Speed Mods Reduce Fuel Flows

[plehrke]

A decrease in drag or increase in thrust obviously increase max speed but it also plays a big part in the range equation (translates to reduced fuel flows). I know most people that make speed mods do their testing at WOT and leaned for max power but they also go on cross country flights with their RVs. I am wonder if anyone has a rule of thumb that for every X kt increase in max speed at max power you could get a Y gph reduction if leaned for best economy?

 

[Andy_RR]

not sure if my maths is 100%, but...

... a 1% increase in top speed represents approximately a 3% reduction in total drag power, assuming the same shaft power / fuel consumption rate.

If you wind it back to the same cruise power you'll see approx slightly less than 1% improvement in fuel consumption.

Presumably if any drag reduction is only a parasitic drag reduction (~95% of drag is parastic at max speed) then you'll realise less of the drag reduction gain as you slow down, since induced drag increases and parasitic drag decreases as a proportion of the total.

A gain in propellor efficiency at maximum speed may not translate into a gain at cruise - it depends too much on where you are w.r.t. TAS and RPM.

Back at the parasitic drag reduction case, if you wind the power back to the same cruise speed, you'll see some of the 3% improvement in fuel consumption. If you throttle back at the same altitude, you'll be worsening your BSFC, so you'll lose some of the 3% You might see more than 3% if you use altitude to reduce power - you'll gain TAS and maintain(ish) engine BSFC. Reducing engine speed also improves BSFC for the same power level, but the propellor efficiency characteristics destroy any certainty as to which way it will go in total.

In summary, 1% speed = ~3% FC, if you get everything in your favour and you resist the temptation to go faster!

A

PS: this is only theory, and possibly bad theory at that!

 

[hevansrv7a]

Getting it Right

The idea that drag reduction saves fuel is absolutely correct!

If you look at the spreadsheet that is part of the article (see signature for link) I published in EAA's on-line journal Experimenter, you will find that you can establish your own airplane's drag curve and make very accurate predictions at all speeds and density altitudes up to 10,000'. You will need to first establish your best L/D speed according to my "method X" which makes prop drag "transparent" and is a good approximation of the zero thrust that CAFE used, but without the device.

I note in passing that at 165 kts TAS induced drag is only about 15 ~ 16% of total drag but at 140 kts TAS it is about 26%. It will be slightly different for you if your L/D speed is not 96 kts. These numbers apply to 8000' density altitude. It will be different at different altitudes.

As you know, induced drag increases as you slow down and parasite drag increases as you speed up. Both change in proportion to the square of the speed.

The spreadsheet includes columns for horsepower and fuel consumption.