How good is an airplane?

The way we typically look at these types of problems is with a FIGURE OF MERIT.

I propose the following figure of merit for any airplane:

Score = ( square(max cruise speed in knots) * max range with 45-minute reserve in nm * payload with full fuel in lb)
/
(dirty stall speed in knots * fuel in gallons)

This score says that it is VERY good to have an airplane that can go fast, and can go far, and can carry a lot of people, and can go slow enough to land safely at many airports, and is not too expensive to fill up with gas once you DO land.

In other words, this score is one possible indication of how good an airplane is.

(note: we SQUARE the speed because the DRAG on an airplane goes up with the SQUARE of the increase in speed, so it takes FOUR times the thrust, or one QUARTER the drag, to go TWICE the speed. Also, we want our airplanes to GO FAST. I mean, X-Plane.com is not about tugboats, right?)

Now, let's look at the figures of merit of a few airplanes in the 4-to-6 seat size-class:

So this is interesting... the Citation Mustang, for example, delivers a horrible score.

Why?

Well, look at the speed compared to the TBM-850... The Citation is barely any faster! Yet, the range is quite a bit worse, the payload worse, and the fuel requirements simply horrible!

This is hardly a surprise to an aerospace engineer... Jet engines are much less efficient than propellers in the 0 to 400 knot speed range... jets only become the best method of propulsion as the aircraft speed approaches the speed of sound.

The TBM also soundly spanks the Piper Meridian. Why?

Well, the Meridian, I believe, has an engine that is turbo-normalized to HOLD it's power to a high altitude, delivering very high cruise speed... and while it DOES use more fuel, it delivers the RANGE to compensate for the higher fuel bill.

The Evolution, though, pulls down the pants on every plane on the chart, and then ties their shoe-laces together and then gives them an atomic wedgie. How does it do it?

The secrets are several:

-Carbon fiber construction... this thing has 750 hp, yet weighs only 2250 lb empty!!! This leaves PLENTY of weight for a BIG payload.
-Efficient wing... this thing has a high-aspect ratio wing that is very efficient.
-Low-drag fuselage... this thing has a very streamlined body.
-Turboprop power and efficiency... much faster than a recip, and much more efficient than a jet for this speed range, a turboprop moves you at 300 knots with low fuel flow. It is only a LITTLE BIT slower than a Citation Mustang, but goes much farther, carrying MORE, on LESS THAN HALF the fuel!

Remember!!!

1: ALL propulsion systems are only efficient if the EXHAUST speed is very close the AIRCRAFT SPEED.

2: JET engines are only efficient if their exhaust speed is very close to Mach 1. (you have to clamp down on the nozzle on the garden hose to get it to kick back, right? You must do the same thing with the jet exhaust nozzle)

3: Therefore, all jet airplanes must go very close to Mach 1 to be efficient. No surprise that all the airliners travel at Mach 0.85! No surprise that the Citation Mustang gets a much lower score than the turboprops.

Now, a Citation-X travels very close to Mach 1, and would probably deliver a very impressive score... but we are just looking at 4-seat planes here. The thing to remember is that since jet exhaust needs to be Mach 1, and engine exhaust speed must be similar to flight speed for efficiency, jets are only good if flying close to Mach-1. For lower speeds, use a propeller. The scores here clearly show this.

One more tidbit of information for you: Since the exhaust speed must be comparable to the flight speed to give good propulsive efficiency, that is simply another way of saying that the propwash or jetwash should be very low to give good propulsive efficiency. The propwash of the Evolution at 300 knots: About 10 knots. In other words, at 300 knots, the propwash is coming out of the prop of the Evolution at only 310 knots... just a 10 knot increase. In other words, if you were standing on top of Mount Everest and an Evolution flew by... the propwash that hit you behind the plane would be only 10 knots, or 11 miles per hour. You are simply NOT going to beat that kind of efficiency!

CAN YOU FIND AN AIRPLANE WITH 4 TO 6 SEATS THAT HAS A BETTER FIGURE OF MERIT THAN THESE PLANES?