Making Bad Designs Good

[wuzak]

I know Cd is coefficient of drag: What's Cd0?

Zero lift drag coefficient.
https://en.wikipedia.org/wiki/Zero-lift_drag_coefficient

As for the numbers are they linear or exponential?

It's a constant at a set altitude. I think.

How many waste gates are normally used in an aircraft turbocharger of that era?

One per turbocharger.

Was the turbos normally faired?

Some were, some weren't.

The P-47's was faired.
https://upload.wikimedia.org/wikipedia/commons/e/e4/GAC_P-47s_of_Brazil.jpg

(under the rear fuselage)

The B-17's not.
http://www.warbirdphotographs.com/Anoka2/B17-Engine-3s.JPG

I believe the XP-37's turbo was supposed to be faired, but I'm not sure if it was fitted.

The XP-39's wasn't faired.

Coling drag = Cooling drag or Cowling Drag?

Cooling Drag.

The capacity of intercooler was found to be sorely lacking, no doubt due to the lack of any control of boundary layer, along with absence of any flap that will control the air flow.

Okay

• What methods were normally used to control the boundary layer?
  
• When you say "any flap that will control the air flow" do you mean a diverter?
  

[/quote]

A) Usually a boundary layer splitter or duct.

B) He means a cooling flap, like a cowl flap for a radial engine.

Was the intercooler an air-to-air cooler or a liquid-to-air cooler?

Always air to air in US WW2 turbo installations. Only liquid to air ones was the V-1650-3, 7, 9, etc and maybe for the IV-2220 for twin stage supercharged engines.

[KJ_Lesnick]

Wuzak

Zero lift drag coefficient.

Gotcha

It's a constant at a set altitude. I think.

Tomo Pauk was listing several different numbers, in order to determine how different they were I need to know if they were linear or not

One per turbocharger.

Why'd they go with four then?

Some were, some weren't.

What's the advantage of each option?

The P-47's was faired.
https://upload.wikimedia.org/wikipedia/commons/e/e4/GAC_P-47s_of_Brazil.jpg

(under the rear fuselage)

Which reduced the drag of the exposed assembly: My guess is that the issue was which was more important (weight of adding the fairing or the drag reduction)

The XP-39's wasn't faired.

Why?

Cooling Drag.

Understood

A) Usually a boundary layer splitter or duct.

This might drift me off topic slightly, but I thought it was a revelation when the P-51 ditched the earlier radiator in favor of a fixed design with diverter?  If this was the case, it didn't seem that new...

B) He means a cooling flap, like a cowl flap for a radial engine.

To vary the airflow through it...

Always air to air in US WW2 turbo installations.

Was this by requirement, the design of the engines, or just a simple efficiency issue?  While I'm aware the air-to-air layout is more efficient theoretically, it is also often bigger and uses more convoluted airflow paths: A liquid-air intercooler is smaller and more compact even if a bit heavier.  In some designs that actually pays off.

BTW: Engine coolant was in the form of either water & glycol or glycol right?

[tomo pauk]

The XP-39 used 4 wate gates because of the placement of the turbo vs. engine - one under another. There was one exhaust collector-manifold per each of 3 cylinders, each manifold with it's own waste gate. Each waste gate was located next to the turbine.
The air-to-air intercoolers were probably far easier to incorporate on multi-engined aircraft, they don't add yet another system that can be go malfunctioning or damaged.

BTW, if you want to discuss the technicalities of ww2 aircraft, while not cluttering this modelling forum with that, maybe you'd like to take a look at ww2aircraft.net forum.

[wuzak]

It's a constant at a set altitude. I think.

Tomo Pauk was listing several different numbers, in order to determine how different they were I need to know if they were linear or not

He listed the cooling zero lift drag for the P-39 and P-38 and compared that to the total zero lift drag of a P-51. The numbers are directly comparable.

The XP-39's wasn't faired.

Why?

Cooling.

Plus, the turbos used for the XP-39 and XP-37 were notoriously unreliable.

A) Usually a boundary layer splitter or duct.

This might drift me off topic slightly, but I thought it was a revelation when the P-51 ditched the earlier radiator in favor of a fixed design with diverter?  If this was the case, it didn't seem that new...

No, it was not that new. The XP-51 used a variable inlet and exit to the radiator duct. I think Allison models retained this sytem.

Merlin models had a fixed entry and variable outlet.

And no, it was not new.

Always air to air in US WW2 turbo installations.

Was this by requirement, the design of the engines, or just a simple efficiency issue?  While I'm aware the air-to-air layout is more efficient theoretically, it is also often bigger and uses more convoluted airflow paths: A liquid-air intercooler is smaller and more compact even if a bit heavier.  In some designs that actually pays off.

I'm not sure that an air to air intercooler was more efficient. Possibly teh opposite, in fact.

Where the air to air system scores is weight and simplicity. It doesn't need additional heat exchangers, fluids and systems (such as coolant pumps).

[KJ_Lesnick]

Tomo Pauk

The XP-39 used 4 wate gates because of the placement of the turbo vs. engine - one under another. There was one exhaust collector-manifold per each of 3 cylinders, each manifold with it's own waste gate. Each waste gate was located next to the turbine.

Do you have a diagram you can put up?

The air-to-air intercoolers were probably far easier to incorporate on multi-engined aircraft, they don't add yet another system that can be go malfunctioning or damaged.

Yeah, I think in principle air-to-air is better as it's more efficient: However, you cannot always focus on one parameter in a design.  Aircraft are notorious for requiring compromises in one area to help another in order to ultimately produce the best design.  The engine already has liquid coolant for itself: You might need more coolant, but it would scale down the size of the intercooler: I'm not sure by exactly how much but it would possibly make for something that could all fit.

BTW, if you want to discuss the technicalities of ww2 aircraft, while not cluttering this modelling forum with that, maybe you'd like to take a look at ww2aircraft.net forum.

This is a forum for concepts, but the fact is that both of us seem to be the type that want realism in even fictitious concepts.  I think you'd be welcome here -- there was a guy named Evan Mayerle who actually worked on gas-turbine engines. 


wuzak

He listed the cooling zero lift drag for the P-39 and P-38 and compared that to the total zero lift drag of a P-51. The numbers are directly comparable.

So the XP-39 without any armament had a Cd0 that was around 2.4127 times as much as the XP-38?

Cooling.

So un-faired facilitates superior cooling of the turbo itself?

Plus, the turbos used for the XP-39 and XP-37 were notoriously unreliable.

I'm curious why they used an F-type turbocharger instead of the B-types used on the P-38's and B-17's.

No, it was not that new.

Seemed odd to me, but the US seemed to be behind the UK and Germany so I figured it was new here.

The XP-51 used a variable inlet and exit to the radiator duct.

And this was to regulate the airflow in and out of the duct?

Merlin models had a fixed entry and variable outlet.

I'm more familiar with this one than the early one (I was surprised to learn that the earlier layout was so different).

So simple, yet so effective.

[tomo pauk]

The diagram is on the pg. 9 of this pdf: http://www.enginehistory.org/Convention/2009/Presentations/SuperchargingAllison.pdf

I've listed just poweplant-related Cd0 figures for the XP-38 and XP-39, and total Cd0 for the P-51D. The Cd0 for a complete aircraft: the XP-38 was 0.0252, for XP-39 it was 0.0329, vs, again, the P-51D was 0.0176. No wonder the XP 38 was immediately sent to NACA to 'cure' it's aerodynamics.

There were no F-type of turbos, if we're strict on nomenclature. The late 1930s turbo was named 'Form F-10', or 'Form F-13', for example. The 'Type B-2' was basically the Form F turbo, with change of nomenclature - no more 'Form', but 'Type'.
The boundary layer tunnel on the Bf 109F was discontinued with, once the 109G emerged - so we can't say that it's creators were satisfied. The extra volume thus earned was used to install bigger coolers for the DB 605 engines.

Thanks for the tip re. Mr. Mayerle, but I prefer piston engines Hence my recomendation.

[KJ_Lesnick]

tomo pauk

The diagram is on the pg. 9 of this pdf: http://www.enginehistory.org/Convention/2009/Presentations/SuperchargingAllison.pdf

Strange layout to say the least: Would it be possible to enlarge the liquid coolant reservoir within the confines of the aircraft and route some to a liquid-air intercooler and the rest to the engine?  I figure the liquid cooling system would be heavier but it could absorb more heat pound per pound even if the radiator didn't change size.

As for the radiator, the issue seems to be detail related issues

• Intercooler capacity: Rectified by using liquid-air intercooler (the coolant can be located anywhere along the line); it allows an extra set of intakes for the cooling air to be removed from the aircraft.  The engine might require more coolant and some extra pumps, but the design would be "denser" which in this case is what the doctor ordered
  
• Cowl-flaps in the back to regulate airflow through the radiator
  
• If diverters were used in the US, they should have been used
  
• If not, a retractable assembly like the P-51 could have been an option provided knowledge existed.
  

I've listed just poweplant-related Cd0 figures for the XP-38 and XP-39, and total Cd0 for the P-51D. The Cd0 for a complete aircraft: the XP-38 was 0.0252, for XP-39 it was 0.0329, vs, again, the P-51D was 0.0176.

Significantly higher.  While I'm at it, I'm curious how the F4U compared (at the penalty of sounding a bit silly)

There were no F-type of turbos, if we're strict on nomenclature. The late 1930s turbo was named 'Form F-10', or 'Form F-13', for example. The 'Type B-2' was basically the Form F turbo, with change of nomenclature - no more 'Form', but 'Type'.

Gotcha

[tomo pauk]

The Cd0 of the F4U was 0.0267, data from 'America's hundred thousand', reference being the NACA Report L5A30, 2/45.
P-39 was at 0.0217 (a bit better then famed Bf 109F-4), P-40 at 0.0242 (in 1940).

Bell was, with the XP-38, probably trying to put 1.5 L of liquid in a 1 L bottle - turbo, 200 gal of fuel, armament was to be a big cannon plus MGs, tricycle U/C, spacious cockpit canopy - basically, all what P-38 did, but on just one engine. Had they went for the tail dragger U/C from day one, there would be enough of space in the front for cooler system(s), of course the turbo need to be faired ASAP.
Going with non-turbo P-39 was a quick way for the USAF to acquire a reasonably modern fighter in a timely manner. For really high altitudes, the P-38 and P-47B were seen as solutions, plus the P-43, and USAF had the money to go with tose two/three. There was also the XP-40H, that almost got the turbo, but that one got cancelled in order for Curtiss to concentrate on regular P-40, P-53/60 and production of P-47G.

[KJ_Lesnick]

Tomo Pauk

The Cd0 of the F4U was 0.0267, data from 'America's hundred thousand', reference being the NACA Report L5A30, 2/45.

Thank you

Bell was, with the XP-38, probably trying to put 1.5 L of liquid in a 1 L bottle - turbo, 200 gal of fuel, armament was to be a big cannon plus MGs, tricycle U/C, spacious cockpit canopy

Had they went for the tail dragger U/C from day one, there would be enough of space in the front for cooler system(s), of course the turbo need to be faired ASAP.

Now that's a far better argument for the removal of the nose-gear: I'm not sure if it was a requirement, but it was a strongly desired feature if I recall.

[KJ_Lesnick]

tomo pauk and wuzak

I'm curious about two things

1. Would the intercooler radiator (the part on the side of the fuselage) need to be changed if a liquid-to-air intercooler were used?
2. Which scoop drew in airflow for the engine?

[tomo pauk]

There is an item that appears twice, on the diagram on the doc 'Supercharging Allison' pg. 9 that I've provided the link, named 'Cold air intake', here the ambiental air enters the turbo to be compressed.
The change for liquid intercooler would certainly mean that then-current air-to-air intercooler is changed.
The reason why both the P-39 and P-38 went for tricycle U/C is that item was featured in the 'merit points' system used then by the USAC - a proposal what has tricycle will be awarded with the points (vs. the proposal that features tail-dragger U/C), gaining more ponts overall, hence being more likely to be purchased by the USAC.

[KJ_Lesnick]

tomo pauk

There is an item that appears twice, on the diagram on the doc 'Supercharging Allison' pg. 9 that I've provided the link, named 'Cold air intake', here the ambiental air enters the turbo to be compressed.

Okay, so that's where the air first goes in...

The change for liquid intercooler would certainly mean that then-current air-to-air intercooler is changed.

At the penalty of sounding very stupid: Where did the air for cooling the intercooler enter (was that the pipe up top, or did the air from the cold air pipe simply flow through the radiator passages and the airflow through the radiator just carry the heat away and out the back?)

The reason why both the P-39 and P-38 went for tricycle U/C is that item was featured in the 'merit points' system used then by the USAC

Yup, hence my reluctancy to remove it... regardless, what competitor was there for the P-39?

[tomo pauk]

Two intercoolers were installed at the sides of the XP-39. Slipstream went in the front side and entered on the back, a quick glance on the pic of the XP-39 shows the layout. As for the other competitors on the C.P. X-609 - I don't know them.

[KJ_Lesnick]

tomo pauk

Two intercoolers were installed at the sides of the XP-39.

I thought one of the scoops was a radiator for the engine, and the other was for the intercooler?

Slipstream went in the front side and entered on the back, a quick glance on the pic of the XP-39 shows the layout.

The layout is rather strange.  For comparison, looking at the B-17's turbocharger diagram (it's on page 4), as well as this diagram: You have...

• Ram-air intake: Where the air is inducted starting in the wing-roots and fed to the...
  
• Turbocharger impeller: Which of course boosts the airflow's pressure; then feeds it to the...
  
• Intercooler: Which cools the airflow down to an acceptable temperature (intercooler derives its airflow from an intake along side the turbocharger intake, which then presumably flows either in between sheets of thin metal in the intercooler; then provides cooling for the turbine), where it then goes to the...
  
• Carburetor: Which mixes the airflow with a fine spray of fuel, which then proceeds to go through the...
  
• Integral supercharger: Where the air is compressed further and then routed into the...
  
• Engine: An R-1820, which burns the fuel-air mixture, drives the propeller; some of the thermal emissions are radiated out through fins and carried away in airflow that goes through the cowl; the exhaust products go through the exhaust manifold and the...
  
• Exhaust Stack: Where it would normally be expelled into the airflow, but being that this is a turbo; the exhaust gasses end up driving a
  
• Bucketwheel: A turbine which is driven by the exhaust gasses from the engine; the turbine is cooled from the (somewhat warmed) air that was used for the intercooler; the airflow is then routed to the...
  
• Wastegate: Who's job it is to regulate the RPM of the turbocompressor.  After this point, the exhaust parts ways with the aircraft
  

...the P-39's arrangement is very weird in that the airflow inducted for the engine is basically used to cool the turbine .  From that point if I look at things right, the air then flows rearwards, then up through the intercooler (admitted with it's airflow configuration, it would be perpendicular the airflow as seems to be the general case for heat-exchangers), before going into what appears to be the intake manifold.

I think I know where I could find that...

[tomo pauk]

Radiator for engine cooling was in one wing, oil cooler was in another.

BTW, in the talk about Airacuda, you've mentioned that G limit on the A-20 was 3,something. Such was also for the Airacuda -perliminary limit was +3.4 G, ultimate +5.6 G. As a fighter, it was a turkey.
As a what if - maybe reverse the engine and gunner stations front-to-back, so the gunners became rear gunners? That way the radial engine can be used, hopefully the R-2800 on a 688 sq ft airplane. Alternatively - install another pair of V-1710s instead of the gunner's stations, all four without turbo, akinn to what Do-26 had. Both of these changes should turn the Airacuda in an interesting bomber.
The Airacuda carried max of 800 gals of fuel (!) in the wings, even with providing them with self-sealing will not steal much.