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Full rich vs leaned: Difference in fuel consumption

Very economical for a twin and shows you that a twin doesn’t burn twice as much fuel as is often claimed

Or just shows that some twins don’t burn twice the fuel of a similar single! (e.g. a Baron seems to be like flying two Bonanzas in formation in terms of fuel burn).

Andreas IOM

A very simple rule of thumb is that you get 1nm/litre from a Cheiftain.

Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

I have some data from a flight done today:

Conditions: Socata TB20 IO540-C4D5D 3B prop, 4800ft +4C QNH 1000 2400rpm

Peak EGT: IAS 146kt 12.5 USG/hr 23" MP
130F ROP: IAS 146kt 14.8 USG/hr 22.5" MP
Full rich: IAS 146kt 17.5 USG/hr 22" MP

To get meaningful results, the RPM and the IAS were held constant i.e. constant power / constant thrust, and the MP was varied slightly to achieve the constant IAS.

So, taking the Peak EGT value as the base, 130F ROP (“best power”) burns 18% more fuel, and Full Rich burns 40% more fuel!

For a comparison, I normally fly (low level) slightly LOP at 23" IAS 139kt 11.5 USG/hr. It is about 50F LOP.

I forgot to try LOP for an IAS of 146kt; clearly the MP would have to be above 23" to achieve that. And the MPG should be exactly the same as for peak EGT, because once you are stochiometric (close to peak EGT) there is no MPG gain if thrust (power) is kept constant. Obviously, you get better MPG if you fly slower and that will be true anytime you are flying above Vbg, which for a TB20 is about 95kt for the airframe (i.e. gliding), though the best-MPG speed is more like 110kt due to the engine efficiency not being constant versus power.

At the above conditions, 146 IAS is 156kt TAS, so we have the following MPG:

50F LOP (with reduced IAS) 13.0 MPG
Peak EGT 12.5 MPG
130F ROP 10.5 MPG
Full rich 8.9 MPG

You might ask: why is Best Power relevant?

  • if I was in a hurry to get somewhere and fuel was not relevant
  • it is needed to reach the operating ceiling (though obviously the fuel flow and speed etc figures will all be different at FL200)
  • a lot of people why 100F ROP or so out of mistaken belief that it is “right”

Full Rich is a complete waste of time and money unless you want to achieve the highest possible speed.

These results will be approximately valid for other piston GA so PPL training industry take note

Shoreham EGKA, United Kingdom
13 Posts
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