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How much of a difference does a CS prop make during high altitude cruise?

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Duncannon

Active Member
In my case it would likely be an RV 9 (o-320), but anyone with experience in the other 2-seat RVs please chime in. But when cruising above 10k feet, maybe the lower teens, will a fixed-pitch prop still perform just fine in comparison to a CS prop, especially if I’m flying solo? I imagine if I were carrying a passenger, I wouldn’t bother climbing to hypoxia levels.

I haven’t had yet had the pleasure of flying in an RV, but when/if I ever get to own one, it’ll be purely for fun and not mission-related (traveling for work or to see the in-laws 3 times a month, etc.)., so I’m wondering if the extra costs of a CS prop simply wouldn’t be necessary, especially as I’d be based at a low-elevation airport with a mile-long paved runway.

I was reading through some old threads, and I recall someone recommending going with a fixed-pitch prop and “using the extra money on nicer avionics”, bput I thought I’d ask just how much of a difference would it make?
 
This decision is heavily dependent on what you want the plane to do, and how you fly it.

For example, if top end speed is the only metric of success, then a properly pitched FP prop for your engine, plane and altitude selection should go as fast as a CS prop. Here is the rub. Few if any of us have this as the only metric of success.

I built my first RV (8A) with a FP prop. It flew just fine but I was not happy with either top end speed, take off acceleration or fuel consumption in cruise. At ~300 hours I replaced it with the Hartzell BA two bladed prop. The transformation across the flight envelope was much more than I anticipated.

I don’t want to start a debate as FP props are very popular, cost less and are much less maintenance. I’m just sharing my experience. Here again decide the mission of the plane. I also suggest you get as much information from builders that have gone from FP to CS (or vice versa) on the same RV.

I’ll also point out that all CS props (just like FP) are not created equal. My flying mission is weighted toward high efficency cruise. For me all four builds sport the two blade Hartzell BA CS prop.

Carl
 
I remember as a kid have a single speed bicycle. Worked just fine, but dang hard to get started on a hill, not really fast on a long stretch unless you peddled your butt off. Then I bought (paper route money) a 3-speed hub bike, never looked back. Then the Cool kid on the block got a 5-speed derailleur, and blew us all off the street…..
 
I was fortunate to have another RV8 builder who finished his RV8 at the same time I did. Both 180 HP, his with a Catto FP which he optimized for top speed and mine a Hartzell composite CP. At 6500 feet we were roughly the same top speed. His initial climb was 1500 vs 2000 fpm for mine and I had a 30F lower CHT. His solo takeoff roll is 12 seconds, mine 5. The engine braking difference is dramatic. I followed him on downwind at 120 mph, turned base inside him landed and took off before he touched down.

Either way they are fabulous high performance planes but the price differential for a typical RV build FP vs CS is less than 10 percent of the total cost with a 30 percent increase in climb performance and a 100 percent increase in the wow factor.
 
Yes the debates are endless. In older threads, there are those who state they would never even think to own a Vans with FP prop, whereas others wouldn’t dream of going CS. As I said, takeoff performance is not an issue, nor would aerobatics or formation flying (not in a 9). It’s simply performance and efficiency at higher altitudes. Of course, the debate between carb vs. FI would also be something to explore here.
 
I had researched this quite a bit before deciding on my FP Catto on my io320 RV9A
What I think, is At high altitude, prop pitch has little effect on top speed. One to three knots. All prop pitch does is change the rpm of the engine at which the top speed occurs. I thought I could get another 10 kts out of the plane with a finer pitch prop,( higher rpm), but could find no data where this occurs.
But where the CS prop shines is during climb. It would allow one to run 2700 RPM, at super high fuel flows ( OK only 12 gpg on a io320) and the plane would climb like a homesick angel. As a reference, my usual climb is a cruise climb at 133kts 300 fpm at 9 g/hr. (Most of my climbs start off in controlled airspace, so not climbing very high.)

As a data point, coming back from vegas last night at 10.5k, dropping the % power from 62% to 55% increase nmpg from 17 to 22, while only reducing speed 5 kts. On my plane, a slight pull of the throttle reduces fuel flow a lot, when lean of peak.

If I climb at max power at max rate, I burn a lot of gas. So much that my range is affected. For long range in the RV9A, efficient climb is important too.
 
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The real advantage of a constant speed prop on an RV-9A is extra drag on descent and landing. The extra takeoff and climb performance doesn't hurt, either. But I put a c/s on mine mostly for the drag. Really, a c/s means you get all the performance, all the time, whereas a fixed pitch does not give full envelope performance.
 
Ed_Wischmeyer said:
The real advantage of a constant speed prop on an RV-9A is extra drag on descent and landing. The extra takeoff and climb performance doesn't hurt, either. But I put a c/s on mine mostly for the drag. Really, a c/s means you get all the performance, all the time, whereas a fixed pitch does not give full envelope performance.
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Exactly what Ed said is my only drawback to fixed pitch. Being dumped off from ILS at 500’ agl 3,500 feet from end means the final descent is steep in a FP RV9A. But it doesnt really matter, because all ILS/GS runways are long. It just means you’ll touch down where the big boys do, a thousand feet down from the end.
 
In response to the OP's question, yes a FP prop will perform just fine at altitude. These are photos of the EFIS in an RV 8 with a Catto three blade FP powered by a Superior IO360. The first shows 178 knots true airspeed burning 7.77 gallons per hour at just under 16000 feet density altitude. The second shows 158 ktas at 5.85 gph at just under 12000 density altitude, 31 statute miles per gallon.0620141308a.jpg0620141322.jpg
 
So the RV8 is about 25 knots faster than the RV9A with the same fuel flow. Barring any airspeed calibration errors.
 
I agree with Carl it depends on what you want to do with it. With that, my first plane was a -7 with IO360 and FP sensenich. I Had roughly the same numbers as the -8 post above except I saw the fuel burn down to 4.9/4.8. Made a few nonstop flights from Texas to Maryland at 17k+. I later switched to a three bladed Catto and noticed a slight increase in takeoff and climb but negligible at high altitude.
 
pro's, acro, formation, take off performance, 'cool factor'

con's, cost, weight, maintenance,

the 85" FP sensi metal prop has been a great prop for me. has served me well for almost 5,000 hrs.
 
I have a FP prop on my RV8 and I cruise at 171KTAS at about 9500DA at 8.7gph. I didn't go higher because I didn't have oxygen. The FP will always be a compromise between climb rate and cruise performance. In my case, I am happy with the cruise number, but I max out at 1500 fpm on a climb at near sea level. However as MikeB indicated in his post above, the CS prop will always climb faster and provides a greater performance adjustability. I only had exprience with the CS prop in the SuperDcathlon and doing a steep climb out is fun, even when it doesn't fly very fast

If your financial mean allows for the CS prop, then go for it.
 
brad walton said:
In response to the OP's question, yes a FP prop will perform just fine at altitude. These are photos of the EFIS in an RV 8 with a Catto three blade FP powered by a Superior IO360. The first shows 178 knots true airspeed burning 7.77 gallons per hour at just under 16000 feet density altitude. The second shows 158 ktas at 5.85 gph at just under 12000 density altitude, 31 statute miles per gallon.View attachment 63582View attachment 63583
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@brad walton, It appears that the MGL's TAS calculation is off a wee bit; first picture yields 178KTAS, second picture yields 158KTAS.

Calculators here -- https://e6bx.com/tas/ and https://indoavis.co.id/main/tas.html

It could be time to run the GPS 4-way calibration and see what that yields as far as IAS error.

Cheers!

B
 
bjdecker, sorry but I don’t understand your point. I ran the numbers from my picture number one through your e6b calculator and the true airspeed displayed on my EFIS came out exactly the same as your e6b, 178 ktas. Picture number two was at a different altitude, different airspeed and different rpm and demonstrates performance at a whole different set of parameters.
 
brad walton said:
bjdecker, sorry but I don’t understand your point. I ran the numbers from my picture number one through your e6b calculator and the true airspeed displayed on my EFIS came out exactly the same as your e6b, 178 ktas. Picture number two was at a different altitude, different airspeed and different rpm and demonstrates performance at a whole different set of parameters.
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My bad - I was looking at the GS number not the TAS. Carry on!
 
bjdecker, I ran the numbers from the 2nd photo through your e6b calculator. True air speed came out exactly as displayed on my EFIS at 158 ktas as well. Perhaps you misread the MGL display?
 
brad walton said:
bjdecker, I ran the numbers from the 2nd photo through your e6b calculator. True air speed came out exactly as displayed on my EFIS at 158 ktas as well. Perhaps you misread the MGL display?
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Exactly, I was looking at the GS instead of the TAS.
 
Duncannon said:
It’s simply performance and efficiency at higher altitudes.
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In that context, generally speaking, an “optimized” FP blade for a given fixed flight condition will outperform a CS in that same fixed flight condition. You can have a FP prop that will outperform a CS on climb, cruise, or top speed - but not on the same flight. So if you take the time to optimize the airfoil, twist and pitch to match your ideal flight condition, then it’s unlikely you will find a CS that will match it.

That said, as soon as you are “not” at that condition, your optimized FP is no longer optimal. Is it “good enough”? Lots of FP RV’s flying, so the generic answer is “probably”. But that is a different question than you asked.
 
I think the FP vs CS debate always trends towards: FP - Pick the your favored segment of flight (Climb, Cruise, etc). CS - adjust and optimize all segments.

I ended up going Catto 3-blade FP and optimized for Cruise. Sure, I climb a little slower. Sure, my WOT is a little slower. But I see 160kts TAS and it's economical.

Part of the fun/agrevation is picking which parameter to optimize. :)
 
If you choose fixed pitch then you can claw back some climb losses by building light. I’ve found that every pound of weight savings yields about 3fpm.
 
Nobody mentioned going with a 180hp 360 (O or IO) for better performance up high. The weight difference is small, but maintaining power above 10k into the teens is worth it in my opinion. Yes I know Vans doesn’t recommend it, but others have had great luck with it. Have you ever had any motorized vehicle and thought to yourself “I wish I went with a smaller engine..)
I have a couple of hundred hours in my buddies -9a with a 160hp O-320, and a Hartzell CS prop. Above 12,500, it struggles to climb. (My 180 FP-4 goes to 17,500 no problem)
If I were to have a -9 with list, it would be 180hp, CS prop, fuel injection, in that order, preferably all three!
 
Dugaru said:
That seems somewhat anemic, based on my experience. I wonder if there’s another issue.
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She seems to run good otherwise.. 1000 hours, recent new mags, plugs ect.. 153-158 KTAS on 160hp.. but doesn’t have the high altitude performance that my O-360 has.. I have had this -9a up to 14,500 but it is mushy.. I don’t recall the TAS as we didn’t stay long there.
 
Taltruda said:
She seems to run good otherwise.. 1000 hours, recent new mags, plugs ect.. 153-158 KTAS on 160hp.. but doesn’t have the high altitude performance that my O-360 has.. I have had this -9a up to 14,500 but it is mushy.. I don’t recall the TAS as we didn’t stay long there.
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Hmm, sounds about right. And there’s no doubt the extra hp makes a difference. Still seems odd to be played out by 12,500. How heavy was it?
 
My 9a had a fixed cruise sensinech prop on it when I bought it. IO-320 w single mag, single lightspeed. I could get 165kn tas at 2550rpm. I swapped to a WW151 constant speed prop. Now I typically fly at 155kn at 2300rpm and 25” manifold pressure. Burn around 7.3-7.5gph. Climb out is much better at the expense of increase chts and fuel flow. Best thing is ability to slow down quickly in the pattern.
Would I do it again? Probably not.
In fact I just bought a 2002 glastar w O-360 and fixed pitch and I doubt I swap it either.
I think a 180hp engine and fixed(ga) prop is the best/simplest combo for a 9a.
 
edhunter said:
My 9a had a fixed cruise sensinech prop on it when I bought it. IO-320 w single mag, single lightspeed. I could get 165kn tas at 2550rpm. I swapped to a WW151 constant speed prop. Now I typically fly at 155kn at 2300rpm and 25” manifold pressure. Burn around 7.3-7.5gph. Climb out is much better at the expense of increase chts and fuel flow. Best thing is ability to slow down quickly in the pattern.
Would I do it again? Probably not.
In fact I just bought a 2002 glastar w O-360 and fixed pitch and I doubt I swap it either.
I think a 180hp engine and fixed(ga) prop is the best/simplest combo for a 9a.
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I love my CS but if I was starting my -9A from scratch I would probably go FP. Cheaper, simpler, and in the mid-Atlantic I’m sure the climb performance would be perfectly adequate.
 
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