brakes?

[Dobs]

Looks like .4 difference in Cd at M .725.

[FLS]

Looks like .4 difference in Cd at M .725.

That's for the 1" flaps on the model. It's indicative but it's not definitive for an actual P-38.

You'll also notice the P-38 dive flaps are a different design than the 1" flaps on the model, they are a V in cross section and I'm sure there's a reason for that but I don't recall any information on it.

[Dobs]

You are correct..V shaped due to the way they worked the hydraulics in to deploy them.


From here is the only dimensions I can find...."8.5 Inchs and 58 inches in length" "Extrudes 5.5" from the wing to the piano hinge"

"One of the interesting new cases of P-38 pioneering is the use of recently added dive flaps to offset compressibility effect which shifted center of lift from fore to aft portions of wing. Due to the unusually high speeds attained by the heavy P-38 in power dives, shifting of the center of lift caused loss of normal control above the "hydrodynamic" speeds — where air reacts much like water — with a resulting tendency of the plane to go into an outside loop. Since installation of the flaps this characteristic has been overcome.

The flaps are fabricated of three layers of aluminum alloy sheet, flush riveted. They attach, by means of a piano-type hinge, along the same line at which the leading edge of the wing is joined to the outer panel. Actuation is electrical, with a high speed electric motor driving actuating screw mechanisms connected to a curved arm hinged to a fitting on the brace or rearmost of the two panels of the flap assembly. When lowered, the flap stands at an angle of 40° from the lower skin surface line, and at its farthest point is 5½" from the wing to the piano hinge by which it is attached to the brace panel. Two actuating mechanisms, side by side at the center, operate each flap, the actuating arms swinging downward through an opening in the wing skin. The flap and brace panels have a combined chord of 15½", divided 8½" to the flap itself and 7" to the brace. Length is 58". The mechanism is bolted to a heavy casting anchored to the lower skin structure and two wing ribs between which it is located."  --http://migrate.legendsintheirowntime.com/LiTOT/P38/P38_Av_4408_DA.html


From a PDF: (http://migrate.legendsintheirowntime.com/LiTOT/P38/P38_redo.pdf)
"When lowered, the flap
stands at an angle of 40° from the lower
skin surface line, and at its farthest
point is 5½ in. from the wing to the
piano hinge by which it is attached to
the brace panel. Two actuating mechanisms,
side by side at the center,
operate each flap, the actuating arms
swinging downward through an opening
in the wing skin. The flap and brace
panels have a combined chord of 15½
in, divided 8½ in to the flap itself and 7
in to the brace. Length is 58 in. The
mechanism is bolted to a heavy casting
anchored to the lower skin structure and
two wing ribs between which it is
located." 

[FLS]

You are correct..V shaped due to the way they worked the hydraulics in to deploy them.

I'm sure you meant to say the electronic actuator shown in the diagram.

I think a V bulge may have less drag than a plain flap.

[gflyer]

Those silly engineers went to all the trouble of installing an extension/retraction system, and weight,  for those silly things when they could have just left em stuck out in the air since all it really effected was an increase in the dive speed. 

[FLS]

The question is how much drag. When you know that you can put it on the wish list.

Why double the Flap weight if it didn't improve it?

[gflyer]

i sure wish the data existed, wouldn't that be nice?  But this sure makes for a lot of good book learning.  I guess no data for the 47 either?

[Dobs]

I'm sure you meant to say the electronic actuator shown in the diagram.

I think a V bulge may have less drag than a plain flap.

Based on what? Your background in aero?

What part of the V is in the airstream? The backside is academic....see the diagram...it explains the shape.

Electronic actuator.  The part that pushes the Dive flap into the slipstream.

See the diagram with Cd levels...it quantifies the drag. 


Tell me how the V shape affects the fact that the flap front face is in the windstream? 8" x 58" piece of metal stuck down 5" into the slipstream=



Toss in the statements from the pilot operating Handbook.  "the flaps also add some drag...permits diving at a much steeper diving angle" "With dive recovery  flaps extended before entering the dive, angles of dive up to 45 degrees may be safely accomplished." 




Note sure what else is needed....

[Dawger]

The question is how much drag. When you know that you can put it on the wish list.

Why double the Flap weight if it didn't improve it?

They quite obviously needed the structural strength to withstand the aerodynamic forces present at .67 Mach.

The V profile would have no effect on the induced drag value and very likely none on parasitic drag as the airflow turbulence aft of the trailing edge would be unaffected by the panel.

As can seen here

https://youtu.be/eBBZF_3DLCU

As for the correct drag value, the FM builder can just model the dive flap lift, drag and pitch values to match the real world known performance. Its not all that difficult.

[FLS]

Based on what? Your background in aero?

That's correct. My opinion is based on my experience with aerodynamics. I'm not an expert but I know that aerodynamics are affected by the rear of the design as well as the front.

They quite obviously needed the structural strength to withstand the aerodynamic forces present at .67 Mach.

The V profile would have no effect on the induced drag value and very likely none on parasitic drag as the airflow turbulence aft of the trailing edge would be unaffected by the panel.

As can seen here

https://youtu.be/eBBZF_3DLCU

As for the correct drag value, the FM builder can just model the dive flap lift, drag and pitch values to match the real world known performance. Its not all that difficult.

I agree that strength could be a reason for the design but we're just guessing.

Your video of a flap does not show the airflow for a dive flap.

Where are you seeing lift, drag, and pitch values?

[Drano]

This discussion anyways revolves around the same misconception that this is a brake. It's not a brake. Wasn't intended to BE a brake. It was intended to correct a specific aerodynamic problem with certain airframes within certain speed ranges that if left uncorrected tended to turn the pilot into a rider of the lawn dart. It just moves the airflow a bit so that doesn't happen.

A brake, As you would see on a dive bomber is a totally different animal. They're not only much larger but they protrude into the air stream much further. The intent of an air brake is to actually slow a dive bomber in a near vertical decent so it can accurately deliver orders before becoming part of the landscape. Different animal. Different intent.

These flaps are on fighters where speed is life. It was found that too much speed might very well not be and so this was the solution. I know some classify the 38 as a bomber but that's not really being serious.

Sent from my XT1585 using Tapatalk

[Dobs]

It is a dive recovery flap which moved the center of lift so the aircraft would remain controllable at speeds above .67 M and worked until .72ish.

The debate, Drano, is about how the current game models them. It just turns on a light and lets you go a little faster....

I was told earlier that the amount of drag they incur is not enough to be of any concern....yet it shows an increase of .4 in Cd in Diagram 16, and the Pilot Operating Handbooks says it allows you to dive up to 45 degrees (whereas before you were limited to 20) if you deploy them prior to the dive start.

The other part of the equation which is not modeled (and holds true for all aircraft) is the increase in drag caused by reaching critical mach..so the 38 goes rapidly into compressability.

[FLS]

Drano the dive flap is not an air brake but it does add some drag, nobody is disputing that. The question is how much. There is also a question of a pitch response. I haven't seen any data that can be used in the flight model but there is anecdotal evidence.

The diagram Dobs posted is for a P-38 model glider testing different angles of a dive flap in a wind tunnel and comparing the drag. It doesn't tell us the drag of the actual dive flap on the P-38.

[Drano]

Oh I get how it works. I fly the 38L pretty much exclusively, for years, and use the flap all the time when heading downhill, especially so at higher alts where your indicated airspeed is gonna pick up REALLY fast in the 38s. If I'm forced into J or G because of ENY or events I'm usually mashing the flap button in dives by force of habit. It's of course not there in the J or G and I've gotten myself into trouble getting faster and losing pitch control than I could in the L. Lose a lot of alt that way! IMO it does work as intended. However, with this caveat: if you have combat trim engaged and hit the flap, combat trim will almost if not entirely counteract its effect. Trim manually and it pitches up. Just how much it does or should is anyone's guess but it DOES happen. FWIW I'm a manual trim guy for this exact reason.

I'd agree that what data exists regarding the flap is almost purely anecdotal. It seems to ammount to they tried it, it worked, added it to later block Js and beyond and just went with it. Pretty much it. Nobody seems to have tested extensively JUST for the performance of the flap in RL. If they had, surely it would have cropped up by now after this horse has been beaten thru to the magma layer year after year and then maybe added to the model.

It just seems to me... And I may be missing the translation... That everyone focuses on the drag, the drag, the drag. That's Always at the center of this discussion. That leads me to believe peeps WANT to use this as a BRAKE, which it is NOT. It was never intended to slow the plane down. The confusion starts when people start referring to it as a brake. You need look no further than the subject line of this thread. Just might tend to give the impression it's gonna do something that it's never gonna.

Sent from my XT1585 using Tapatalk

[Dobs]

So knowing the deployment of dive recovery flaps moves the center of lift enabling control of the aircraft at higher speeds.....this is not the topic of discussion.

Knowing that it isn't a dive brake is not the topic of discussion.
(THIS IS A DIVE BRAKE)


What is the topic of discussion is that deploying them (RL) causes drag and enables you to dive up to 45 degrees without slamming straight into compressability.  How much drag? .4 change in the Cd according to the diagram...what that means to the coder is up to them.  Presently I turn the dive recovery flap on when I take the runway....because it has ZERO effect on the Flight model other than allowing you to go a little faster. 

So yes we do understand they are not a brake, but trying to get them modeled where you can actually point your nose somewhat down hill without wearing out your trim button (and yes I fly manual trim as well).....

The other issue is compressability is modeled at .65 vs .67.....  .65 was the placard limit to keep you out of compressability effects. Dive recovery flaps enabled you to go all the way to .72 before entering compressability.

Good input Drano (from a fellow 38 driver), and I know there was a lot of posts to read through...above is a summation of the discussion (not trying to be dick, just keeping it brief).