Best Range Speed - AEO vs OEI

[GipsyMagpie]

Looking through the performance manual and discovered best range speed with all engines operating is faster than one engine inop. Why?

I believe the normal technique is to do a tangent to the power/drag curve from the origin. From my manual it intercepts the curve nicely at the best range speed OEI.

Thanks in advance.

[Darkhorse30]

It takes the same amount of horsepower to fly the same speed given the same gross weight and atmospheric conditions, whether AEO or OEI. There may be some small differences in the power required because of the installation of two engines, but not much. Specific range is calculated by dividing Vtrue by Fuel Flow. Since turbine engines are more fuel efficient at their higher power settings (in other words more hp per pound of fuel flow), two engines operating have to develope more horspower to get the best most efficient specific range. Of course drag increases by the square of airspeed and hp increases by the cube, so it is a trade off found by using flight test data to calculate the values. If the name of the game is to go as far as you can, fly OEI. Maybe not the safest thing to do.

[Shawn Coyle]

Since I've had to correct at least one professor who wrote a book on helicopters on this, I'm not surprised that this misconception still exists.

You cannot use power for determining best range speed. You have to use fuel flow.

Whether you use True Airspeed and correct for wind, or go straight to groundspeed is another matter.
But you cannot use power for the vertical axis - especially in turbine engine helicopters. Turbines become more efficient at higher power settings, and this seriously affects the calculation of range.

Darkhorse is right - if you want to squeeze every last mile, go OEI (and hope you can go as fast as you can on two engines). Better yet, climb high (assuming no wind, or even better a tailwind).

[ShyTorque]

Not forgetting that OEI really does mean one engine inoperative, i.e. completely shut down, rather than one just throttled back, which is a waste of fuel.

This may have other implications (e.g. only one electrical generator working so no IFR), or where where the aircraft transmission is severely torque limited when OEI, resulting in a badly reduced cruise airspeed. A strong headwind en route can then completely mess up the plan.

[RVDT]

Specific range is the term.

Wf/distance.

In general most aircraft OEI specific range is better although there are a few exceptions and certain conditions as mentioned can make it a limiting factor.

There are also other possible factors which may need consideration.

On some aircraft models all the fuel on board may not be available to you should you have to have an OEI alternate.

EC135, A109.....................:E

[WhirlwindIII]

Pounds per nautical mile (minus unusable fuel) - FMS does it nicely.

[HeliComparator]

Generally agree with Dark and the sheep

Turbine engines are most efficient (in terms of power to fuel burn ratio) at high power. Therefore its likely that close to OEI max continuous power speed gives the best range. Being AEO at the same speed would mean engines running really slow, poor efficiency - more economical (in terms of power to fuel burn) at higher power settings/speed. Go to higher altitude and its even better.

In the case of the EC225, best range speed AEO is Vh minus about 5 kts - ie around 145kts - but this is because the bleed valve starts to open only a bit below max continuous cruise power esp at low level. Bleed valve partially open = big loss of efficiency.

HC