<VV> jets for power

[Tony Underwood]

At 09:00 hours 09/21/2005, NicolCS at aol.com wrote:
>I'm not a piston engine pilot or any kind of pilot for that matter.  I made
>the silly assumption that the full rich setting on an airplane carb yielded
>12:1 or so, which is the max power afr.


Not a silly assumption at all.   Logic would suggest such.  However 
with airplane piston engines, especially larger ones, there's more at 
work than just max power.


>Evidently, it's richer than that.  Why
>doesn't a piston airplane exhibit black exhaust smoke as it does on a car
>engine when they are over rich?

They DO.   Ever see anything with PW R-2800s working hard at takeoff 
pulling a heavy load?    With the throttles WFO and mixture at full 
rich, an R-2800 smokes like a diesel.   Other big radials do it too 
when the throttles are propped open and mixture is full rich.    They 
actually leave a trail of black smoke behind them.

It's insurance that the engines won't get hurt when serious demands 
are being made of them, especially in this day and age when really 
good gas is hard to find and expensive, and those big radials NEED 
good gas for max output.    Most guys never push those big radials to 
full throttle anymore, they ease their way off the ground whenever 
possible.    Overhauls for airplane engines are *really* expensive.


>Why not grey smoke as when an a car engine is
>passing unburnt fuel out the exhaust?

What's coming out the stacks of those big piston aircraft engines 
isn't unburned fuel, it's incomplete combustion product...  contains 
a lot of soot.   It also won't allow the engine to ping under hard 
load, not good for a big displacement airplane engine.

Bomber pilots during the war would stretch engine power by use of the 
mixture and throttle settings via the temp gauge while working the 
nacelle cowl flaps, juggling engine power vs heating while running as 
lean as possible without dropping power or running the engine temps 
too far into the red.   B-17 pilots discovered that they could run 
those Wright radials well into the red without damaging them (much) 
from overheating, and a good pilot could juggle mixture and throttle 
and cooling so as to minimize drag from the nacelle cowl flaps 
hanging out in the breeze while also saving as much fuel as possible 
(may well need every drop to run full-tilt the Hell away from the 
target area after you bomb it).   It became a bit of an art, although 
the mechanics didn't much like having to overhaul the engines that 
came back with blistered piston crowns and galled skirts resulting 
from the abuse that the pilots put them through.

According to scuttlebutt via an old 91st Bomb Group mechanic I 
recently spoke to, if a squadron "wrench" ever griped about the 
severe wear and tear on the engines, he usually got "OK, *you* fly 
the next mission" from the pilot.

"It didn't stop us from complainin' though."

It was not unusual for a "seasoned" B-17 to wear out 20-30 engines or 
more during the course of its tour, provided it survived long enough 
to do so.

...and people think Corvair engines have it tough...?



tony..