Engine/supercharger tech question about ducting/intakes

[BarnOwlLover]

There were several different types of engine/supercharger intakes, but did the more curves they have harm efficiency with the ducting like it seems to with say modern auto racing engines? I've been told that forced induction helps overcome that to a degree, but is still having a straight shot into the carb/supercharger more effective than having a curved duct designed to reduce frontal area?

 

[ThomasP]

re

. . . but did the more curves they have harm efficiency with the ducting like it seems to with say modern auto racing engines? . . .

As a general rule, yes.

Once aircraft started being able to fly faster than about 150 mph the advantage of having a forward facing intake (in effect forced induction via RAM effect) became noticeable, even on normally aspirated engines (ie no supercharger). At 150 mph it did not add much speed but you might gain 500 ft in FTH at 10,000 ft vs an air intake that drew air from inside the fuselage. Since the RAM effect increases with the square of the velocity, if you gain 500 ft at 150 mph you will gain 2,000 ft at 300 mph. Since the air gets thinner as you gain altitude, you start noticeably gaining Vmax as well.

It was also noticed that having multiple curves in the intake reduced this gain in FTH - mostly due to the momentum loss caused by the change(s) in direction and some loss due to friction, both of which would cause a loss in air velocity which in turn causes a loss in pressure. While the effects of multiple curves in pipes transporting fluids or gases, and RAM effects (to a significant degree) were understood in the scientific community as far back as late-1800s, it did not seem to filter down to the aircraft manufacturers (in practice) until the late-1920s at the earliest.

The Hurricane Mk IIA is a good example that is often used. The FTH for the Merlin XX was about 18,250 ft if stationary. This means that the Hurricane Mk IIA Vmax would have been about 318 mph TAS at 18,250 ft if the air intake had been drawing air from inside the fuselage. Because of the forward facing air intake and RAM effect it gained almost 4.000 ft in FTH with the Vmax being 340 mph TAS at 22,000 ft. The increase in air pressure at the entry to the supercharger meant that the supercharger had to do less work compressing the air. The forward facing portion of the air intake was kept as short as possible to keep the effects of high AOA to a minimum, and the curve was optimized for minimum pressure loss (kind of). The intake was rated at 80% efficiency at the time, which was pretty good for the time.

NOTE 1. The amount of gain in FTH will depend on what altitude you are operating at as well as on the increase in speed.

On 2-stage systems like the P-38 (ie turbocharger + supercharger) the turbocharger is more limited by the temperatures involved (ie the temperature effects of the exhaust gases on the turbine) and maximum turbine rpm than any significant increase in FTH due to the RAM effect at the air intake (I think). But having fewer bends in the induction piping and in the piping between the turbo and the engine mounted supercharger would help reduce pressure losses between stages. I am not sure how much the RAM effect of a forward facing air intake would have on the workload done by the turbocharger at turbo FTH and full power.

 

[GrauGeist]

I have always wondered how much the ducting on the P-47 effected the system's total potential output, versus a system like on the KI-87, which had the turbo-supercharger up front, in the engine cowling.

 

[MIflyer]

When Stanley Hooker, an aerodynamicist, went to work for RR, he was just looking around to get familiar with the operations and noted some drawings that looked interesting. He asked if he could examine them and was told to go ahead. He then wrote a report describing how the angle of the vanes in the Merlin supercharger was incorrect. He was then told he was the new head of supercharger development at RR.

Hooker also pointed out that the air intake for the the Merlin was not optimized and was told they would fix that just as soon as they could finish not losing the Battle of Britain.

Take a look at the intakes of a F4U, which used wing root intakes for both the air-to-air intercooler and the carburetor air, thereby putting the air intake back closer to the engine intake. This proved to be better than the approach used for the F6F, which took the air in at the front and had to duct it back to the engine, but Grumman said the F4U approach tended to promote carb icing as compared to the F6F and even if that cost performance they preferred it.

The F4F used intakes at the front of the engine for both carb air and intercooler air, which led to the need to put the oil coolers a bit out in the wings, since they still had to leave enough cooling air for the engine itself.

Note that both the P-38 and P-39 put the carb air intake for the V-1710 way back next to the engine in order to minimize losses, while the P-51 and P-40 put it up front
and had to duct it back.

One P-39 pilot ferrying to AK for delivery to the USSR shot himself down by tossing a stale ham sandwich out the window, which plugged up the carb air intake.

 

[Shortround6]

When dealing with flows bends are more important than straight runs. 10-20ft of flow through a straight constant cross section duct/pipe is going to cause a loss compared to 5-10ft but a single 90 degree bend is going to be worse. A lot depends on the sharpness of the bend.

I am guessing that the intercooler/s (?) are in the belly bulge with the inlet being the large diameter tube inside the cowling and the outlet being the sloped duct just forward of the wing leading edge. Actual duct/s from the engine nacelle inlet behind the propeller and going under the engine are missing. Description says that intercooler air entered there, went through the 16 blade cooling fan and then entered the intercooler duct/s. Perhaps that is where the engine inlet air was also.

An awful lot depends on the size/shape of the needed ducts and the size/shape of the needed bends.
The US liked to have a certain amount of distance between the engine and the turbo to lower the exhaust temperature just a little bit (100-200 degrees?) to extend turbine life. Like not having blades fail throwing the turbine out of balance leading to catastrophic failure.

P-47 installation. Long but large ducts and pretty straight lines.
Now move turbo to the upper right fuselage behind the engine. Put the intercooler in belly ahead of the wing and you wind up with a lot of pipes/ducts going up and down and trying to fit in gentle shallow bends gets hard.
Differences are guess work. Americans had high temperature alloys for their turbos and their turbos often had at least the life of the engines if not twice the life at times.

There were several different types of engine/supercharger intakes, but did the more curves they have harm efficiency with the ducting like it seems to with say modern auto racing engines? I've been told that forced induction helps overcome that to a degree, but is still having a straight shot into the carb/supercharger more effective than having a curved duct designed to reduce frontal area?

We may be talking about two different things?
Early Spitfire/Hurricane inlets were pretty much a large radius 90 degee bend in a flattened pipe.

Carb intake is pretty good for flow into the carb. Pretty bad for external streamlining. Extending the intake forward is not going to do much for either. Putting sort of teardrop fairing after the carb inlet does nothing for the intake performance but may help the drag of the airframe?
Allisons with down draft carbs could run the intake over the top the engine and not add to the frontal area although surface drag may have been a bit higher.
There is also the question of the manifolding from the supercharger to the inlet ports on the cylinder heads which had nothing to do with the drag/frontal area or pressure rise due to ram. But some engines had a lot better manifolding than others and here using a bit more boost from the supercharger could make up for poor design.

 

[GregP]

I have always wondered how much the ducting on the P-47 effected the system's total potential output, versus a system like on the KI-87, which had the turbo-supercharger up front, in the engine cowling.

I used to have a 2002 Camaro SS. They made supercharger and turbocharger systems for it. The one where the turbo was back by the trunk seemed to work the best.

The theory was that the duct was long enough so that when the turbo spooled up and pressureized the duct, the volume helped protect it from pressure variances. Works that way too for BIG turbochargers, like for a large ship. They have an entire room that acts as a pressure plenum to keep power pulses from affecting the big turbos. Like something for a 1.5 million cubic inch Wartzilla-Sulzer engine ... bore = 38 inches; stroke = 98 inches. Dispalcement is 1,829 litres (111,590 cubic inches) and makes 107,390 horsepower at 120 rpm.

So, I'd bet that once the P-47 system was pressurized, the length didn't affect it much at all. That has been sort of corroborated by P-47 pilots who almost universally have told me that the turbo system was very reliable and damage resistant. That is, small bullet holes didn't really affect boost capability and were easy to patch.

Cheers.

 

[icepac]

I've tested a lot of rear mount turbo systems on our dyno dynamics twin retarder RWD and none worked as well as having the turbos close to the exhaust.

The other problem with them is the fact that the pipe between the head and the turbo get's insanely hot and there have been a few fires.

 

[Shortround6]

GE turbos were huge and were coupled to huge engines. Airflow was around 10,000lbs for an Allison and higher for the late model P-38 engines.
They were also built with 1930s/early 40s metallurgy. Early P-38s had steel plates to shield the Pilot from turbo explosions or thrown blades.
Later P-38 turbos had operating RPMs several thousand RPM higher than the earlier P-38s.

A late war/post war Allison with a turbine recovery system (turbine was coupled to the engine for power recovery) used water injection into the exhaust manifold/ducting to keep the exhaust temperature in the safe zone at high power settings.