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For the purposes of aircraft performance (best endurance/range speeds, Vx, Vy, etc.) would a propfan be considered a power producing engine or a thrust producing engine?

I would imagine it would be power producing (with some exhaust thrust like a TP400), since they are closer to a turboprop than a turbofan. However, these engines always seem to be rated in thrust rather than power. I always thought that was more of a marketing gimmick, since manufacturers want these to compete with turbofans, but I haven't been able to lock down a concrete answer.

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    $\begingroup$ These answers should help. $\endgroup$
    – sophit
    Nov 19, 2023 at 5:20
  • $\begingroup$ Both. "Power producing" and "thrust producing" are only approximations. That said, propfans are improved turboprops. $\endgroup$
    – Therac
    Nov 19, 2023 at 7:53
  • $\begingroup$ The original definition of power by Watt was derived from a horse turning a mill wheel (against its friction). Clearly P = W/t = F × d/t. The depiction here is actually showing linear force or thrust. One might even imagine the force increasing the height of the object as excess thrust. The mill wheel description is in the same article, along with the math, under "History". $\endgroup$ Nov 19, 2023 at 15:55

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I happen to have an engine deck representative of a GE-36. It is tabulated in terms of thrust, so I'll have to do some manipulations to get a power curve to compare with.

Here is a chart showing full throttle thrust lapse (vs. Mach) for a GE-36 propfan at 35,000 ft. I've multiplied thrust lapse by Mach to give something related to power.

GE 36 Thrust Lapse and Power trend

From this, it is clear that neither is constant with Mach. So it doesn't behave as a classical thrust producer or power producer.

I divided the fuel flow by each of those curves to result in something proportional to TSFC and to SFC.

GE 36 Specific fuel consumption trend

I'm not specifically sure what is happening below Mach 0.3. Often specific fuel consumption shows behavior like that -- but usually when thrust or power go negative or close to zero (not happening here). In general, it is much better to interpolate in fuel flow rather than SFC exactly because of stuff like this.

So lets just ignore everything under Mach 0.5 -- we are at 35,000 ft and full throttle after all. (It might actually be better for me to pull back the throttle a bit, but I'm not going to spend that much time on this).

Above Mach 0.5, the PSFC curve is surprisingly flat. It would seem reasonable to treat the engine as power producing.

Long answer -- behavior of actual engines is much more complex than 'thrust producing' and 'power producing' idealizations. You should work to build an engine deck that models off design performance for the engine cycles you're considering -- this means variation with airspeed, altitude, and throttle (at least).

No, I won't give you the GE-36 deck I have.

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  • $\begingroup$ Interesting; It seems to always be taught (at least at a basic level) that best endurance for a piston engine is speed for minimum power required, and that best endurance for a jet is minimum thrust required (L/Dmax). This goes off of the assumption that less power = less fuel consumption for piston engines, and less thrust = less fuel consumption for jet engines. I guess the crux of my question was whether an aircraft with this engine would experience best endurance at min power required, min thrust required, or somewhere in between. $\endgroup$ Nov 19, 2023 at 20:14
  • $\begingroup$ You're working with the idea that these rules are based on less power=less fuel or less thrust=less fuel. Both are always true. These simplified rules are actually derived from the slightly more complex idea that fuel consumption is proportional to power (PSFC) or fuel consumption is proportional to thrust (TSFC) -- and that the constant of proportionality is independent of airspeed (and throttle). We can calculate both fuel consumption metric, so we're interested in which is most constant with airspeed. Blue line, second chart. $\endgroup$ Nov 19, 2023 at 21:08
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Thrust is a Force and Power is Force × Velocity.

Let's look at the application of those terms with respect to propulsion.

All aircraft are moved by thrust force.

why do we even talk about "power"?

It seems to make sense when the units of Power as Force × Velocity are rearranged to:

Power = Force × distance/time

Now we can say Power = Torque/time

Anything with a rotating mechanical output is most aptly described in this manner. The torque turns the wheel, the wheel transfers the thrust to the ground. Thrust is measured by:

(Torque/wheel radius)/time = propulsive Force/time

So why don't we say Thrust/time?

It seems this is a more proper comparative definition to Power output. In reality this is what all engines generate.

In aviation, Torque turns propellers against their drag. The prop airfoil produces thrust (thrust/time).

A jet produces thrust without creating rotational motion, or does it?

a jet engine must create "power" to turn its compressor.
a fan jet must create "power" to turn its fan

So, power is Torque/time. Thrust is (more correctly) Force/time.

In all applications involving Power, the distance term will cancel out leaving thrust.

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