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2 blade vs 3 blade props - pros/cons & performance

C

CFItom

Member
I hope I posted this in the proper location.

I don’t want to start a controversy here. But I would like to know the basics of the different considerations regarding the choice of 2 blade and a 3 blade propeller.

I would like to keep it simple and keep it to fixed pitch.

What are the general advantages and disadvantages of each.

It would seem a three blade will almost certainly be more expensive given similar construction materials. Such as 2 blade catto and a 3 blade catto. And the 3 blade will weigh more than the 2 blade.

I also assume that the three blade will generally have a smaller overall diameter and thus give a bit more ground clearance.

But what about performance? can you compare 1:1 the same pitch?

Is one more quiet exterior noise than the other?

Maybe another way to ask is. Why would I choose a 2 blade over a 3 blade? (Or vice versa)?
 
Certainly won't be any lack of unsubstantiated personal opinions on this topic.

So I'll start!

Three blades are sexier
Three blades are smoother

Now I'll duck and cover :)

ETA: To make it less personal opinion - What plane and engine are you going to hang this prop on?
 
CFItom said:
I hope I posted this in the proper location.

I don’t want to start a controversy here. But I would like to know the basics of the different considerations regarding the choice of 2 blade and a 3 blade propeller.

I would like to keep it simple and keep it to fixed pitch.

What are the general advantages and disadvantages of each.

It would seem a three blade will almost certainly be more expensive given similar construction materials. Such as 2 blade catto and a 3 blade catto. And the 3 blade will weigh more than the 2 blade.

I also assume that the three blade will generally have a smaller overall diameter and thus give a bit more ground clearance.

But what about performance? can you compare 1:1 the same pitch?

Is one more quiet exterior noise than the other?

Maybe another way to ask is. Why would I choose a 2 blade over a 3 blade? (Or vice versa)?
Click to expand...

I think the generally accepted theory is that 3 blades are smoother. And specifically the new 3 bladed composite props are very smooth and very light compared to metal 2 bladed props. But they come with some draw backs, mainly that they are expensive. But also I've heard complaints about struggling to get the lower cowl off with a 3 blade. There's a couple of guys who've come up with a "technique" for dealing with that.

On the Van's series, I've heard over and over again that the Hartzell Blended-Airfoil 2 bladed CS prop is the fastest. And, it's a whole lot cheaper than the composite props (2 or 3 blade). the drawback is the weight. And I've been told that onn the RV-8 with centerline seating you pretty much have to put a 30lb AGM battery in the aft location to get the CG far enough aft to make the plane handle nicely.

If you haven't seen it yet, here's the Kitplane's 2025 Propeller Buyer's Guide https://www.kitplanes.com/2025-propeller-buyers-guide/
 
As a general rule, aero engineering says that with identical blade designs (comparing apples to apples), the two blade will be faster, the 3 blade will pull (climb) better. That is a serous generalization however, and the truth is usually in the testing. Testing older blade designs against newer blade designs, the new three blades are fester AND pull better than the older two-blades - evidence that the wizards who design props are learning…..
 
The popular belief seems to be that the 3 blade is smoother and quieter but looses a couple knots in cruise relative to the 2 blade. I have a WW 300 3 blade on my 8 (IO-360 PV) with the battery on the firewall and no CG issue. Chose it because it was the lightest (36 lb with the spinner) and it was also the least expensive composite CS I found and it looks great! I have no comparison to a 2 blade CS but it is much smoother than the 2 blade FP on my 7. No problem getting the lower cowl on or off as long as you leave 3/8 to 1/2" gap to the spinner. A Cardinal on our field changed from a 2 blade to a 3 blade CS and it was much quieter on climb out.
 
+1 on the comments above:
1. 3 blade on any RV looks good
2. 3 blade FP catto is a very good prop matched to your RV of choice however they are substantially lighter than the design basis props such that in some cases (RV-7, RV-9) you will have to add weight up front to keep the CG in a reasonable forward range to accommodate range with baggage.
3. Getting the lower cowl off with a 3 blade can be a real PITA, think about this and solutions during the build.
4. For fixed pitch the RV performance is so good that it probably doesn't really matter which one you pick so the "cool" factor comes into play which is a personal choice.
5. The composite FP props do seem to have a few non trivial other advantages, they slow down quicker (even with a heavy crush plate) which makes managing speed in the pattern or when flying with buddies with CS props a little easier, they tend to have smaller hubs which have less inlet blanking tendencies when climbing out steeply on hot days (helps CHTs), and they may save your motor in the event of a prop strike.

These are of course opinions which you are welcome to ignore:cool:
Figs
 
MTV-9
1; look badass
2; lost approx 5 knots on top speed
3; climbs better
4; quieter and smoother
5; lost 12# on the nose if I recall
6; flat pitch closed throttle on decent, like throwing out a drag chute
7; no issue with cowl

1755727203898.png
 
I made two major mistakes on my first build:
- I built an RV-8A instead of an RV-8 (because I was told by someone at Van’s that I did not want a tail dragger and I did not know any better).
- I used a FP prop on it (because someone at Van’s said that is what I wanted and I did not know any better).

I fixed both mistakes. The FP prop went away at ~300 hours, replaced with the two bladed Hartzell BA prop. This one change transformed the plane in all aspect of flight. I used the same prop on the next two builds. The third build was an RV-8 that I’m flying today.

I’ve flown side by side with an RV-8 with the same engine and weight, but he has an expensive three bladed MT prop. I pulled away but his RV-8 does look sexy sitting on the ramp…

Pay your money and take your chances!
Carl
 
I went from a really nice 2 blade Sensenich ground adjustable to a 3 blade Wirlwind CS. I did it for better high density altitude take off and climb performance. The take off and climb is MUCH better and cruise is 5 KTS faster.

[Mod note: Vendor bashing text deleted per VAF rules; S.Buchanan]

prop 3.JPGprop 2.JPG
 
Last edited by a moderator:
uk_figs said:
+1 on the comments above:
1. 3 blade on any RV looks good
2. 3 blade FP catto is a very good prop matched to your RV of choice however they are substantially lighter than the design basis props such that in some cases (RV-7, RV-9) you will have to add weight up front to keep the CG in a reasonable forward range to accommodate range with baggage.
3. Getting the lower cowl off with a 3 blade can be a real PITA, think about this and solutions during the build.
4. For fixed pitch the RV performance is so good that it probably doesn't really matter which one you pick so the "cool" factor comes into play which is a personal choice.
5. The composite FP props do seem to have a few non trivial other advantages, they slow down quicker (even with a heavy crush plate) which makes managing speed in the pattern or when flying with buddies with CS props a little easier, they tend to have smaller hubs which have less inlet blanking tendencies when climbing out steeply on hot days (helps CHTs), and they may save your motor in the event of a prop strike.

These are of course opinions which you are welcome to ignore:cool:
Figs
Click to expand...
Comment on Fig’s second bullet point:
As he suggested, if you have a tail dragger you may have some CG issues to deal with because of the light weight prop on the nose. However, my 9A with the Catto three blade has no such issue (mark a + for the nose wheel crowd!).
 
Just as a comparison, according to the Sensenich folks I talked to at SNF last spring about their GA carbon fiber two blade VS their new (at the time) GA carbon fiber 3 blade: in flight above about 9,000’, the 2 blade did better (slightly faster). Below that, including runway performance, it was hard to tell any difference. The three blade had a perception of sounding smoother, but that was due to non-quantitative personal analysis. They hadn’t confirmed any vibratory analysis, technically, at that time. I don’t think you’ll see any meaningful differences in a fixed pitch prop. The three blade may give you very slightly better runway and initial climb benefits, but suffer very slightly in cruise performance at the altitudes we typically fly enroute. I didn’t have that choice when I selected my Sensenich 2-blade, and I’m not sure I would have done anything different. The three blade definitely looks cooler sitting on the ground with the engine not running. The two blade definitely looks cooler in flight walking away from a like three blade in cruise (when you can’t see the number of blades). Question is - when do you want to look cool? I think Sensenich makes a very good product with their Ground Adjustable props, and I don’t think you can go wrong with either choice.
 
andoman said:
3-bladed props are a PITA to ship…
Click to expand...
Not so with a three blade GA prop (Sensenich, Whirlwind). The blades ship in about a 4 foot box, with the blades, hub, bolts, and everything you need. Not so with a CS prop, or a one piece three blade Catto, but the OP was inquiring about fixed pitch, so you have options if shipping is a deal breaker.
 
This data is not from an RV but from a Mooney 201 with Lyc. IO360. My first Mooney had a Hoffmann HO-V123K-180R 3 blade wood composite prop. Now I have basicly the same aircraft, but with a McCauley B2D34C214-B/90DHB-16E two blade prop.

McCauley B2D34C214-B/90DHB-16E two blade prop
49,5 lbs
no continuous operation between 1500 and 1950 RPM below 15” MP.

Hoffmann HO-V123K-180R
49,6 lbs
smooth with no RPM resriction.

The weight is the same, but the Hoffmann was smoother and had no RPM restrictions.
I lost abbout three knots top speed.
I don’t care about the top speed. It is quite negligable an a typical one to three our cross-country flight.
The Hoffmann was an old blade design from the 70ties. Newer props may perform better, but the same goes to the newer two blade props.


What I liked most about the Hoffmann, was the braking when pulling the throttle. I fly a lot of rallyes, where on landing you must put the main wheels on a 3 meter (about 10 ft) wide strip painted across the runway. With the McCauley, consistency was replaced with luck.


It depends on the mission, but if I had the choice, I would go with a (any) three blade prop, because of the better braking. In Europe we have a lot of really nice airstrips with less than 1500 ft.
 
The vintage of the design matters a lot. CFD modeling has gotten much better and (for a while now) has included modeling beyond 2D. The cruise speed penalty for a well designed 3 bladed prop has pretty much closed to zero on the newer designs, while maintaining the climb and other advantages.

Ultimately, your (the OP's) choice will be complex across many attributes that will have different levels of importance. Mission requirements, component complexity, cost, OEM outlook (nobody wants an orphaned component), others mentioned in previous replies. Enjoy the research, self-education, and selection process. It can be one of the more fun parts of this endeavor, IMO.
 
Three-bladed props need to be protected from the piano hinge teeth when removing the lower cowl. (I slotted some pipe insulation and slide it over the leading & trailing edges; slotted pool worms would work too.)
 
Bo1000 said:
Three-bladed props need to be protected from the piano hinge teeth when removing the lower cowl. (I slotted some pipe insulation and slide it over the leading & trailing edges; slotted pool worms would work too.)
Click to expand...
Not if you split the lower cowl!:cool:
 
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