VAF Forums - Hartzell 72" or 74"?

Quote:

Originally Posted by 13brv3

Greetings,

I finally made an engine decision for the RV-8, and have an O-360 on order. The plan is to use the new blended airfoil Hartzell, but I notice that it is available in two diameters, 72" and 74".

Well it is funny that you asked. I called Hartzell on the very subject and just got some great data from them.

In another thread, it was stated 2600 RPM gave higher speed than 2700 RPM. I have always found an extra 100 RPM gave a slight 1-2 mph speed advantage, at least in my two RV's. It makes sense from the fact a Lycoming makes about 2-5 HP more for every 100 RPM (in general round numbers). More HP means more speed in general. Well yes if the prop efficiency does not change (too much). Factors affecting prop efficiency: air density, RPM, aircraft speed, blade angle and blade design (airfoil/twist/tip/shape). HP and airframe drag characteristics also affect prop efficiency indirectly by virtue of total forward speed and prop wash effects. Stay with me, it is worth it.

I asked for data on the HC-C2YK with the 7666-2 & -4 blade. I wanted to find out if lower RPM at high speed cruise, at altitude, could give a higher speed than higher RPM. Now keep in mind this is max speed not max range or MPG. The following is REAL data from Hartzell.

They sent me data with tip speeds of MACH .70, .75, .80, .90. The data gives blade angle, Ct and efficiency. You need to know "J" and "Cp", Advance factor and Coefficient of power and do lots of interpolation between J, Cp and Tip speed.

J=V/n * d
(V= velocity, n=rpm, d= prop diameter)

Cp= Hp/p * (n^3) * (d^5)
(Hp = shaft HP in watts, p = air density at altitude, n^3 = rpm to the 3rd power, d^5 = diameter to 5th power)

You can see HP, RPM, diameter and air-density are all factors easily plugged into the equations (in the correct units). HP or power can be derived from Lycoming data. V or velocity is not known directly, but you can make some assumptions or measure it in flight. The big factors in efficiency of a particular prop is rpm and diameter. Small changes in other factors have less effect (less not none).

Well what did I find? Could a RV go a few miles per hour faster at 2,600 rpm than 2,700 at WOT, at 8000 feet DA, 74" dia prop, at a tip speed approx 0.85 mach, at approx 195 mph?

(From Hartzells' data) Prop efficiency at 2,600 rpm is 0.794. Prop efficiency at 2700 rpm is 0.790, at approx the same conditions (but with 3.2 more HP and assumed 1 mph faster). 2700 rpm is 0.40% less efficient. Well does the extra 3.2 more HP is enough to cancel the small loss in prop efficiency? MAY BE/MAY BE NOT NOTE: A 3.2 HP increase should increase speed about 0.78% (based on airframe drag alone) or 195 mph to 196.5 mph. Since prop efficiency reduces .40% (with 100 rpm increase) we should still increase speed of about .38% (0.78-0.40) or 0.74 mph (195*0.0038). In this case 100rpm will not slow you down.

I know 0.74 mph is a small point, but flight test of the builder I mentioned showed a speed drop of 3 mph going form 2600 to 2700 rpm. This may be attributed to the fact of different prop, airframe (RV-10), engine and typical flight test data errors. He did good work and not criticizing, but it does not match my experience.

Well this gets tricky. It is all related and theoretical prop data does not predict speed for small changes in RPM. This is what the Hartzell engineer wrote:

Dear Sir,

You are correct in that the propeller thrust goes up slightly as the RPM is advanced due to the increased power. If we were only concerned with propeller thrust, it would be very easy to explain things. In reality, however, it is net thrust that propels the airplane. The fuselage, landing gear, wing roots and tail surfaces are all immersed in the propeller slipstream. The slipstream velocity is higher than that of the aircraft and it contains individual blade wakes, so those parts of the airframe that lie within the slipstream experience a drag increment - or thrust decrement - that is not only dependent on the applied power, but also on the airframe design and operating conditions. It is possible that, when adding RPM (power), the conditions in the slipstream change such that the slipstream drag increment is greater than the additional thrust, resulting in a net performance loss.

Since the performance differences are so small, we can only guess as to the exact causes and the experience on an RV may be different than on another airplane design. For this reason, propeller performance is normally presented as "uninstalled", without considering the effects of slipstream losses. The attached files can be used to estimate the prop thrust and efficiency of a two blade propeller using F7666A-2 blades (74 inch diameter) under any reasonable flight condition. The MAPINTRP program determines what the propeller performance is. (after) Comparing the results to what your aircraft is doing, the differences are most likely attributable to installation losses.

Best regards,

WHAT IS BEST 72" or 74"

To answer my own question, yes a slight RPM decrease could improve speed in the exact right condition or can decrease speed. Because of my experience I have found a slight speed increase with 72" dia props. A 74" prop on a different model plane could be a case where rpm increase results in a slight airspeed loss. A prop design matching the airframe is critical, this is where the Hartzell "Blended" airfoil comes in.

To answer you question, yes at high speed, in general, a smaller dia will improve prop efficiency. Also at slow forward speed (CLIMB) the larger diameter will increase (thrust) efficiency:

In the above cruise example with a 74" dia prop, 195mph, 8000ft DA, efficiency was .794/.790 for 2,600/2,700 RPM respectively. With a 72" prop, efficiency is .814/.807 (tip speed .825M). The 72" efficiency goes up about 2% over the 74" prop.. With a 72" prop the speed increase with 100 RPM (2600 to 2700) will almost be Nil (.15 mph), but you are already going 1.2 mph faster than the 74" in the first place. So with a 72" prop, airspeed is almost a wash going from 2600 to 2700 rpm. Therefore unless racing, 2600 rpm would be better (less noise and FF) for about the same speed. This is in-line with my flight test. Of course we all tend to run lower RPM's for normal cruise (2350-2500).

Bottom line props are complex and the only way to determine the best RPM for best forward speed is to flight test it. Also measuring 1 mph in flight test is hard to do (and each time you change power you need to adjust mixture).

What about CLIMB? 100mph, tips speed 0.80M , 180hp@ 2700 RPM and sea level:

Prop efficiency in climb:
.................Prop Dia.
RPM........72"...........74"
2600......0.7468......0.7543
2700......0.7470......0.7544

Now notice at 74" is NOW more efficient than the 72". Also notice both the 72" and 74" dia props are more efficient at 2700 than 2600 by a tiny amount! With 5 more HP at 2700 rpm and higher prop efficiency there is no doubt about RPM to use. For sure HP rules in climb. HP has a bigger affect to climb than HP does to top speed. HP is almost proportional to ROC. With a 180HP engine @ 2700 rpm, you will have 175HP @ 2600 rpm (sea level). 180/175= 1.0285 or about 2.8% more climb based just on HP. Prop efficiency is greatest with a 74" prop @ 2700. Note: 72" prop is 0.74% less efficient than the 74" at 2700 RPM, but this is only worth about 16 fpm. HP rules! MATH IS FUN :D

In conclusion more RPM will make more noise, burn more fuel and may give a small increase in speed for many normal conditions; however this is really an issue for racing; since we don't fly around on a routine basis at 2700 rpm or even 2600 rpm for that matter. For max efficiency range, especially at high speed slower RPM is better in general regardless of prop diameter. Of course airframe efficiency (best L/D) is better at slower speed and engine efficiency (power for fuel burn) may improve slightly (friction) at lower RPM (efficiency = HP/FF not just FF, piston engine efficiency really does not change much except at full WOT, which is better than with a partially closed throttle or throttle body). That's why WOT at an altitude (+/-8000) which gives 75% or less power (so we can lean) is the best efficiency.

The higher you fly the better range (to a limit). With the need for O2 above 12,500', a practical range for cruise altitudes is 080-120, depending on air-temp/gross weight. Despite the promise of turbo chargers, many things work against you as you climb higher (with a piston/prop powerplant), not to mention complexity, weight, maintenance and expense of the turbo system.

For me I run a 72" prop because I am interested in top speed than climb. (Really I got a great deal and I am cheap. :eek: ) I might suggest you get a 74", since the Blended F7496 comes in 74". There is the practical consideration of ground clearance; if you plan on flying in dirt, shorter blades are less likely to pick up rocks. Older RV-4's (short gear) IMO need a shorter prop. Hartzell even makes a 68" blade for the Lancair's. Since we are talking about .5% more efficiency for cruise, .75 mph (shorter prop) and about 0.8% greater efficiency for climb, 20 fpm (longer prop), it's a toss-up.

George

Note: This is very specific data. It ONLY applies to the Hartzell C2YK-1BF/F7666A-2 & -4 on a typical two seat RV. From the data I see the Hartzell prop produces great performance / efficiency for a range of flight conditions we fly. The "Blended" prop, C2YR-1BF/F7496, is even better and designed for 74" (I think). For the record at 2300/2400 rpm cruise efficiency is .831/.827.