]

Nails what down, exactly?

The technical reason behind the design of the flap.

The only open question now is wither Mach Drag is included in the AH P-38 model or not when entering transonic speeds.

[FLS]

We still don't know how much drag we should see in subsonic flight or if we should see a pitch change when deployed in subsonic flight. We don't know how much drag the flaps add at compression either. We have a wind tunnel test on a 3 ft model wing and advice to pilots that extending the dive flaps after compression causes a pitch up and leaving them down limits your speed.

[GScholz]

Yes, we don't know anything about the drag, except that there was some (obviously), and that it isn't modeled in the game right now. Unless the P-38 ran on magic and unicorn juice those flaps must produce drag at any airspeed.

[colmbo]

These calculations represent a stick-fixed trim change that, for a wing loading of 60 pounds per square foot at 25,000 feet at Mach numbers of 0.600 and 0.890, will correspond to approximately a 3.6g pullout; however, at lower altitudes the normal acceleration will be somewhat greater.

That's a lot stronger pitch moment that I expected.

[GScholz]

Just tested it again.

At 35k, 382 TAS, M .58 the flaps do nothing whatsoever.

I then took it to 40k and dived it past M .72 in a long shallow dive, but experienced no Mach tuck effect. Lots of shaking and controls stiffened up, but no increase in dive angle. Can anyone confirm that Mach tucking and compressibility effects are actually modeled in the game, or perhaps only superficially simulated as loss of control authority?

[colmbo]

I then took it to 40k and dived it past M .72 in a long shallow dive, but experienced no Mach tuck effect. Lots of shaking and controls stiffened up, but no increase in dive angle.

Were you hand flying or using auto-angle?  Any chance you cancelled out any pitch down that was present?  I don't expect that mach tuck is modeled, the shaking and loss of control is sufficient for AH needs.

Here's a question…why does the 38 roll left at high speed?

[GScholz]

I used auto pilot, which only controls trim IIRC.

So compressibility effects are not really modeled in AH?