ES Performance differences with Hartzell and MT props?

[Dan OBrien]

This was posted LancairTalk, but thought I'd port it over in case readers over here have knowledge of Hartzell v. MT Prop performance.

I put the batteries in the back of my ES and chose the lighter MT prop on the front because I was told the ES tends to be nose heavy.

Lou Rubin asked:

"Dan
Did you lose and performance with the MT? I would love to save 20 lbs off the nose. I have a foward CG?"

Lou,

I never had a Hartzell on, so I don't know.

I put two Odyssey batteries in the back and an MT prop on the front hoping to take nose heaviness out, and at first I thought I over-did the adjustment, as I can't get anwhere near the front part of CG envelope. But honestly, I think I've got it about right. I have a very small amount of nose down elevator when trimmed in cruise with full fuel and only me--not sure how that compares to others--and I don't run out of nose up elevator trim when landing.

Someone knowledgeable told me some years ago that the ES is slightly faster in cruise with the Hartzell than with the MT (the person was talking like a knot or two or three), but that MT did a little better in climb. I don't know whether these points are true, but I think it is pretty hair splitting for modern constant speed propellers of the same number of blades and size. Why do I say this?

Years ago I got Hartzell to give me some performance data for the propeller used on the ES. When I plugged in book cruise numbers for the IO550 ES (the claimed book # I knew at the time was 191 KTAS at 75% power at 10,000 feet), I found that that would mean a Hartzell propeller efficiency of 0.887, which is quite good. (Design textbooks say constant speed props have efficiency in the 0.8 to 0.9 range; Gudmundsson's "General Aviation Aircraft Design" book suggests using 0.85 for the Cirrus--he was part of the design team and now teaches that stuff at Embry Riddell). This means that the % power with the Hartzell after accounting for losses due to the propeller is 0.665 [=(0.887)(0.75)]. Suppose the MT propeller for the ES has an efficiency in cruise of 0.85, which is what Gumundsson suggests using in the design process, which is 3.7 percentage points less than the Hartzell number I calculated. Percent power would fall to 0.638 [=(.85)(.75)]. It seems unlikely that MT would be any lower than that in cruise if Hartzell really is at 0.887 and Gudmundsson says use 0.85.

So what does this mean? I have a little spreadsheet I use to predict performance at different % power settings. (Basically, one inputs an observed TAS and altitude at a given % power setting, the program calculates a drag polar, and then you can change power settings and altitudes to generate KTAS predictions at different settings/altitudes using standard performance equations.) The analysis is approximate for a variety of reasons, but for small changes like this, it's going to be very accurate. That analysis predicts that if your airplane goes 191 KTAS at 75% power at 10000 feet with a propeller that has an efficiency of 0.887, then lowering the prop efficiency to 0.85 will lower your speed to ~188 KTAS.

So even if the difference between the Hartzell and MT were pretty large -- almost 4 percentage points -- we're talking about small potatoes here, basically 12 nm in a four hour flight, which is about 4 more enjoyable minutes in the air (but maybe lower bragging rights).

And my bet is that the MT actually is not 3 kts slower than the Hartzell in cruise, but I really don't know. It would be interesting to have MT's performance data for comparison, but I doubt they'd share it. When I asked the Hartzell folks at the landing about it, they didn't want to share (though they did so some years back!)

Happy and safe flying,
Dan

[Ryan Riley]

Dan, thanks for sharing. I have the MT prop and have been happy with the smoothness. As noted in your post, with the Odyssey batteries in back, I have no CG issues. The only drawback to these props are they seem to weather faster than metal props.

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[Dan OBrien]

Ryan, just for clarification, by "weather" do you mean pick up ice or wear out?

[Ryan Riley]

Dan, how they wear.

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[J.C. Peterson]

And my bet is that the MT actually is not 3 kts slower than the Hartzell in cruise, but I really don't know.

Dan,

I’ve tested quite a few Hartzell, metal and composite, and MT props over the years on IVs, ESs, Legacies, Evos, and Epics, including 3, 4, & 5 blade combos depending on the aircraft. I can say with near certainty that the MT is likely at least 3 knots slower on a ES for a given blade count. The speed differential isn’t linear in the data I’ve collected with higher TAS aircraft having a larger split.

The most dramatic contrast I’ve seen was on an Epic going from a 4 blade metal Hartzell to a 5 blade MT. Climb was very slightly improved, cruise efficiency at a given power setting was slightly improved until higher power settings. However, the max cruise airspeed delta was in excess of 15 knots TAS. It equated to a greater than 5% speed reduction. By the way, MT also promised the prop would be faster than the existing Hartzell, and when confronted with the data then refused to make any kind of concessions on the purchase.

I am far from an aerodynamicist, but my observations are the MT blades nearly always have a substantially thicker airfoil profile. In comparison, the 5 blade composite Hartzell on Evos and Epics is actually thinner than the metal alternative. Those composite props are also the most efficient props in every metric tested. It’s a very generalized summary, but thinner airfoil designs seem to be better.

[Ryan Riley]

JC, how do they compare on smoothness and noise? I’ve always heard that MT props are smoother, which mine is, but I have only flown the MT on my bird. How about the weight difference?


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[J.C. Peterson]

JC, how do they compare on smoothness and noise? I’ve always heard that MT props are smoother, which mine is, but I have only flown the MT on my bird. How about the weight difference?

A properly-balanced Hartzell can be every bit as smooth as an MT. Many mechanics don’t get the balance as perfect as possible. They settle for “within spec” which can be noticeably different. Of note, MT does not do a dynamic balance on their props before shipping them. They do a static balance only. I’ve had several MTs on PT6s so badly out of balance I thought the prop bolts hadn’t been fully tightened. The line that MT’s props are smoother is a sales pitch that has been perpetuated.

Noise is very subjective mainly in terms of quality of sound. dB readings that I’ve done showed little between props. I personally find many of the MTs to be “buzzy.” I’m guessing it’s somewhat due to the squared-off blade tips. At low prop speeds you can often hear a characteristic chirping sound outside the aircraft when an MT-equipped aircraft taxies by. I’ve not noticed the same sounds, on-ground chirp or in-flight buzz, on the older round tip blades, but they were lower performance than the newer MTs.

The weight difference depends on which prop is selected. The Hartzell composite props I believe are essentially the same as the MT of a given blade number. What is often overlooked is reduced weight on the nose may be a disadvantage for Cg reasons. IVs and Evos, for example, need the extra weight as far forward as possible to be able to fully load the aircraft without bumping up to the aft Cg limit. Piston engine hot starts are also quite a bit more challenging with the lighter props because of the reduced flywheel effect.

In my opinion Hartzell knocked it out of the park with their new composite props. The blades are incredibly thin yet still tough enough for the real world. Climb rate and speed are improved compared to other similar props. I really only see one downside: cost. They’re comparatively speedy, but the difference seems to be lessening as more props are being produced.

[Dan OBrien]

Very interesting info JC, thanks!

Maybe this explains why my 10:1 compression doesn't seem much faster (if at all) than the regular 8:5 compression ESs, although it does some very efficient lean of peak.

[George Wehrung]

Great write up JC! I am trying to decide if I do a prop upgrade or not. With my engine eating oil I may top it and that might be a good time to get a new prop.


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[Ryan Riley]

Great write up JC! I am trying to decide if I do a prop upgrade or not. With my engine eating oil I may top it and that might be a good time to get a new prop.


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When did that happen? How much is it eating and how's your compressions??

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