Limiting Speed Prop

I have a Cherokee 140 with a 160 hp engine and I am trying to improve short field takeoff & landing performance and cruise speed performance. This has led me to investigate constant speed, adjustable pitch propellers. Aside from not finding any STC constant speed props available to replace my fixed pitch prop in my application, I also found some operational disadvantages of installing a constant speed prop on my plane.

The constant speed propeller has a governor that has direct control of blade pitch during takeoff, climb, cruise, and decent power settings. The operator uses the prop control to adjust governed RPM and the pitch is set automatically to maintain it. This means the tachometer can no longer be used as a leaning aid or as an indication of power output. The addition of both a manifold pressure gauge and an engine EGT or engine analyzer are needed in order to fly and operate the engine properly. Power is set using a combination of manifold pressure and RPM selected, and fuel mixture is adjusted while referencing the engine analyzer or EGT indicator as the mixture is leaned.

The fixed pitch propeller is simpler to use, but it may only be optimized for one primary flight condition and one secondary condition at a reduced power setting. It is less efficient under all other flight conditions, thus providing less available engine performance for the aircraft. For simple airplanes with cruise and climb speeds relatively close together, the fixed pitch prop is ideal. If the cruise speed and climb speed are not close together, a fixed pitch prop must be selected that compromises aircraft performance in one way or another. The designer usually has a choice of climb performance, cruise performance, or a compromise of performance somewhere in between. This is why adjustable pitch constant speed propellers were designed.

The constant speed prop allows the engine to develop and maintain full RPM and power during takeoff, constantly adjusting pitch to maintain full RPM as the aircraft accelerates and the airspeed through he propeller increases. Constant speed propellers allow the pilot to set the engine to maintain a constant high RPM climb and a constant mid RPM cruise. But constant RPM means the engine must be monitored in different ways to maintain safe operating conditions. This means more indicators to use and maintain, and more pilot workload to insure the engine running properly.

Ground adjustable pitch propellers can be adjusted while the aircraft is on the ground, but not in the air. The pilot must pick his compromise and stick with it for the duration of the flight. These props are easy to use, need no additional instrumentation and are ok for short hops from short runways, or long hops from long runways, but not very good for long hops from short runways. The constant speed propeller and governor also out performs the adjustable pitch propeller during the takeoff and initial climb, because it keeps the engine at full power and maximum thrust throughout the ground acceleration phase of the takeoff. A fixed pitch prop setting would not be as efficient at all times throughout the takeoff procedure.

Wouldn’t it be nice if we could have the best of both systems? A system with the governor tracking capability of the constant speed prop for takeoff and the ability to adjust and set the prop to a desired constant pitch for cruise. No need for a manifold pressure gauge, engine analyzer or EGT unless you wanted to install them. This system is called a limiting speed propeller system.

Limiting speed governors have been around for a long time. The limiting speed propeller would also allow the engine to develop and maintain full RPM and power during takeoff, constantly adjusting pitch to maintain full RPM as the aircraft accelerates and the airspeed through he propeller increases. It would also let the pilot select a constant pitch that would allow the engine to maintain a high RPM climb and set a constant to pitch to maintain a mid RPM cruise. At these constant pitch settings, the engine would perform just like it had a fixed pitch propeller but the pilot could adjust that pitch setting while in flight to match actual flight conditions. The tachometer, altimeter and the airspeed indicator would give the pilot all the necessary information needed to operate the engine properly, efficiently, and make TBO.

Here is how it would work:

During takeoff the mixture, prop, and throttle controls all go forward to the stop. The engine goes to full RPM and power and the governor continuously adjusts the pitch to hold it there during acceleration, rotation, and initial climb at Vx or Vy. The pilot then backs the throttle just off of the stop and lowers the nose and accelerates to in-route climb speed. He then eases the prop control back to drop the engine speed about 100 RPM. The drop in RPM takes the governor out of the picture and the pilot now has direct control of propeller pitch. At every 1000 feet of altitude gain the pilot leans to peak RPM, then he readjusts the pitch if needed to keep the RPM about 100 RPM or so below governed speed.

If the desired cruise altitude is 5500 feet, the pilot would lean to peak RPM at 5000. At 5500 feet he would adjust the prop control to drop about 300 RPM, then lower the nose and accelerate to cruise speed. He then adjusts the throttle and elevator trim to hold it there. He then readjusts the prop control to the desired RPM, and adjusts throttle to maintain altitude. That’s it. Now the aircraft is cruising steady at cruise speed and 5500 feet. A quick mixture check to peak RPM periodically is all that is needed.

If the desired cruise altitude is 9500 feet, the cruise configuration is slightly different. The engine is automatically producing less power due to reduced air density so the throttle will remain at the in-route climb setting, it will not be reduced. At this altitude and near wide open throttle, the more RPM on the engine, the more power is being developed. The pilot would lean to peak RPM at 9000 feet. At 9500 feet he would adjust the prop control to drop about 200 RPM, then lower the nose and accelerate to cruise speed. He then adjusts elevator trim for airspeed and prop pitch to change the RPM to hold altitude.

The aircraft’s weight, cruising altitude and airspeed basically dictate how much power the airframe will use. If you trim for the desired airspeed at cruise altitude, the engine will automatically develop the power required to hold that altitude at a certain RPM. If the aircraft climbs, lower the RPM. If it descends, raise the RPM. If it hits full RPM and won’t hold altitude, the airframe too fast and is drawing more power than the engine can deliver. Slow down some and try again.

The prop control need not be readjusted unless the cruise speed changes or a new climb is initiated. It can be left alone until the aircraft gets back to the traffic pattern. When the aircraft gets to the pattern the prop control is pushed forward to just short of the stop. Pattern power may be adjusted by RPM. During a go-around applying full throttle power will top out the RPM putting the governor back in the system. The system will automatically adjust pitch as needed to maintain full power on the climb.

Once the pitch is set, the tachometer can be used to check the mixture setting so long as the prop control is not disturbed during the leaning process. The altitude and airspeed will determine the power required, so a manifold pressure gauge is not necessary. The tachometer can be used as the mixture setting tool, so and engine analyzer is not necessary. The limiting speed system is simpler in operation with less equipment to mount in the panel and maintain. I like that.

I wrote this because I want a prop company to make this product available to us. The limiting speed prop is the perfect upgrade if your aircraft is currently set up with a fixed pitch prop, especially if you want to takeoff shorter, climb better and still go fast. It is as easy to build and install as a constant speed prop, requires no additional instrumentation, and is simpler to use and still get good results. So how about it?

Thanks in advance for your replies. Pete

I think what you are asking for is something that is operated more like the old style conventional variable-pitched prop, e.g. the prop control varies the pitch but there is no governor function that automatically varies the pitch to maintain RPM. These planes are flown just like an airplane with a constant speed prop, and you need the same instrumentation, the only difference being that you have to jockey the pitch control to maintain a desired cruise prop speed. A constant speed prop does that work for you.

The bottom line is I see no advantage to a setup as you describe over a constant speed prop. Read the discussion a few threads below on conversion to a CS prop. If you have a variable pitch prop, whether it's a conventional adjustable pitch or a constant speed, you still have the same weight and instrumentation penalty.

This may be a bit belated (I just found it from a search engine), but I don't see the process of managing a constant speed prop as being such a problem.

I happen to own the first 140 to get the Hartzell prop. The STC is serial number 0001. The plane was purchased by one of Hartzell's principals in 1969, and was the basis for the paperwork.

Personally, I find the process of operating the prop to be an intriguing element of flying the plane as a whole. Once you get used to using the additional instrumentation, it's just not a big deal. In any case, the benefits are well worth it. This plane climbs like crazy, with no other modifications to the stock configuration.

I am aware of one CS prop on a Cherokee 180 in Southcentral Oregon. My understanding is that it was a field approval. Same guy is working on an STC to convert the plane to a taildragger using a fuselage mounted gear system ala Cessna 180.

Can't imagine why on the TD, but the CS prop intrigues me (after coming from a Cardinal). Only thing is, I don't need any more weight in the nose of this thing.

Dirk, I'm not running the Powerflow exhaust, but I increased power enough with the 160 mod and drag reductions to allow me to repitch to 62 inches and still climb better than stock.
JimC