Engine Power/high CHT vs weight (physics question)

[jrwilson]

Hi All,

Had a debate with my mechanic this week regarding engine power.  My CHTs were high on a recent flight and the mechanic was rather insistant that the CHTs were higher because I had more weight in the plane on  this flight, therefore, the plane was using more power and making more heat.  I disagreed with that idea, as horsepower is unrelated to weight.

I explained to him I maintain a constant airspeed during climb (120) and accept the decreased rate of climb the extra weight will give.  I have a JPI 700 and we downloaded the info and it showed the temps were up.  The plane had been in for new spark plugs and an oil change right before the flight.  (I think the baffles didn't get sealed up real well, which caused the extra cht, as I taped it up and it was cool again).  So I think the CHT issue is resolved, but still want to win the debate  

Mechanic says the extra weight will cause engine to work harder during climb and cruise.

Ok, so Force will be affected by weight (F=mass * accell), but if I'm at 120 constant speed, aren't I unaccelerated?  The work (power) formula uses force, but not sure if it really applies well to this situation.  My thought is percent horsepower is what matters, which is unnaffected by weight (percent full MP * percent full RPM = percent horsepower).

Will extra weight make an engine run hotter, even during cruise?

[Ned Gravel]

Here is one data point.  But it is telling.

Takeoff and climb at 2200 lbs (375 below max gross).  CHTs are all between 320 and 350.  WOT, 2700 RPM, 120 mph, cowl flaps closed (don't need them open at these temps).  Drop back to 2500 RPM at the transition altitude (whatever ATC has assigned to me).

Takeoff and climb at 2575 lbs (loaded for Oshkosh camping). CHTs were all between 320 and 350.  WOT, 2700 RPM, 120 mph, cowl flaps closed (did not need them open at these temps).  Dropped back to 2500 RPM at the transition altitude of 4000'.

In conclusion, no change.

CHTs do not appear to be affected by the airborne weight of my aircraft (anywhere between 1620 lbs empty and 2575 lbs at max gross).  I can confirm that my CHTs are very much affected by the repair of my doghouse (or baffling in your case).  Mine used to run as high as 415 in the bad old days before my overhaul was completed and the doghouse rebuilt.  Now they are in the bottom half of the 300s - and some days even cooler.

Your mechanic may be missing something.  I would get a second opinion from a knowledgeable A&P (someone who knows Mooneys) Mine is just one data point.  YMMV.

 

[conom06d]

Nice, another member from the Sacramento region!

 

I feel that if you were following a specific Feet/Minute climb this would affect the CHT's depending on weight; however if you are maintaining the desired airspeed and sacrificing the climb rate, as you stated, your CHT's shouldnt change

[carusoam]

Thanks for throwing the engineers a bone!!!

From an energy balance point of view....

Energy in (fuel burned) - Energy out (exhaust and cooling) = heat retained (temperature).

Fuel burned:

WOT, max rpm, max fuel flow.  The engine is not going to know there is extra weight in the back.  All of the fuel is turned into heat.

Cooling:

Since you climb at constant speed and accept given rate, cooling is maintained at a constant (air speed through the dog house, geometry of the doghouse and OAT must be kept the same to make this statement true.)

What will be different.  climb rate (at constant speed), Angle of attack while cruising.

If you wanted to maintain the same airspeed under different load conditions, more power would be required at heavier loads.  Same airspeed, would result in higher cylinder temps. (theoretically speaking).

I am thinking....Outside air temp, density altitude, baffle geometry (changes) would be more likely to change cylinder temps than a few hundred pounds in the cabin.....

Best regards,

-a-

[jrwilson]

Thanks for the input guys.

Either way, temps are back down, so I'm happy.

[Piloto]

Your mechanic is right. Everything else constant added weight will increase the CHT. That's because the AOA is higher in order to carry the extra load. This reduces the cooling efficiency. You can improve the cooling by lowering the AOA (lower climb rate).

José

 

[DaV8or]

Quote: Piloto

Your mechanic is right. Everything else constant added weight will increase the CHT. That's because the AOA is higher in order to carry the extra load. This reduces the cooling efficiency. You can improve the cooling by lowering the AOA (lower climb rate).

José

[KSMooniac]

Climb is a function of excess power, not AOA, for a given airspeed.  If the OP kept a constant IAS during climb, then only climb rate will differ with weight and AOA will be constant.  In cruise, AOA will vary with weight, and thus IAS will also vary.  Heavier = slower.

I don't know what kind of Mooney is under discussion, but I suspect a J or later and the baffle seals were not in the proper position after maintenance since the high CHT came right after a maintenance action.  (I have no idea what "taped it up" means, though!)  Go with the simplest explanation first...something changed as a result of the maintenance causing an anomoly.