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Fixed Pitch vs Constant Speed Prop

Ed_Wischmeyer

Well Known Member
This is the first time I've flown at WOT at altitude. The first time I was going over 2700RPM and was unsure about staying over for a little bit of time. This time I wanted to measure top speed so I stayed at WOT long enough to do that.

So at 2750 RPM, 21.4 MP, 73% power, 9.7GPH, 138 IAS, 162 TAS, 8670 DALT

When I compare this to Vans web site 75% power and 8,000 solo weight they state 187MPH or 162.5 KT. This matches up pretty well and I'm happy with that.

So the questions from a newbie here is... If I'm at full throttle producing 73% power, I presume with a fixed pitch prop there is no more in the tank so to speak. This is going to be my maximum speed. So is the only way to get to Van's top speed number with a constant speed propeller?

This is with an O-320 carburetor running one Mag and one EFII ignition. The prop is a 3-blade Catto 67x70 carbon composite.

Fixed pitch props can, in general, match the pitch range of a constant speed prop from most coarse pitch (cruise) to most fine pitch (climb). So, in general, a constant speed prop won't gain you any extra speed over a fixed pitch prop. The advantages of a constant speed prop are uniformly good performance over the speed range of the airplane, more drag on descent and landing, and, if your plane is tail heavy, more weight up front.

However... the Sensenich metal fixed pitch props have a 2600 RPM redline, so the engine won't be able to achieve max RPM, hence, won't achieve max power.

When I flew over West Texas a few summers ago with the autopilot holding altitude, I had to retard the throttle in the updrafts to avoid overspeeding the prop, and then add power in the downdrafts. Presumably would not have to do this with a constant speed prop.”
 
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Question

So the question comes up, does the RV series have sufficient climb performance so that a fixed pitch cruise prop still provides adequate climb performance? I am getting to the point in my build where I need to decide on fixed vs CS prop. I was all set on a fixed, but just curious how much climb would I lose in, say an IO320 RV9?
 
So the question comes up, does the RV series have sufficient climb performance so that a fixed pitch cruise prop still provides adequate climb performance? I am getting to the point in my build where I need to decide on fixed vs CS prop. I was all set on a fixed, but just curious how much climb would I lose in, say an IO320 RV9?

My first RV (RV-8A) started off life with a FP prop. I replaced it at 300 hours with a Hartzell BA CS prop. It was a totally different airplane from climb, to cruise to top end.

I tell builders that the one big mistake I made on my first RV was that FP prop. I did not repeat that mistake on the next two RVs.

But - factor in your mission. If you want a joy ride once a week for a hamburger get whatever you want - any prop will still make your RV climb better than most Cessnas. If you want high efficiency cross country cruise, get the very nice Hartzell CS BA prop from Van?s. Flavor of RV (other than the 12) makes no difference.

Keep in mind the efficiency of a prop is more than FP vs CS. I?ve flown one RV with a custom composite CS prop that put the load on the engine but at RV speeds it was not as efficient at translating that engine power to thrust as the Hartzell (less speed for the same fuel flow). I suspect trying to match a FP prop to peak at whatever flight condition you are aiming for may have a similar issue.

Carl
 
Unless you blow the engine, you will not get more than that power at that altitude.

You have one of the best FP props on the market.

You are happy with the speed you are achieving in the cruise, so fly on and enjoy !

We sit at 23/23 at 8,000' in our RV7 behind an IO-360 and MT 3 blade CS prop. Guess what, the power is mid 70% range, so we are similar. I see around 140 IAS - maybe 160 TAS ish. Quite frankly, I am not bothered, I know I am cracking on in an awesome aeroplane :D

My buddy has an RV6 with an O-360 and a Prince P-Tip prop. He sits beside me at 2400 rpm in the cruise at the same speed as me at 23/23.

Sit back, relax, enjoy the view and look forward to the destination arrival at 200kts downwind........;)
 
So the question comes up, does the RV series have sufficient climb performance so that a fixed pitch cruise prop still provides adequate climb performance? I am getting to the point in my build where I need to decide on fixed vs CS prop. I was all set on a fixed, but just curious how much climb would I lose in, say an IO320 RV9?

You only get what you pay for.

I have always had a CS prop on my RV and really want a CS prop on any RV that I own. Of the RV series of aircraft, only the RV-12 would be acceptable to me with a fixed pitch prop.

Check in with Tonya and Scott Card. They have a constant speed prop on their RV-9A and they also fly formation. They can report first hand how they do with only 160 HP and constant speed prop with other RVs having larger engines and fixed pitch props.
 
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Not all Sensenich aluminum props have a 2600 RPM limit. If you check the Sensenich spec sheets on these props, and page down the the ones designed for Vans aircraft, the O-360 powered airplanes, and some O-235 airplanes, including the RV9, dont have that RPM limit on the prop. Last page identifies details.

http://www.sensenich.com/wp-content...luminum_Propeller_Applications_1349891787.pdf

I don?t know why that is. The O-360?s usually have prop hub requiring 1/2? bolts, and I think most O-320?s require 7/16? bolts. That might not have anything to do with it, and what about the O-235? It sounds like your prop is set up pretty good for your airplane. 2600 RPM seems to be your only limitation.

I have a 72FM (71?) Sensenich with 85? pitch on my O-360 powered RV4, and my limit is Lycoming?s recommended limit of 2700. However, mine turns up 2900 at WOT level at 8000?, so I?m underpitched, and I only ran it that way to determine what I needed. Initial acceleration is slower than my CS RV formation leads, but once airborne I accelerate quickly and can climb usually much better than my counterparts, most likely due to my lighter weight.
 
So the question comes up, does the RV series have sufficient climb performance so that a fixed pitch cruise prop still provides adequate climb performance? I am getting to the point in my build where I need to decide on fixed vs CS prop. I was all set on a fixed, but just curious how much climb would I lose in, say an IO320 RV9?

Hot summer-temps climb I get about 1400fpm at takeoff (2000msl), winter temps I get about 1800fpm. Catto cruise-pitched prop gives me about what Van's indicates my top speed should be. Happy camper, especially since I appreciate simplicity and value. YMMV.
 
Really?

If is true you are getting anywhere near 1400 fpm and didnt make a mistake and add an extra zero, I think I will be thrilled with the performance of a Catto fixed pitch.
 
In my humble opinion, the only benefit of a CS prop is takeoff distance and climb performance. The downside is cost and weight up front.
 
In my humble opinion, the only benefit of a CS prop is takeoff distance and climb performance. The downside is cost and weight up front.

Cost yes, weight not necessarily. I am a bit on the tail heavy side of the CG envelope with my constant speed Whirlwind (composite) prop.

Erich
 
Yep, benefits: takeoff distance, and INITIAL climb performance. At cruise climb airspeeds (125-135 KIAS) I can outclimb ALL of my CS buddies, with 1500-2000 fpm all the way to 8000? - WOT ~2550 RPM.

Downside: Cost, weight, and increased maintenance.
 
If is true you are getting anywhere near 1400 fpm and didnt make a mistake and add an extra zero, I think I will be thrilled with the performance of a Catto fixed pitch.

That's about right, yes. We like to nitpick and argue on this site about a few knots here and there among friends, just giving each other a hard time - but the clear performance difference between ANY of the RV's and the spamcan world is a real eye-opener, regardless of FP versus CS.

On my 9A with constant speed, I have my default autopilot climb/descent rate set at 1000 fpm, because I can engage that on an IFR missed approach, open the throttle, and not worry about the airspeed in the slightest as I climb away. During summer temps like now I usually see 1500 or so fpm when solo on climbout, and during the winter I can hold 2000.

Come on in, the waters fine! :D
 
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In my humble opinion, the only benefit of a CS prop is takeoff distance and climb performance. The downside is cost and weight up front.

This is exactly true. If you want a C/S prop, that's great. That's one of the freedoms we have. I've been flying RVs for almost 30 years and I would never have a C/S prop on one. Van himself always recommended a light weight F/P prop, but customers wanted C/S so he sells them.

Any RV with a F/P prop will out perform almost any other aircraft of similar type so much that a C/S is not really necessary. Yes, they have the advantage of giving you max power on take off and if you don't mind the extra money and weight, that's your business and no one else's. You build what YOU want.
 
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Previous thread here of some friendly competition to get some very unscientific climb performance for different engine and prop combos.
My options I considered for about the same money when I built my 6A was CS O320 or FP O360. I am happy with my choice as you never can go wrong with HP.
 
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In my humble opinion, the only benefit of a CS prop is takeoff distance and climb performance. The downside is cost and weight up front.

For a little more money and a little more weight, you can get a parallel valved stroked O-320 (aka O-340) or a stroked O-360 (aka O-375). Saves the weight and cost of a CS prop and gives a boost in performance.

On the other hand, the angle valve engines really should have a CS prop to get good performance, according to the torque curves.

V
 
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I was going to write a few paragraphs, but I remembered Section 11 in my builder's manual which explains the advantages better than I could. One of the most compelling reasons for me was fuel economy.

The primary reason for using CS props is performance. The ability to control the prop blade angles permits
the pilot to maximize engine and prop performance for any given flight condition. Below is a description of
the performance offered by CS props for several flight conditions:
? Take off: Setting a low propeller pitch reduces the prop load on the engine and permits it to rev up to full power RPM. The low prop pitch angle is also more efficient. A CS RV can expect take off distances to be reduced between 20 to 40 percent from that of a fixed pitch prop. This is a significant performance difference, but is not a major factor because RV take offs are short even with fixed pitch props. In other words, the CS prop is not as necessary on an RV as it is on most production airplanes with comparable top speeds, which would have unacceptable take off performance if equipped with fixed pitch props.
? Climb: For the same reason as the take off the CS prop will improve the climb rate and climb angle.
Climb rate will increase by approximately 10-15%, depending on the climb speed.
? Cruise: This is the flight condition at which we feel the CS prop offers the greatest advantage to an RV. Most RVs will spend the majority of their flight time in cruise, so any benefit gained will be of greater value. Though the fixed pitch prop is operating at its best in the cruise condition, it is still a compromise. But, there is a wide variety of conditions which occur under the general heading of ??Cruise??; anything from full throttle & RPM (at altitude), to just enough power to maintain minimum power flight. ??Rated Cruise Speed?? for production aircraft is quoted for conditions under which the maximum permissible continuous speed can be achieved. This usually occurs (for non-supercharged engine) at about 8,000 ft. and at maximum permissible continuous RPM. This combination produces about 75% of maximum rated power. Under this condition, the CS prop will offer little advantage over the fixed pitch prop, other than what little it may gain from better blade efficiency. The CS prop offers its main cruise advantage under reduced power cruise conditions. Engines operate at peak efficiency when the throttle is full open. This is because the air flow control vane in the carb or injector throttle body is completely open and offering the least resistance to airflow. This reduces what is known as ??pumping losses?? within the engine. There are two primary means (from the pilots vantage point) of reducing power output of an engine. One is to reduce the RPM of the engine and the other is to reduce the manifold pressure. With a fixed pitch prop, the only means of reducing RPM is to retard the throttle setting. In so doing, the control vane (butterfly) in the carb partially closes, manifold pressure is reduced, and engine efficiency drops. With a variable pitch (CS) prop, the RPM can be controlled through adjusting the blade angle, and thus the propeller load on the engine. The throttle can be left full open (in its most efficient position) and the power output can be reduced by lowering the RPM. This reduced RPM, full throttle condition achieves both reduced engine friction because of the lower RPM, and minimum pumping losses. Fuel efficiency will be improved, but speeds will drop because less than full cruise power is being used. Above we mentioned 8,000 ft. as the optimum cruise altitude. This is because it s the lowest altitude at which an engine will develop no more than 75% power at rated RPM. With a variable pitch prop, selecting low RPM can cause the engine power output to be 75% or less at altitudes of less than 8,000?, making it possible to utilize the efficiency of a continuous full throttle opening at altitudes well below 8,000?. Just leave the throttle wide open and pull the RPM back to a number which, according to the Lycoming manual, produces 75% power or less.
? The bottom line when analyzing prop performance and efficiency in cruising flight is fuel consumption. One can expect rates of 1/2 to 1 gph less than with fixed pitch wood props. Savings might be as high as 1 1/2 gph under extreme conditions with 180 HP engines.
? Descents: Constant speed props offer two completely different advantages over fixed pitch props during descents. First, they can offer added speed during long, slow descents from cruise altitude. With fixed pitch props, power descents are not practical because the added speed causes excessive RPM requiring power reduction. Constant speed props will control the RPM as speed increases in the descent. Now, red line speed, not RPM becomes the limiting factor. On the opposite end of the scale, when the throttle is retarded to idle or near idle power, the CS prop can be moved to low pitch which will offer noticeable aerodynamic braking action.
? Aerobatics: During aerobatic flight, the braking action of a CS prop can help to control speed build up during the diving portions of maneuvers. And, of course, its slow speed thrust advantages are helpful during the climbing, particularly low speed climbing, portions of the maneuvers. On the negative side, the added inertia of the CS prop causes greater stress on the engine crankshaft and slows down maneuverability to a small degree.
 
Thanks for mentioning the low RPM/ Full throttle condition for efficient cruise. Many overlook that significant advantage in these discussions. No matter how good a FP prop is in its design envelope, it can't provide max RPM at zero airspeed AND very low RPM at full throttle cruise - just not going to happen. For those of us serious about efficient cross country, that ability is a mission requirement.
 
Gotta see the funny side of this

Thanks for mentioning the low RPM/ Full throttle condition for efficient cruise. For those of us serious about efficient cross country, that ability is a mission requirement.

Says the man with a IO540 stuffed into a homesick angel that was a RV during her mortal years.

Please forgive me, if I Have offended anyone
 
My fp prop 160 hp stats

Today 90 degrees in Chicago with full fuel TOW approximately 1550 lbs, used at most 800ft of runway, initial climb 1200 fpm at 100 kts. At cruise I routinely throttle back to stay below 180 mph yellow arc. How much better can a c/s prop do while staying within airspeed limitations vne 209? It was a big decision back then but I?m very pleased with my Sensennich prop.
 
If on the fence with CS vs FP prop, take a look at what your CG is with full baggage and what the amount of baggage you can put in the airplane and still be forward of the aft limit.

Without a constant speed prop on my RV-6, I would not be able to take more than a few pounds of baggage.
 
W&B with a light prop

Van himself always recommended a light weight F/P prop, but customers wanted C/S so he sells them.

I know that Vans say this ... also not to mount too much into the panel.

But if you do so, then you end up with a really tail heavy plane!
I do not understand this ... because Vans recommend it that way ... The Engine should be a bit more forward, or you have to install a thick crush plate in front of the prop. Done this on a IO-360/Catto/RV-7A to bring things back to balance.
 
If on the fence with CS vs FP prop, take a look at what your CG is with full baggage and what the amount of baggage you can put in the airplane and still be forward of the aft limit.

Without a constant speed prop on my RV-6, I would not be able to take more than a few pounds of baggage.

That can be addressed in other ways. I have a catto and also have a 19 lb crush plate that gives me a full 1" of aft CG increase. inexpensive and low maintenance. Also, I am a bit under pitched. This allows me to climb at 25/25, right where the CS props are climinbing. That said, at higher altitudes, a CS prop would be likely helping some. At 8000', I am turning 2750 and get speeds and fuel burns very close to those with CS props. I am WOT and LOP in this condition, so no pumping losses. I can also go ROP and turn 2800 for more speed, though never do (good amount of anecdotal data to say that the 320's can handle 2800 all day long - remember that lycoming allows a +/- 4% tolerance in RPM indication - that means 2810 actual RPM is in spec). By being under pitched, I can still turn 2700 RPM at 13,000 and get the same speeds as 8000," though I am at peak or slightly ROP in this condition.

I also have a CS prop on my 10 and really enjoy how it expands the envelope. However, it is not a night and day difference from the FP on my 6 with an IO-320. With the right prop, it is a very good compromise when you factor in the cost/maintenance component.

Larry
 
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When building a -7 you can address the CG issues with a light weight FP prop by putting an 0-360 on the 0-320 mount which moves the engine forward a couple of inches negating the need for the heavy spacer/crush plate combination. I wish I had known this when building my -7.

The choice of FP prop also matters, I swapped out a metal sensenich prop for a Catto which took 28 lbs off the front which necessitated the need to add weight back with the crush plate etc. to bring the CG back in line.

I regularly get 1800-2000 fpm climb with the Catto and WOT turn 2750 at altitude so a pretty good all around performance with no complexity which suits my mission goals.

The CS comments ref take-off acceleration, slowing down, aerobatics etc. are all valid and come down to personal choices/preferences. Either way you end up with a really good performing plane :D
Figs
 
Dollars to Doughnuts...

Guys,
This is an age old discussion that will never be truly settled as everyone (thank God) has their own opinion.
Way back when (post 9/11) my F16 squadron had purchased (or built) 13 RV's, 11 RV4's and 2 RV8's all flown/owned by F16 Pilots. We operated them in just about every realm you could imagine for several years before desert deployments took their toll and most were sold.

The props were nearly equally split between 6-CS and 7-FP. The skill sets present allowed us to do day and night formation, formation aerobatics, cross country, ACM and my favorite, 1V1 or Dog Fighting. We all flew each others airplanes and got to closely compare performance between the various engine prop combinations. First and foremost for us was that nothing on Earth compared with the F-16C Block 30 Big Inlet that we depended our lives on every day. That said...

As far as our sport-planes went the overall consensus was the RV4 with 0-360/CS* was King Kong in the takeoff Climb/Cruise cross country scenario but the lighter weight 150HP wood prop RV4 was a clear and consistent winner in 1V1's, aerobatics and fun factor where a more aft CG, lighter nose really make a difference in nose rate, flywheel effect and reposition. It also was a consistent DogFight champion over the CS. Amazingly enough, on long trips the arrival time difference between the higher HP CS and the 150FP was only a few minutes.
The fastest of them all? Our Squadron Commander's RV8 with a 0-360/ Sensenich Metal FP.

25 years, 3700 RV hours and Three airplanes later I still have a Catto FP 2 blade on my RV6X.
Personal preference...:)
V/R
Smokey

*That RV4 was built by RV legend Art Chard with RV6 wing tanks, perfectly balanced controls and one of the best RV's I have flown ever. It sported a LyCon 190HP 0-360 and was owned by my late good friend Brian Wolf.
 
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The fastest of them all? Our Squadron Commander's RV8 with a 0-360/ Sensenich Metal FP.


*That RV4 was built by RV legend Art Chard with RV6 wing tanks,

Good to know Smoky as my RV8 is equipped like your Sq/CC's.

The wings of my RV4 were built by Art Chard in '87.....never leaked.
 
Guys,
This is an age old discussion that will never be truly settled as everyone (thank God) has their own opinion.
Way back when (post 9/11) my F16 squadron had purchased (or built) 13 RV's, 11 RV4's and 2 RV8's all flown/owned by F16 Pilots. We operated them in just about every realm you could imagine for several years before desert deployments took their toll and most were sold.

The props were nearly equally split between 6-CS and 7-FP. The skill sets present allowed us to do day and night formation, formation aerobatics, cross country, ACM and my favorite, 1V1 or Dog Fighting. We all flew each others airplanes and got to closely compare performance between the various engine prop combinations. First and foremost for us was that nothing on Earth compared with the F-16C Block 30 Big Inlet that we depended our lives on every day. That said...

As far as our sport-planes went the overall consensus was the RV4 with 0-360/CS* was King Kong in the takeoff Climb/Cruise cross country scenario but the lighter weight 150HP wood prop RV4 was a clear and consistent winner in 1V1's, aerobatics and fun factor where a more aft CG, lighter nose really make a difference in nose rate, flywheel effect and reposition. It also was a consistent DogFight champion over the CS. Amazingly enough, on long trips the arrival time difference between the higher HP CS and the 150FP was only a few minutes.
The fastest of them all? Our Squadron Commander's RV8 with a 0-360/ Sensenich Metal FP.

25 years, 3700 RV hours and Three airplanes later I still have a Catto FP 2 blade on my RV6X.
Personal preference...:)
V/R
Smokey

*That RV4 was built by RV legend Art Chard with RV6 wing tanks, perfectly balanced controls and one of the best RV's I have flown ever. It sported a LyCon 190HP 0-360 and was owned by my late good friend Brian Wolf.

Probably the most valid prop comparison notes for RV-8 and earlier aircraft that we've seen.

If you want an above-average performing RV.......build it light!

Thank you, Smokey, good to hear from you!
(I still remember the fully-armed F-16 low pass down DCU's 18 shortly after 9/11. :) )
 
Glad to see Art Chard mentioned. I got to know him. Great craftsman. Never got his due from the Van?s group. Was instrumental in getting the early RV series going. Worked for Van for many years. A man with quiet confidence.

Bob Grigsby
Flying my Cub a bunch.
3B almost done
Will call the Shaman in to try to post pics someday.
 
Thanks for the feedback

Thank you all for the feedback. This has been a fun thread. I think I will still baseline a fixed catto. BUt if I have to add a gazillion pounds of nose weight I might considered a constant speed instead. Good suggestion to do a weight and balance. I will do that as soon as i hang the engine.
 
I have a catto and (...) I am a bit under pitched. This allows me to climb at 25/25, right where the CS props are climinbing. That said, at higher altitudes, a CS prop would be likely helping some. At 8000', I am turning 2750 and get speeds and fuel burns very close to those with CS props. I am WOT and LOP in this condition, so no pumping losses. (...) By being under pitched, I can still turn 2700 RPM at 13,000 and get the same speeds as 8000," though I am at peak or slightly ROP in this condition.

Larry, what?s your static full throttle RPM? I have a 320 with a Catto and I?m a solid 300 RPM slower than you at 8k WOT/LOP cruise, and climb around 2300. Was the underpitch by design, or just how it happened to work out?

I believe my prop has been overpitched since the beginning (even after one repitch) and trying to decide whether changing props or repitching again has a large enough performance benefit to be worth chasing after.
 
Larry, what’s your static full throttle RPM? I have a 320 with a Catto and I’m a solid 300 RPM slower than you at 8k WOT/LOP cruise, and climb around 2300. Was the underpitch by design, or just how it happened to work out?

I believe my prop has been overpitched since the beginning (even after one repitch) and trying to decide whether changing props or repitching again has a large enough performance benefit to be worth chasing after.

Sorry, I have never really measured static RPM. I got here by chance. I bought a used prop from an RV-9 owner (gen 2 set up for a 320). I talked with Craig Catto about a re-pitch and could probably pick up a couple knots by adding an inch or two of pitch at 8000'. However, the ability to still get 2700 at 11 and 12K' is important to me, as I like to fly higher for the fuel efficiency it provides and turning 2700 at 12K allows me to get the same cruise speeds that I get LOP at 8K. Also, the climb with the 320 at higher altitudes is not that great and adding pitch would reduce that further. For clarity, I could probably turn 2850 at 8000' if at best power mixture. I simply haven't done this since phase I testing.

Remember that adding pitch adds MAP, so as RPMs go down from pitch, the MAP goes up, keeping fuel flow similar. Same is true in reverse. IMO, you want a prop that delivers 8000' cruise at target RPM when LOP with a 320 (assuming you routinely fly LOP). Otherwise, you will not be able to maintain similar speeds at 12K and peak or ROP.

If you are turning 2450 RPM at 8000', you are WAY over-pithced and IMO would pick up a decent amount of speed at cruise from re-pitching. The higher you fly, the more speed increase you will see.

Clearly this logic doesn't apply if you have a prop with a 2600 RPM limit. My Catto is rated for 3000. Opinions vary, but I believe that optimum is slightly under-pitched compared to the industry standard.
 
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