UdoR wrote:
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.
The EGT probe temperature is a measure of the average gas temperature. If RPM rises there will be more exhaust strokes per minute so if the exhaust gasses keep the same temperature the EGT should rather increase?
When in the cruise for hours with the autopilot on and not much else to do than entertaining idle thoughts, I have thought about this a lot without figuring out a likely reason. At least I’m glad that I’ve not missed something obvious.
UdoR wrote:
When RPM rises the speed of the gas in the exhaust increases.
When the RPM increases the effective timing changes. i.e. the position when peak pressure occurs will happen further into the stroke.
Ted wrote:
When the RPM increases the effective timing changes. i.e. the position when peak pressure occurs will happen further into the stroke.
Which means that – given that MP and mixture is constant – the peak pressure will also be lower, meaning that temperatures will be lower. That could well explain a lower EGT!
Airborne_Again wrote:
That could well explain a lower EGT!
Aye. Then it should not matter whether the engine is fuel injected or carburetted.
Regarding lift in an updraft I don’t agree to the assumptions that it should be less or equal. It is more, and it has to be.
Consider still air and no pilot reaction, then in an updraft the aircraft will climb with more or less the climb rate provided by the updrafting air:
This is nothing else than lift exceeding drag.
Now consider a pilot reaction to equal out climb rate. This may be done by either reducing thrust or increasing speed, the following example shall show in basic principle with increasing speed:
The increased speed a) “consumes” potential energy to convert it to kinetic energy, and in a stable condition like in a long-lasting updraft b) increases drag in order to equal out the increased lift.
And the change of AoA in an updraft is not negligible. If it was negligible, the aircraft would not climb. You can say that in explicite numbers the angle might change by only 0.1 degree or something, but still the result is that the aircraft climbs. When having excess energy from the engine you can generate that additional lift by means of thrust. But in an updraft, this additional energy giving more lift to the wings is “donated” by mother earth.
You can go to an extreme with this in a paraglider. In an updraft which is strong enough you don’t need any horizontal speed, because all the lift needed to keep the paraglider in the air and climbing is provided by the updraft. In a glider you do the same when flying “dolphine style” (don’t know how it is called exactly in english), you fly as slow as possible in an updraft with a speed close to stall speed, but in an updraft the horizontal speed against air flow may be a lot slower than in still air (not as extreme as in a paraglider, but still noticeable). Same is true for any aircraft.
UdoR wrote:
This is nothing else than lift exceeding drag.
You mean lift exceeding weight? It isn’t, except when the aircraft first encounters the updraft and accelerates vertically. With a stable rate of climb (descent), the the total aerodynamic force equals weight. It doesn’t matter if the aircraft is in an updraft or not. (Newton’s first law.)
And the change of AoA in an updraft is not negligible. If it was negligible, the aircraft would not climb. You can say that in explicite numbers the angle might change by only 0.1 degree or something, but still the result is that the aircraft climbs.
What I meant was that the change in the vertical components of the drag (and lift) is negligible with the small change in pitch. Even if the aircraft pitches down 3° to remain level in the updraft, the vertical component of the drag is not more than 5%.
Airborne_Again wrote:
What I meant was that the change in the vertical components of the drag (and lift) is negligible with the small change in pitch. Even if the aircraft pitches down 3° to remain level in the updraft, the vertical component of the drag is not more than 5%.
Creating an equal negligible change in EGT 
Airborne_Again wrote:
SEP with fixed pitch prop, in cruise, leaned to peak EGT.I have noticed that when flying level with constant power in an updraft
Power is torque*rpm. Consider the torque curve is more or less flat at cruising rpms, the power will increase when the rpm increases. What does EGT show? It’s a mix of lots of stuff, and the position of the sensor is important too. The only thing you can be sure of is with increasing rpm and power, the exhaust gas velocity increases. If there is a flame front (of some sort) in the exhaust, then that front will change position, which could be either way, smaller or longer distance from the sensor, depending on where the sensor is.
