The Lycoming -320 may not be a state of the art power plant but has performed valuable service in some of the best known, popular GA aircraft. Including the Super Cub, Cessna 172, homebuilts, and even the Twin Comanche.
It is the power plant of the Warrior, and the Reno Rare Bear racing team used an SOP in their company hack Warrior, to operate it always full throttle – this seemed to keep the engine happy in the Nevada heat as they overhauled it at 3600 hours, and found that it was still within overhaul limits.
While the POH includes data on cruise performance at 55 , arguably these engines are happiest at 65 to 75% power. The mentioned Reno Warrior probably ran at full throttle as this gave the engine extra fuel cooling in the Nevada heat, it also hopefully got the aircraft above the low level thermal turbulence.
At 2500 RPM the Warrior will fly at between 110 KTAS and 120 KTAS, and around 70% power, using 36 litres per hour. TAS improving with altitude.
In the UK flight schools and clubs seem to recommend cruise at 2300 RPM, and use 32 litres per hour as a block planning tool. This has resulted in the quite nice taper wing of the Warrior to be associated with 95 KTAS type cruise speeds.
Looking into the POH and selecting typical UK cruising altitude of 2,000 feet under the London TMA, this 2300 RPM is equal to 55% to 60% during summer months. Also, typically the aircraft mixture is full rich, so block planning at 32 litres is probably sensible. Using POH best economy, with basic carburettor engine leaning technique, the cruise consumption is 26 litres per hour. TAS at 2300 RPM best power is between 95 and 100 KTAS. Deduct 2 or 3 knots for no wheel fairings. TAS best economy is around 90 to 95 KTAS.
Full throttle 75% best economy occurs at FL90 ISA and results in 115 KTAS and 32 lph. Full throttle 75% best power occurs at FL80 ISA and delivers 126 KTAS at 38 lph.
Time to climb to FL80 ISA is 16 minutes at 79 KIAS.
In practical terms the Warrior will not cruise above FL50 in the UK due to CAS, but at FL50, 2500 RPM best power, results in 112 KTAS at 67% and 34 lph. Taking 12 minutes to climb to FL50.
Maybe in nevada, they are always flying so high that full throttle means 75% power or less. Is flying full throttle at 2000ft DA reasonable ?
I read some will fly the 320 WOT, then use a little carbheat and lean to 2500 rpm. A sort of LOP.
(Just registered to respond to this, so “hi, everyone” etc.)
I’m very low time, so take this with a pinch of salt, but I’ve been cruising the school/club aircraft at 65-70% power settings and they do settle to over 100KIAS, drinking less than 30 litres per hour. This is on aircraft without fairings, usually covered in bugs and dirt.
Today I flew a decent length leg at 4000ft (where it was still), 15C outside, very slightly shy of 2500rpm and I showed about 103KIAS. This was with the plane about as dirty as it gets – I know a private owner would be less likely to let it get to that state.
At least in today’s aircraft I wasn’t able to hold 79KIAS in the climb all the way to 4000ft due to oil temperatures – this may be a consideration when planning if your objective it to minimise flight time.
@Winston welcome and thank you for the data point. It works out to 110 KTAS and around 70% power. Wheel fairings would add a couple of knots, and a good wash and polish probably another couple of knots.
