P-38 stall-speed and elevator response

[Bremspropeller]

Hi there,

 

I just checked the POH of the Lightning and it quotes several stall-speeds at various weights and "clean" and "dirty" condition.

 

 

I can't seem to replicate the "dirty" figures (haven't checked in "clean" yet) - mostly due to running out of elevator (trimmed fully nose up).

Trying to reach those speeds by adding a little power (and propwash) just leads to the airplane accelerating away...

The lowest figure I have reached yet (10% fuel, no ammo) was 83'ish mph and a general tendency to drop a wing.

 

I have recognised several times while fighting, that the airplane is severely hampered by elevator-authority - especially with combat flaps deployed.

I think this needs some tweaking.

 

 

[Bremspropeller]

Anybody else have similar experiences?

[Toots_LeGuerre]

One of the things they teach in R/L flight school: 'Never trim a plane to stall'.

Might not be able to overpower the trim with the controls when it starts flying again.

[Bremspropeller]

Not an issue here.

There's not enough elevator-authority to keep the plane locked in stall with neutral trim or even with the trim all the way cranked back.

Also, the actual aircraft is supposed to fly with little trim-requirements in both pitch and yaw, which is not reflected in game.

 

Clean, the airplane is supposed to go straight ahead over the nose with very little apparent pitch-change.

In game, there is no "break" - the airplane just mushes over the side with no associated pitch-down.

[busdriver]

Something I noticed early on was the inability to trim hands off for an approach. I never bothered to test the stall numbers. Guess I have to now. 

 

In the clean configuration, half fuel load, somewhere around 100+ mph, I get a pronounced left rolling tendency. Not much of a stall break.

 

In the dirty configuration, half fuel load, somewhere around 90+ mph, I get a slight left rolling tendency. Not much of a stall break.

Edited June 24, 2020 by busdriver

[Barnacles]

On 6/17/2020 at 1:59 PM, Bremspropeller said:

Anybody else have similar experiences?

My gut instinct is that the p38 seems a bit of an outlier when compared to the other planes, with respect to elevator authority. It really doesn't have any.

 

[Bremspropeller]

7 hours ago, busdriver said:

Something I noticed early on was the inability to trim hands off for an approach. I never bothered to test the stall numbers.

 

Yeah, that also had me raise an eyebrow flying it on the first day...

 

5 hours ago, 71st_AH_Barnacles said:

My gut instinct is that the p38 seems a bit of an outlier when compared to the other planes, with respect to elevator authority. It really doesn't have any.

 

Running out of elevator is now my #1 reason for dying in the P-38 - especially with combat-flaps out.

[unreasonable]

Not specific to P-38 elevator authority, but to greater or lesser extent planes in BoX stall at IAS greater than test or manual IAS figures. If they were modeled to stall at these numbers - as the Tempest was on initial release - they would have absurd CLmax figures.  Reason - BoX has no instrument errors so IAS = CAS. 

 

Working out what the actual instrument error at the stall would have been is non-trivial but in many cases it was clearly quite high.  See the UK tests with a trailing pitot on the Bf109 for an example, where the error was estimated at 20mph.  (Pictures from RAE test on Kurfurst's site).

 

 

 

The usual PEC tables in manuals never go down this far.  So trying to match game stall IAS with manuals is futile, unless you also have this kind of additional information.

[Bremspropeller]

That's a good hint with the CAS = IAS issue.

The elevator-effectivity remains underwhelming, though.

 

The best argument for re-tweaking the elevator is the current inability to trim a stable approach-speed.

[LColony_Kong]

Another related problem that I suspect has to do with this elevator authority problem is medium speed dogfights (220-350mph indicated at sea level or the equivalent at altitude) where the P-38 seems incapable of turning well. The P-38 is one of the best turning planes in the game in sustained turns with flaps down, but I have found that regardless of flap position it has immense problems generating turn rate at higher speeds. This does not seem to have to do anything with compression, since it is at speed far far below where that would be a factor. It just feels like there is not enough elevator authority and the plane feels like its flying through molasses (even with maximum pitch up trim). I have found that in private dogfights with an opponent I beat every single time in a Mustang (opponent was in a tempest), that I had severe problems with the P-38 after the initial pass and could not turn well enough until the fight got extremely slow.

[DD_Fenrir]

I have a great deal of time in the GBS P-38 now and find that fuel weight has a significant impact on the elevator authority and critical AoA flight characteristics. 

 

Bear in mind that the J model carries more fuel than the airframe originally was aerodynamically expected to carry; as such if the trim and elevator design was un-changed from the H model and you are fighting or landing at fuel loads significantly above that at which the designer had originally calculated was the highest expected landing weight then you will run out of trim authority. 

 

What these figures actually were... well.

[Bremspropeller]

32 minutes ago, DD_Fenrir said:

Bear in mind that the J model carries more fuel than the airframe originally was aerodynamically expected to carry; as such if the trim and elevator design was un-changed from the H model and you are fighting or landing at fuel loads significantly above that at which the designer had originally calculated was the highest expected landing weight then you will run out of trim authority. 

 

If there was a significant change in handling-chracteristics, it would have been mentioned in the POH. It isn't, though.

Also, if there were trimmability issues, this objectional handling would have raised issues our you would read about it in anecdotal evidence. I haven't read anything about that.

 

The additional 55gal outer wing tank is very close to the CoG, so any trim change is very minimal.

The POH advises using the outer tanks first (after freeing up volume for the carburator return-line and using up external fuel), but doesn't mention objectional pitch-handling. One can safely assume this is to reduce roll-inertia for possible combat.

 

 

[DJBscout]

@DD_Fenrir does the 38 turn better with more or less fuel? I assume less? What approximate amounts should I be taking if I'm going to fly the 38 as a fighter and not a bomb truck?

[LColony_Kong]

On 6/16/2020 at 7:32 AM, Bremspropeller said:

I have recognised several times while fighting, that the airplane is severely hampered by elevator-authority - especially with combat flaps deployed.

I think this needs some tweaking.

I agree, there is something odd going on with the P-38s elevator authority. At low speeds under many conditions it flys like it is a fly-by-wire plane and you can pull the stick to max deflections without departing. I have also noticed that at higher speeds that are well below compression speeds that the plane does not like to respond, which is odd because once you get it slow it out turns everything that isn't a Spitfire.

[354thFG_Rails]

I have noticed the pitch authority problem as well from the get go. with full nose up trim and flaps full down it's hard to get the nose to pitch back up while on approach. I've adjusted my landing approaches so i dump full flaps on short final. In a dogfight i don't go full flaps down because it has a very bad nose down tendency. Not sure if this was the case in real life, my guess is now. I only deploy maneuvering flaps when i really need them. only playing with the pitch trim to get it to come around quicker. It also seems that the dive recovery flaps give it more of a nose up tendency as well. again not sure if this was the case in real life as well. To me the elevator authority should get sluggish at slower speeds. As it is in game there is barely enough to get the nose to come up.

[Bremspropeller]

Still unresolved, so bumped.

[1PL-Husar-1Esk]

That FM statement might interest Mr. @Holtzauge as  he made great performance analysis of WW1 aircrafts in his recently published book and now is working on WW2. Without the analysis data any chance of FM revision is not to be made.

[Holtzauge]

1 hour ago, 1PL-Husar-1Esk said:

That FM statement might interest Mr. @Holtzauge as  he made great performance analysis of WW1 aircrafts in his recently published book and now is working on WW2. Without the analysis data any chance of FM revision is not to be made.

 

Yes, I'm working on WW2 stuff currently but quite focused on the Bf 109E and Spitfire Mk.I during the BoB right now since that is what my current book project is about. 

 

And yes, the P-38 should turn well: Especially with the Fowler flaps dropped some. I have the J-variant modeled but have not worked with it for some time.

 

When it comes to the P-38 stall speed, I think @unreasonable answered that quite well above: Usually the manual just states the instrumented IAS which is usually way off from the real CAS in most planes.  Have to say I'm a bit worried that he has not posted anything since March, so I do hope he is well?

 

Acceleration is actually one of the easiest metrics to ballpark gauge if things are as they should be: Just look at the power loading. Aircraft should pretty much stack according to this and aircraft with the same power loading should have about the same accelerations times as long as one is not comparing speeds close to the max speed for either plane.