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CS/FP comparison?

Highflight

Well Known Member
Can anyone offer any more specific comparison between a CS Hartzell prop on a Lycoming 180hp engine and a Sensenich fixed pitch prop?

I know the generalities and what to expect in performance differences, but I'm wondering if anyone has had an opportunity to do an actual flight comparison of the two props on the same aircraft.

I'm reasonably sure that others besides me are wondering if the $3200 difference in cost between the two are actually worth it.
From my way of thinking, a fixed pitch prop set up for cruise should be very close in normal cruise speeds to a CS prop, but that climb performance suffers. However, "poor" climb performance with a fixed pitch RV is probably still twice that of any GA aircraft.

Thoughts?

Vern
RV7-A QB
 
Highflight said:
From my way of thinking, a fixed pitch prop set up for cruise should be very close in normal cruise speeds to a CS prop, but that climb performance suffers. However, "poor" climb performance with a fixed pitch RV is probably still twice that of any GA aircraft.

Yep, that's about right although the CS prop will be just slightly faster in cruise because it can pull more RPM. I had a -6 with a FP metal prop and my friend has the same plane with a CS prop. He can WAAAYYY outclimb me. At cruise, we are nearly identical with him just able to inch ahead of me. Another benefit of the CS prop is that he can slow down faster and descend faster than me as well.

It's a judgement call as to whether you want to spend the extra bucks. If you like to keep it simple or live near sea level, you can probably get by without it. If you live in higher elevations or JUST have to have the fastest RV in your group, then go CS.
 
C/S versus Fixed Pitch

Keep in mind that the C/S requires a prop governor and drive gear. My engine didn't have these items so that increased the initial cost difference. Additionally, there is extra maintainence of a C/S, greasing at annual, and inspection periods and overhaul periods of a C/S. I'm not flying yet, but one reason I went with C/S was to achieve lower prop speeds at cruise, thus lower noise and lower vibration levels, for the same speed as a Fixed pitch. Take off will be "snapper" too, but I guess you know that. Additionally, Hartzell's new blended airfoil is supposed to be more efficient and less noisy than their old prop plan form. Good Luck.
Marty
 
Is it worth it...

You didn't say what you like to do or what you value.

The performance gains are fairly well documented.

If you plan on flying formation the braking effect should not be underestimated. If you are wingie with a wood prop you'll go right by lead if he decreases RPM, same issue on formation landings. The difference is startling at first and increases the risk slightly since aircraft performance is different. Being able to put the brakes on can make you fake a rejoin that you've botched...

There is an advantage in acro as well because you don't have to worry (as much) about overspeeding the engine, though it does add weight.

I'd imagine that you'll get your money out on resale.

If you ding your prop I think I'd rather have a wood prop :)

Chuck
 
What Do You Want?

I mulled CS vs FP for issues other than the obvious acceleration advantage of the CS. Our typical mission is cross-country. If that were all, then FP for sure as cruise is nearly the same - but it is buzzy; CS rpm can be very much less and easier on the nerves, and having what amounts to a transmission is better than tooling around stuck in third gear on all flights. And just like no one ever complains about too much horsepower, a CS is just plain more fun. A practical consideration is that a CS will bias your empty c.g. much more forward than a FP, which makes your baggage weight allowance less of a concern if the kitchen sink and stove regularly travel with you.

John (Love That Acceleration) Siebold
 
When I started building my 7A I was planning on a Sensenich prop to keep costs down. Just talking around with other builders and Van's people convinced me that the C/S prop would be better for my needs. A well pitched fixed prop will certainly give you great cruise and reasonably good climb. The big advantage of the C/S prop for me is being able to slow the plane for short fields and having a lot of extra power to do a go around, if needed. The responce is a great deal quicker with a C/S prop, than a fixed. The RVs are very slippery and hard to slow down. That big C/S prop becomes a big air brake to slow you in the pattern and again on touch down. It really makes the RV so much more managable in all situations. Think of it like having an semi automatic transmission at your disposal or having a manual transmission stuck in high gear all the time. Or low gear if you are pitched for power. If you feel you don't need the versatility of a C/S prop and do little short field flying or high density altitude flying, go for the fixed. You spend your money and you get what you pay for. Buy what you need. I think the C/S prop is worth it. :) :) :)
 
Good comments. Almost all my flying is intended to be cross country, but that also means higher altitudes sometimes in search of a tailwind.
Plus, I know first hand how additional but unnecessary noise will raise environmental stress levels.

Not too concerned with short fields so much, nor formation flying, but from the comments I read here, it looks like I'm going to have to (yet again) bust the budget and go with a CS prop.

Jeez, it seems that every time I ask a question, it costs me more money. :D

But while on the subject, what are the reasons to go with hydraulic or electric CS prop?

Vern
RV7-A
 
I'm not an expert, but I believe hydraulic are much more responsive than electric. The only reason to choose electric is if your engine doesn't have a mounting pad for a prop govenor (e.g. Subaru engines).

Dave
 
Constant-Speed vs. Fixed Pitch

f1rocket said:
Yep, that's about right although the CS prop will be just slightly faster in cruise because it can pull more RPM....... At cruise, we are nearly identical with him just able to inch ahead of me.....

A CS prop will not necessarily be faster nor "pull" more RPMs. It depends on the fixed prop. Where the CS prop shines is climb, landing and overall efficiency. In the case of pulling more RPM, this is true on takeoff and climb. The higher RPM allows you to make more power. The chart from Hartzell shows this real well: http://www.hartzellprop.com/kitplane/kitplane_certified_prop.htm Under performance click chart.

A good CS "how it works" Ref:http://www.avweb.com/news/maint/185020-1.html


Constant-Speed vs. Fixed Pitch
Hartzell,
"Constant-speed propellers offer higher efficiency over a wider speed range compared to fixed-pitch propellers. With a fixed-pitch propeller, builders have to choose between a "climb" pitch and a "cruise" pitch. Choosing one results in a propeller system that compromises the other. The pilot has only a limited range of engine speed variation available to control the thrust of an aircraft equipped with a fixed-pitch prop. The ability to vary blade angle in a constant-speed propeller allows the engine to develop more power since rpm and power can be controlled independently at any air speed. This enables you to optimize both cruise and climb for maximum efficiency and performance."

A CS prop is limited by the governor, usually set @ 2700 RPM max. You can't exceed that RPM limited no matter what. Which is a nice thing; you don't have to worry about it. However, all things being equal, if you race a fixed pitch plane turning 2900 RPM, you might be at a speed disadvantage w/ a CS prop turning 2700 RPM, more RPM = more HP, more Go (HP=torquexRPM/5252).

There are many good fixed props to choose from today. A good option is the metal Sensenich, without the maintenance issues of wood, made for the RV and at a good price. Also, the factory can "tweak" or re-pitch a metal prop to fine-tune it for your needs. The advantage of wood is smoother operation, less likely to damage crank with a prop strike, and lighter. The down side, finnish is subject to erosion in rain and they req periodic re-torque. The Catto prop is also popular, but don't know anything about them except many like them. However one gent had a failure of his Catto and appeared to be a defect in a joint/bond.

RVator, 6th issue 20004, pg 11, Ken at Van's switched from a fixed to CS prop. He sounds like a grumpy old man. (Yea I said it) :rolleyes: His impression is a little negative in tone but admits he has not flown the new config much. Also he states with the heavier prop his CG moved Aft. :confused: Confused? Me to. Despite his CG calc math error more weight on the nose will move the CG fwd. Can be a good thing or a bad thing.

Read all the many opinions, articles and look at your wallet. Only suggestion is consider a purchase of an engine with CS capability even if you go fixed Pitched Prop to start. That way if you can change in the future if you want, and it will be a selling point and have higher resale. I would not use an electric prop due to maintenance of brush or slip rings. If your engine has a solid crank, go fixed. Hydraulic pitch control is reliable and req's little routine maintenance.

Cheers G RV-4 RV-7 (project O-360/CS prop) :D
 
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If he put more weight in the nose and the CG moves aft, his calculator is broke!
 
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