Context: SEP with fixed pitch prop, in cruise, leaned to peak EGT.
I have noticed that when flying level with constant power in an updraft – also in very weak ones (100 fpm or less) – the EGT will drop 25-50°F. I have thought about an explanation for this but can’t find one. As you have to lower the nose to maintain altitude, airspeed will increase and so will engine rpm which will cause a slight decrease in manifold pressure as the throttle position is constant. But why should all that cause the EGT to drop? Needless to say, when you come out of the updraft the EGT will increase again.
That is a puzzle. I can’t think why it should be.
Additional question – does the fuel consumption increase while in the updraft? Or does it remain the same, with the drop in manifold pressure offsetting the increased rpm?
In an updraft two things happen. The angle of attack changes, so that you generate more lift – but also the airflow to the induct changes a bit. Maybe with the changed inflow of air to the engine more (or less) RAM air effect is generated? The other thing is that in an updraft the air is typically warmer than its surroundings and thus a bit less dense.
Another guess: When the RPM rises, but the fuel flow is constant, then the fuel/air ratio should be altered. So that more air is pumped through the engine but mixed with a smaller fuel/air ratio, so being a bit leaner. … So it might depend whether you fly LOP or ROP and whether it is a carburetted or injected engine…
Another option?
P.S. I never watched the EGT like this, only noted in a glimpse yesterday that EGT was affected as I flew in rain.
UdoR wrote:
In an updraft two things happen. The angle of attack changes, so that you generate more lift – but also the airflow to the induct changes a bit. Maybe with the changed inflow of air to the engine more (or less) RAM air effect is generated?
If I keep a constant altitude, lift is also constant. Airspeed increases so the angle of attack will decrease, but at cruise speed the angle of attack is already so low that the 5-10% increase in airspeed has negligible effect on AoA.
The other thing is that in an updraft the air is typically warmer than its surroundings and thus a bit less dense.
That’s true in a thermal, but I’ve also noticed this effect in other situations (like waves in an inversion) where the OAT doesn’t show any change.
Another guess: When the RPM rises, but the fuel flow is constant, then the fuel/air ratio should be altered. So that more air is pumped through the engine but mixed with a smaller fuel/air ratio, so being a bit leaner.
I should add that at least in the last instance I noticed this was in a carburetted aircraft, so the engine pumps fuel/air mixture which shouldn’t change when the throttle and mixture control have the same positions.
So it might depend whether you fly LOP or ROP and whether it is a carburetted or injected engine…
I always fly at peak EGT.
I don’t have an answer but maybe it’s easy to think in terms of the target pEGT value?
- How pEGT changes when you hit hot updraft air OAT vs cold stationary air OAT?
- How pEGT on fast descent ASI (&VSI<0) compares to pEGT on slow cruise ASI (&VSI=0) at various altitudes?
What does the p in pEGT stand for? Not even google knows anything.
It may also be a secondary effect. EGT is an intermediate value, or more precisely it is the temperature of the EGT probe rather than gas temperature. When RPM rises the speed of the gas in the exhaust increases. This could lead to a lower indicated temperature.
I can’t think of any setup on how to proof this in a fixed pitch prop setup.
pEGT to mean Peak EGT, the max value of EGT you can get when moving the mixture knob for fuel flow FF, I get near 1525F on standard days
I am sure there is a general function of engine thermo-mechanics: MP, RPM, PA, DA, OAT as well as airframe aerodynamics: ASI, VSI, AOA…
Never heard of pEGT…
I don’t know the answer but basic physics tells you all engine temps – except oil which on a Lyco is thermostatically controlled to a specific temp value – will scale with OAT, and similarly EGT will scale with the general engine temp (starting with the temp of the fuel system, starting with the fuel servo).
EGT will probably also scale with exhaust back pressure and such. EGT is not a temperature of “anything”; it is an average of a stream of much hotter gas pulses going past the sensor, so any change in the conditions in the exhaust stack will change the reading.
Accordingly, when you have decent engine instrumentation, you quickly realise you never see the same thing twice 
If on autopilot, ALT hold, in an updraught
“If I keep a constant altitude, lift is also constant”
In an updraft, I think lift from wings will be less. In a downdraft lift from wings will be more, to keep constant altitude.
Maoraigh wrote:
In an updraft, I think lift from wings will be less. In a downdraft lift from wings will be more, to keep constant altitude.
Ignoring the effect of the vertical component of propeller thrust for a moment, if the aircraft has constant vertical speed (in particular, zero vertical speed) the total aerodynamic force must be equal to the aircraft weight, otherwise it can’t have a constant vertical speed. If the aerodynamic force on an object is zero, it will fall no matter how strong an updraft it may be in.
You’re right that – as lift is defined as the force perpendicular to the aircraft flight path and drag is defined as the force parallell to the flight part – when the aircraft pitches down in an updraft, part of the drag will have an upward vertical component so the lift can be reduced. But I would think that with a pitch down of only a one or a few degrees this effect will not be noticeable.
On the other hand, when you pitch down, the vertical component of the propeller thrust will also change in the downward direction. That effect should be the same as the effect on drag.
Anyway, this is an interesting discussion in its own right, but doesn’t have any bearing on the EGT.
