Two points
(1)There is no simple way to know exactly how much power the engine is producing, unless you have instrumented the crank with strain gauges to measure torque.
(2)Don't get too hung up worrying about prop efficiency. The values you see may or may not be accurate, so there is no point to focusing on them. Instead, look at more basic performance data, such as speed vs fuel flow, etc.
Excellent points and I agree but two comments:
(1)The engine manufacture has figured out what % power its making with MAP and RPM. With accurate gauges and temp you can approximate the power with out strain gauges to measure torque,
(but torque and RPM is the classic measurement or definition of power).
(2)I think we should get hung up about it in that a MT prop is 8 mph slower than a Hartzell BA prop on the same engine and airframe. THAT IS efficiency.
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George,
Speed in knots = RPM (hundreds) * Pitch (inches) / 12.15
TAS of 188.8 kts. In reality I get 171 kts (2700 rpm).
What am I missing? Regards,
Your equation is a ball park. It assumes ideal screw (prop) I guess? There's a big diff from 188.8 to 171 kts, like 34.5% difference in thrust? 171 kts is right at Vans spec, so your prop is fine but no where near 90%. Your simplified equation makes assumption, but its too ideal by 10% obviously. I've seen it before; its a ballpark. From Vans data the Sensenich [72FM8S9-1 (90)] was 1 mph slower than the BA Hartzell @ 8k, 2,500 rpm, so you're in the +80% range? Thats good.
Kevin is right there is theoretical efficiency and real world efficiency. Theoretical is easy to compare with a computer program and mathematical equations (more complicated than your example). Here is a
LINK for the basic definition of Prop equations; getting the coefficients is easier said than done. As Kevin said, you think measuring or knowing power is hard, measuring thrust in flight is really hard (but it has been done). The math to calculate the Ct (coefficient of thrust) is too involved to talk about here. Somewhere the engineers use a mix of math and flight test data to validate or correct each other. With the parameters, coefficients you can calculate theoretical efficiency, which is very simple J * Ct/Cp, where J is the advance coefficient. J = v/nD; v-airspeed, n-RPM, D-diameter. So in general less RPM or Dia efficiency goes up. Higher airspeed efficiency goes up (if RPM and Dia are constant). Of course Ct/Cp is changing and that's more involved, but again you can see RPM and Dia have a big effect on efficiency. Like a wings L/D curve a prop has a Thrust/Drag curve so to speak. The wild thing about a prop is its spinning and the blade is at different velocities and angle of attacks along its length. The prop is twisted and highly tapered.
The problem with the theoretical is the prop is affected by the airframe. The only way to determine "installed" efficiency is flight test. That's what we really care about, flying and climbing faster. The prop does not exist in a vacuum. It's interconnected with the efficiency of the engine & airframe. One prop does not fit all. A prop has to made specifically with the engine, airframe and mission in mind. There are other factors like cost, weight which figure in.
The BA prop has the advantage of being made for the RV, a derivative of a prop that had been made for 30 years for high performance single engine planes like the Mooney. The little tweaking Hartzell did squeezed about 5% more efficiency out of the existing design, when used on the RV.
The older Hartzell is generically a 78% efficient prop, the BA prop is about 82.5% in high speed cruise. I say the BA prop is 82.5% efficient, there are other conditions where it's more efficient, may be close to 90%, but this is a real world condition, not an extreme or impractical corner of the envelope. You can say a prop is 90% efficient, but if you never fly in that condition or the engine & airframe don't allow it, it does not matter. 90% is the theoretical assumed max. To get 82.5% is pretty amazing for a practical real world condition with an everyday prop. Often racing props are very thin & have narrow structural margins and are one trick ponys.
Measuring prop efficiency between props is hard to do, since prop efficiency is tied to the airframe, engine efficiency & flt conditions. The best way to measure the difference between two props is have two near exact planes flying side-by-side. Measure climb rate and speed difference, with the same power (MAP/RPM); this is proportional to efficiency. That's why its hard to verify prop makers claims, there's just no easy way to do apple-to-apple checks. We have the work of Randy, John and Van, who tried several props on their planes, taking good flight test notes.
A constant speed prop gives us control over RPM, which is a big player in efficiency. In general the slower the tip speed the more efficient the prop is. Tip speed is a function of: RPM, Diameter and airspeed. Airspeed adds to the resultant tip speed vector. However there is a down side of low RPM, the engine makes less power and therefore you go slower, but the prop is more efficient. (Why not a gear box PSRU? long story)
In general airframes are more efficient at slower speeds, less parasitic drag. If you are willing to fly at best long range cruise speed (L/D max x 1.32 approx), you can increase range and endurance, but it might take an hour longer to get there. The nice thing about a C/S prop is it works at high and slow speed, since you can adjust RPM, which indirectly changes props pitch or AOA. The prop is reasonably efficient over a broad range. A fixed prop will play along, but its peak efficiency falls off faster from ideal form the designed flight condition, which is usually cruise. Where a fixed prop is right at its sweet spot, its as efficient as a Hartzell. Sensenich did a good job of making a prop w/ overall excellent cruise performance with good takeoff/climb. The Hartzell will smoke the Sensenich in climb and takeoff. However if your airframe is clean and light, you will have such good t/o and climb pref it will not matter much. I'm not down on fixed props. If you can afford the cost a C/S prop, its best efficiency wise, but the Sensenich is a good compromise and less expensive. Wood props offer lighter weight and use to be cheaper. However many of the fancy fixed wood/glass props cost as much or more than a Sensenich metal prop. Performance wise I think the Sensenich is the better performing fixed prop. The other manufacturers are not lining up to prove how good their prop is compared to the Sensenich. Hummmm wounder why?
A fixed pitch prop is efficient only at one condition, less efficient in all others. On take off RPM is to low or in cruise w/ WOT, RPM might be too high. The c/s prop has an ideal condition, usually its cruise, but losses are less in other conditions, like takeoff & climb, where pitch is reduced (high RPM), allowing the engine to make more HP. Even at high RPM, efficiency is OK on takeoff and climb because airspeed is low. Cruise on the other hand, calls for a lower RPM for better efficiency at higher airspeed. You'll go faster w/ higher RPM to a point, but its like driving 100 mph verse 65 mph down the freeway, one is more economical. Both will get you there but one burns more gas (and might get you a ticket).
