RV7Ron

RV7Ron

Well Known Member
Hey guys, new to the forums, soon to be -7 builder...visiting Vans on April 10 and plopping down a check for the empennage.

I plan to put a IO-390 out front with a C/S prop and fly out of Denver. What are the performance advantages and disadvantages between a two-bladed prop and a three? We can ignore the added cost/maintenance side of it for this discussion as I am mostly concerned about performance pros/cons. Does a three have better climb performance and reduced performance in cruise? I know I have some time to decided on this, just wanted to educate myself with your help. :D
 
Generally speaking, a three-blade prop is less efficient than a two-blade. If performance is your ONLY concern, then go with a two-blade. However the various three-blade models offer other advantages in terms of vibration, looks, and ground clearance to name just a few. In some instances, the three-blades are nearly the same as two-bladed in terms of speed, but not as fast.

BTW, I have a three-blade prop on my Rocket and I did it for weight, looks, and ground clearance.
 
I think it is generally accepted that a 3 blade prop has better climb but slightly less top end than a 2 bladed prop. There of course could be exceptions to the rule.

Saying that, even flying from Leadville, CO on a hot day with any CS RV loaded to max gross should be a none event.
 
Thanks guys, that helps...I didnt consider some of the other things you mentioned...less vibration, quieter, more ground clearance. I do like the looks of them too.

Randy, what kind of three blade do you have on your Rocket?
 
Vibration is definitely different (less is relative), Quieter is most a function of tip speed, i've flown with some real quiet 2-blades at 2200RPM (Cessna 182 for instance). Our 3-blade on the Cardinal is actually noiser at RPM than the older -7666 2-blade, but as soon as the rpms are down to 2300 it shines, 2500+ is a real racket though.

I'd say with the -390 the best combo is going to be with a Blended airfoil Hartzell, but if you want to take the cruise hit, a 3-blade MT will work too. It WILL reduce you top speed significantly, and hasn't been proven to climb better.

Anyone know if the WW-200 is approved for the -390 yet? I think it was HP limited to 200?
 
RV7Ron said:
Thanks guys, that helps...I didnt consider some of the other things you mentioned...less vibration, quieter, more ground clearance. I do like the looks of them too.

Randy, what kind of three blade do you have on your Rocket?
Click to expand...

I have the MT.
 
There may be one other esoteric consideration. Some aircraft cowlings are engineered so that the prop pulse coincides with induction, theoretically providing slightly higher MP in flight. For a 4-banger, this requires a 2 bladed prop to work.

But then, I haven't really ever heard of someone switching to a 3 bladed prop and noticing a measurable loss of MP - so it may be B.S.
 
My RV8 has a three blade Hartzell and I like it for ground clearance and noise. I can't tell that the top speed performance is compromised and like all 200 horse RV's it climbs like crazy. I fly with a number of RV8's and 4's with 180-200 hp and I am just as fast or faster. I am fairly light at 1105 lbs.

I don't think you would be giving up anything significant performance wise with the three blade. I am a bit nose heavy but I like that for when I haul big people in the back.
 
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