Do constant speed props need to be pitched for a certain airspeed?

[DMD3.]

It’s my understanding that when mounting a fixed-pitch prop on an aircraft, it has to be pitched for a certain airspeed. If you removed a Lycoming O-360 from a Beech Sundowner and mounted it to a Vans RV or Glasair, you couldn’t use the same prop or you’d overspeed the engine. Likewise with removing something like an O-320/360 from a Glasair and putting the engine (and the prop) on some slow STOL aircraft. Full throttle MIGHT enable you to taxi at the speed of a brisk walk.

Is the same true with a C/S prop? If you took a 915is off a Zenith and on a JMB VL3, could you use the same prop if it were C/S?

 

[Dan Thomas]

It’s my understanding that when mounting a fixed-pitch prop on an aircraft, it has to be pitched for a certain airspeed. If you removed a Lycoming O-360 from a Beech Sundowner and mounted it to a Vans RV or Glasair, you couldn’t use the same prop or you’d overspeed the engine. Likewise with removing something like an O-320/360 from a Glasair and putting the engine (and the prop) on some slow STOL aircraft. Full throttle MIGHT enable you to taxi at the speed of a brisk walk.

Is the same true with a C/S prop? If you took a 915is off a Zenith and on a JMB VL3, could you use the same prop if it were C/S?

The CS prop adjusts its pitch, but it also has fixed washout along the blade, and that washout will be made to make the prop most efficient where it spends most of its time: in cruise. It needs to be matched to the airframe like a fixed-pitch prop.

Moreover, there are stops in the prop hub that limit the blade travel. Stick a CS prop designed for a slower airplane onto a fast one, and you'll hit the high-pitch stop long before the airplane reaches its cruise speed. Again, the prop needs to be matched to the airframe.

 

[DMD3.]

Ok that pretty much answers my question. The appropriate thread title shoud’ve been “Can a C/S prop be out of pitch for a certain airframe”.

 

[DMD3.]

One other question I have is, could you swap C/S prop & engine with an aircraft with only marginally different performance? An example would be an RV 6/7 and a fixed-gear Glasair (cruise 10-15 kts faster than an RV).

 

[tsts4]

My understanding is props are matched to engines. Aircraft designers then look for the combination that gets them the performance they desire. So yes you can swap between airframes where usually the limitation is engine weight and/or HP, not the engine-prop combo. I suppose the added weight of a C/S over a fixed pitch prop might also need to be taken into consideration for some aircraft. Best bet is to stick with whatever combo(s) the designer recommends.

 

[Dan Thomas]

My understanding is props are matched to engines. Aircraft designers then look for the combination that gets them the performance they desire.

The airframe designers take a bunch of factors into consideration. It's not just the airspeed; there's the ground clearance, the cooling requirements that sometimes demand more or less pitch closer to the blade roots, vibratory modes that are dependent on the type of engine mount, shockmounts, airframe structure, a bunch of stuff. Not as simple as it appears. Homebuilders have sometimes learned this stuff the hard way.

 

[Bell206]

A more technical view on propeller selection:
https://aerotoolbox.com/thrust-cruise-speed/

 

[Jeff767]

The CS prop adjusts its pitch, but it also has fixed washout along the blade, and that washout will be made to make the prop most efficient where it spends most of its time: in cruise. It needs to be matched to the airframe like a fixed-pitch prop.

Moreover, there are stops in the prop hub that limit the blade travel. Stick a CS prop designed for a slower airplane onto a fast one, and you'll hit the high-pitch stop long before the airplane reaches its cruise speed. Again, the prop needs to be matched to the airframe.

The mechanical pitch stops can be adjusted. Most CS props can be used over a fairly wide range of applications.

 

[Dan Thomas]

The mechanical pitch stops can be adjusted. Most CS props can be used over a fairly wide range of applications.

Yes, the stops are adjustable, within limits. But props for faster aircraft have more twist in the blades. It has to be that way, or the tips do all the work while the roots drag. The ideal variable-pitch prop would have the roots twisting more and the tips less, rather than the entire blade moving the same amount. This would take some fancy technology. I imagined a flexible prop blade mounted on a rod, fixed to the hub and running through a tube inside the blade. The rod would be fixed to the blade at the tip. Pitch increases or decreases to the blade would result in some twisting of the rod to let the tips increase or decrease, with the roots increasing and decreasing much more, due to the flexible blade. The whole affair would have to be designed to be resistant to flutter, and strong enough to handle engine firing pulses. Not simple at all.