Originally Posted by dbaflyer
That would explain the difference in those two flights. Makes sense.
My options are to accept climbing at 120/125 KTS to keep CHT in check or increase cooling flow and/or increase fuel flow for a given RPM.
Cooling has a number of factors, but at the fundamental level it's about the available pressure difference between the upper and lower cowl volumes.
At 85 knots and 2000 ft, standard day, available dynamic pressure is 4.44 inches of water. A really good system will convert 85% or so to increased static pressure in the upper plenum, a poor system as little as 65%. So, a fellow might have as much as 3.8" or as little as 2.9" in the upper plenum.
Let's assume the lower plenum has an exit so large that it is at freestream pressure, no restriction at all.
Now look at the Lycoming cooling chart I've attached below. Assume a standard day, 60F for our purposes. Go up (red) to the 435F at 75% power line, then across to the sea level and 5000 ft curves, then down to fiind the required differential pressures. Surprise....it takes about 5" to maintain 435, and that's not at full power.
Go back and run a line (green) for an extrapolated (guessed) 400F at full throttle, and you'll find it takes about 8".
See the problem? Although the airframe does indeed have a Vy of 85 knots, there is simply not enough available freestream dynamic pressure to cool the Lycoming design at full power/low speed on a continuous
basis. Sure, improvements are possible, but unless physical laws are different in your universe, Vy can only be used short term. Dynamic pressure is a function of velocity squared.
Caveat; the above considers only freestream velocity. A cowl which does a good job of harvesting the velocity of propeller outflow will result in higher upper plenum pressure at high power, low speed. That mostly means moving the enlarged inlets outboard, forward, and upward.
Ok, fueling. First thing, determine how far rich of peak EGT it is at full throttle. If 250F or more rich of peak, drilling a jet for more fuel flow is just a dirty way to make a power reduction.
Are there other measurements I could take to see how efficient my incoming and outgoing air flow is?
Yes, if you're serious and willing to fly an accurate test program. You'll record pressures in the upper and lower plenums at three or more true airspeeds.
I would really like to improve this some. Not looking for 85/90 KT climbs but climbing at 100/110 seems reasonable. It's kind of disappointing to have an RV with a Vy of 85 KTS but needing 120 to keep the engine happy.
If you want to climb at 100 and stay under 400, it can probably be done, but there is going to be a lot