Cowl flaps operation

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Artur
Staff Sergeant
Posts: 306
Joined: 12 Jun 2015, 08:37

Cowl flaps operation

Post by Artur »

I am fresh for Bonanza as i have just 24 hours on the counter but have question about cowl flaps and cyl heads temp.
Currently i do cross country flights in Africa.
Just landed, outside temp is 40C, my cowl flaps are fully closed, mixture is lean (40%) so no fuel cooling effect, and i do very long taxi ( over 10min ) - yet my cyl head temp is only 125C and going up in extremely slow pace - such slow that i do not feel i need to open cowl flaps at all.
Is it normal ?

Jarek
Senior Airman
Posts: 180
Joined: 31 Aug 2018, 14:59

Re: Cowl flaps operation

Post by Jarek »

While CHT simulated by A2A are rather on the low side (200 is normally expected at full power), but I would not find it much abnormal in these quoted conditions.
1. Power = CHT Temperature. Running on low rpm puts much lower thermal stress on the engine than climbing at 2500 rpm and 90kts speed where cooling air need more room to "escape". So on the ground at 1000-1200 rpm it should reach its equilibrium within green with flap open or not. On idle, when air flow generated by prop is very low, this CHT may increase. it is recommended to have cowl flaps open when engine is started, even if OAT is at zero C.
2. Combustion efficiency drops with temperature (less air fills cylinders), the highest thermal load is expected around ISA/Room temp. at which the engine reaches its rated power. Deviation up from rated conditions will decrease effective power and thermal load at fixed mixture setting. The formula is:

P = Ps * sqrt ( (273.15 + Ts) / (273.15 + T) )

Where:
P = power at actual temperature
Ps = power at standard temperature from engine power chart
T = actual temperature *
Ts = standard temperature *
* temperatures in degrees C

Similar corrections are applied for air humidity. The higher vapor content, the less power is produced. Thus the "dry manifold" term used on power charts.

Artur
Staff Sergeant
Posts: 306
Joined: 12 Jun 2015, 08:37

Re: Cowl flaps operation

Post by Artur »

Jarek - appreciate ur detailed answer.

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