Two blade prop vs 3 blade?

[Buckaroo]

I see some CT's with two blades. Just curious how would two vs three be different in static rpm, general performance, fuel gph etc?

 

I've heard the less blades equals better efficiency in most applications!

[Roger Lee]

"I've heard the less blades equals better efficiency in most applications! 

Old school thinking. Static rpms can be quite different by 150 rpm. length, flexibility or lack of figures in, blade twist, blade width, ect.

 

I did special research project about 4 years ago now. 14 different props from different Mfgs. and different configurations. These were tested with 4 identical CTSW's and they were flown at the same time. They even took off in pairs side by side. Without getting into all the specifics the 2 vs 3 blades are similar in climb and top speed PROVIDED they are set to the same WOT rpm in level flight. The best all around balanced rpm for a ground adjustable prop for us is 5600-5650 rpm WOT in level flight at YOUR AVERAGE ALTITUDE. 2 blades have a wider frequency vibration in the frame vs a 3 blade and tends to be a tad smoother. The one prop that suffered over the others in climb was the Warp Drive and I would contribute this to it's flat wing like design. Top speeds were fairly equal.

 

If you have a 2 or 3 blade I'm sure your plane is good to go, just make sure the WOT rpm is where it should be and this has nothing to do with static and needs to be flown to get real time numbers. You should never be below 5500 rpm at WOT in level flight. This is over pitched and this has zero redeeming qualities and cause losses in all you flight envelope.

 

I tried hard to make this answer in 2 sentences, but that didn't pan out. If you need more info call me otherwise this can be a dissertation type post.

[WmInce]

"I've heard the less blades equals better efficiency in most applications!

Old school thinking. Static rpms can be quite different by 150 rpm. length, flexibility or lack of figures in, blade twist, blade width, ect.

 

I did special research project about 4 years ago now. 14 different props from different Mfgs. and different configurations. These were tested with 4 identical CTSW's and they were flown at the same time. They even took off in pairs side by side. Without getting into all the specifics the 2 vs 3 blades are similar in climb and top speed PROVIDED they are set to the same WOT rpm in level flight. The best all around balanced rpm for a ground adjustable prop for us is 5600-5650 rpm WOT in level flight at YOUR AVERAGE ALTITUDE. 2 blades have a wider frequency vibration in the frame vs a 3 blade and tends to be a tad smoother. The one prop that suffered over the others in climb was the Warp Drive and I would contribute this to it's flat wing like design. Top speeds were fairly equal.

 

If you have a 2 or 3 blade I'm sure your plane is good to go, just make sure the WOT rpm is where it should be and this has nothing to do with static and needs to be flown to get real time numbers. You should never be below 5500 rpm at WOT in level flight. This is over pitched and this has zero redeeming qualities and cause losses in all you flight envelope.

 

I tried hard to make this answer in 2 sentences, but that didn't pan out. If you need more info call me otherwise this can be a dissertation type post.

Roger,

 

Based upon your experience in that testing, and assuming some of those airplanes had tundra wheels, was prop clearance a legitimate issue with two blades? What is the longest length you would recommend for two bladed props?

[Anticept]

The general rule of thumb is this: given all other factors equal, less blades are marginally more efficient. The trailing streams interfere less with one another with fewer blades, which manifest in a slightly higher top speed. A three bladed prop, however, can translate more torque into forward motion, so it has a slightly better acceleration and climb.

 

Notice what I said was bolded? That never happens. The manufacturers will make subtle changes that closes the gap significantly between two and three blade props.

 

What DOES make a difference, though, is that 2 blades are usually cheaper, and three blades are usually quieter (kindof, it's more because the beating of the air against the cabin happens more frequently, so you think it's quieter, but it may not be true in all cases).

[Buckaroo]

Great information and very informative! Thank You!

 

Roger can you elaborate on "never be below 5500 WOT in level flight"?

 

I know one thing! Three blades look much cooler!????

[WmInce]

The general rule of thumb is this: given all other factors equal, less blades are marginally more efficient. The trailing streams interfere less with one another with fewer blades, which manifest in a slightly higher top speed. A three bladed prop, however, can translate more torque into forward motion, so it has a slightly better acceleration and climb.

 

Notice what I said was bolded? That never happens. The manufacturers will make subtle changes that closes the gap significantly between two and three blade props.

 

What DOES make a difference, though, is that 2 blades are usually cheaper, and three blades are usually quieter (kindof, it's more because the beating of the air against the cabin happens more frequently, so you think it's quieter, but it may not be true in all cases).

Corey,

 

Where you been? We missed you.

[Anticept]

Personal issues. Nothing health related, just distractions and indecision on how I want to continue with the path of life that I'm on. Left me in a bit of a dead zone for a while and didn't have motivation to do anything.

[Tom Baker]

Roger can you elaborate on "never be below 5500 WOT in level flight"?

 

 

 

 

 When flying the airplane in level flight you can advance the throttle all the way forward, after waiting a little bit for the engine to build RPM the RPM number should ne be less than 5500 and not more than 5650. If this is not the case, the propeller pitch needs to be adjusted to bring within those 2 numbers.

[Buckaroo]

So this is a prop pitch test? You still can travel at 75% power and up to 5500 but not travel above 5500 correct?

[Anticept]

The redline is 5800 RPM. 5500 is the intermittent limit (5 minutes)

[gbigs]

So this is a prop pitch test? You still can travel at 75% power and up to 5500 but not travel above 5500 correct?

 

The POH says:  4300 RPM economy cruise, 4800 RPM most efficient cruise, 5500 RPM max horizontal cruise Vh. 

 

The prop is ground adjustable so there is a tradeoff between climb and cruise and density altitude.  

 

Given how low GPH is compared to other aircraft, efficient to max cruise is not much of a fuel penalty.   We routinely cruised around 5000 to 5350 on long cross-country flights and the engine seemed perfectly happy there.  It is true you can get vibration/harmonics at lower RPM so for this reason the higher RPMs may be optimal for both noise and vibration.  But there is no upside to running the engine at max cruise for long periods.  And the engine temps may like you more on hotter days if you run around 5200 or so.

 

Also, the POH does note the plane has a tendency to side-slip a bit in level flight and thus drain one wing tank faster than the other.  They advise reversing the slip once in a while to balance the fuel usage.

[Buckaroo]

Thanks I will be doing many cross country's as soon as I can get to the plane through the snow!☃

[FlyingMonkey]

 When flying the airplane in level flight you can advance the throttle all the way forward, after waiting a little bit for the engine to build RPM the RPM number should ne be less than 5500 and not more than 5650. If this is not the case, the propeller pitch needs to be adjusted to bring within those 2 numbers.

 

I agree with this, but I don't think the 5650rpm number is hard and fast.  When I first re-pitched my prop from the factory dog settings it would run 5700rpm or even a smidge higher at WOT, and I loved it.  It climbed great and cruised fast too.  I flew it that way for a year and a half.  I later thought I might be leaving a little speed on the table to I re-pitched slightly to a 5650ish setting WOT, and I did pick up 3 knots or so, but lost a little climb.  It's so close to perfect now I'm not going to mess with it again.  We set Bill Ince's within 50rpm of mine and he's very happy with it also. 

 

I'd say within the limits Tom mentioned, aim for more fine (higher rpm) instead of lower.  You will still be within whiskers of perfect climb/cruise, but you'll be biasing toward climb.  You might lose a couple of knots in cruise, but nobody ever got killed because their airplane had too much climb performance.  If you use 5650rpm as a target on a Neuform three blade prop, you will be happy with the results if you get close.