Fixed Pitch vs Constant Speed Prop

[Scott Hersha]

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.

[airguy]

Originally Posted by PilotjohnS

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!

[Mel]

Originally Posted by snopercod

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.

[plehrke]

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.

[vlittle]

Originally Posted by snopercod

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

[terrye]

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.

[Toobuilder]

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.

[PilotjohnS]

Originally Posted by Toobuilder

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

[JimWoo50]

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.

[RV6_flyer]

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.