War Emergency Power (or Tell Me About Pressure Carbs)

JoeSelch

JoeSelch

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In the past week I've come across the term "war emergency power" twice - and for the first time ever.
First was the Traffic Alert thread (OMG, there are other planes in the sky!!! http://www.pilotsofamerica.com/forum/showthread.php?t=47220&highlight=miles )
Now, as I'm researching pressure carbs.

I'm considering purchase of a plane so equipped. Have been reading cautionary tales re diaphragms drying out and exorbitant rebuild costs...

What's the reality of flying with a pressure carb? How to keep it in good shape. Cost to repair/rebuild. Using a wobble pump. Etc.

The plane I'm looking at is a simple, old-school aerobatic mount - with inverted fuel and oil. (Do not yet know which make/model carb or whether there is engine driven electrical [which *could* power an electric fuel pump].) Would like to know what I *may* be getting into.
 
Selling your "rocket ship" AA1? :D
 
Back in the day of piston aircraft and iron men, most military aircraft had boosted engines. Either superchargers or turbochargers were used to increase manifold pressure and generate more power. More manifold pressure = more air and fuel into the engine = more power, pressure, and heat.

But there was a limit. Depending on the aircraft, the limit might be 60" of manifold pressure, which is about double normal atmospheric pressure at sea level. That limit was set to keep the engine from prematurely wearing out or failing.

In most applications, there was a copper wire stretched across the throttle quadrant which limited throttle movement and prevented the pilot from unintentionally exceeding the day to day limit.

But if there was a Messerschmidt on your tail shooting holes in you, making TBO wasn't your first priority. Surviving was. In that case, you could push the throttle a little harder, break the copper wire, and push the engine up to "War Emergency Power", which took the engine well past its normal boost (and power) limits. IIRC, the P-51 had a recommended limit of 5 minutes at WEP. After that, a tear down was suggested.

In today's GA world, it probably means "I pushed everything (throttle, prop, and mixture) to the firewall and would have pushed them farther if I could have."
 
Can't specifically address pressure carbs, but I have always understood WEP to be a matter of pushing the limits of the engine above the normal rated parameters. Kind of like pushing the engine to or even above the normal redline for emergency/combat reasons. Basically a 'here is where you normally should operate......don't go into this range unless you have a darn good reason'. Similar to what we call Battle Over-ride on ships and combat systems (when you push a B/O button you are essentially bypassing the protective safety features to keep the equipment running as long as it possible can under combat circumstances).
 
Last pressure carb I played with was nearly the same thing as the bendix/rsa fuel injection systems, it just squirts the fuel in in a different place.
 
Sorry... Wasn't asking about WEP, even though it's more fun. Really do want to know more about living with a pressure carb.

And, no, Anthony... I'm adding to the fleet. :)
 
kyleb said:
Back in the day of piston aircraft and iron men, most military aircraft had boosted engines. Either superchargers or turbochargers were used to increase manifold pressure and generate more power. More manifold pressure = more air and fuel into the engine = more power, pressure, and heat.

But there was a limit. Depending on the aircraft, the limit might be 60" of manifold pressure, which is about double normal atmospheric pressure at sea level. That limit was set to keep the engine from prematurely wearing out or failing.

In most applications, there was a copper wire stretched across the throttle quadrant which limited throttle movement and prevented the pilot from unintentionally exceeding the day to day limit.

But if there was a Messerschmidt on your tail shooting holes in you, making TBO wasn't your first priority. Surviving was. In that case, you could push the throttle a little harder, break the copper wire, and push the engine up to "War Emergency Power", which took the engine well past its normal boost (and power) limits. IIRC, the P-51 had a recommended limit of 5 minutes at WEP. After that, a tear down was suggested.

In today's GA world, it probably means "I pushed everything (throttle, prop, and mixture) to the firewall and would have pushed them farther if I could have."
Click to expand...


Many WWII aircraft also had a Water Injection booster, I presume for cooling purposes, and some like the ME-109 a Nitrous Oxide booster. Yes, the same things street racers use.

Joe, maybe you could put in a nice NOX set up, and see what it can really do. :yikes:
 
Anthony said:
Water Injection booster
Click to expand...

I assume that you are speaking of ADI, Anti Detonation Injection, and it did just what the name suggests.

for an example the Wright 3350- used it for 5 minutes at full throttle (65 Inches) to stop detonation it was a alcohol water mix, it required separate tanks and pumping system, and dumped the mix directly to the first stage of the supercharger.

The rolls Dart (turbin) used it as a coolant on hot days to cool the intake air, to replace horse power lost to heat.
 
Pressure injection carburetors used a different system of metering fuel compared to a venturi carb and fuel injection. Venturi's have issues we're all familiar with such as carb ice, poor mixture distribution due to the butterfly, mixture variation due to the butterfly, floats and float bowls creating problems with flight beyond certain pitch and roll limits etc. The pressure carb was the early attempt to remove some of these problems by getting rid of the venturi, the float bowl and float and metering more accurately over a wider range of throttle settings. They were pretty good but considered to be very complex and difficult to adjust. For the military use they got things down pretty well by the end of the war and these carbs helped keep the engines working in any attitude and at all altitudes. Fuel injection came along shortly after the war for civvie's and thus pressure carbs never really caught on. Fuel injection got rid of some of the issues pressure carbs had and helped make more efficient fuel distribution a reality since it no longer depended on moving mixture through long bent intake tubes. The fuel is injected at the cylinder rather than having to move down the tube with the air. Pressure carbs didn't help that much so for all the good things they did it just wasn't enough better to make them continue. If you found a plane with a pressure carb its not likely mainstream current but likely an older high performance type???

Frank
 
One thing to be aware of WRT pressure carbs is that they have about $1000 worth of rubber diaphragms and the original versions of those get stiff with age which makes the carb perform poorly. I suspect that by now most have been replaced with newer synthetic types which are supposed to have a longer troublefree life (time will tell). AFaIK all the originals were black and the newer (diaphragms) are orange so you should be able to tell at a glance if you've got a potential problem.

Also FWIW, there's an OWT that says you shouldn't leave the mixture control in ICO for extended periods because this will supposedly "stretch" the main diaphragm. I looked into that issue when I had an E series powered Bonanza with a PS-5C pressure carb and found that pushing the mixture control back in after shutdown resulted in that diaphragm being pushed forcefully in one direction by a spring while leaving it in ICO pushed it the other direction but less far from the center operating point.
 
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