The International Cessna^® 170 Association

I was in Salinas last Friday dropping my Champ off for some work at Gold Coast Aviation - Larry Lujan. While having coffee with Larry he mentioned that he has obtained an STC for a three bladed prop for the Cessna 170.

I saw a three bladed prop on a Lycoming powered C-170 at Oshkosh a good many years ago. It really looked pretty nifty, but I think it would be kind of heavy and quite pricey.
I hope the guy in Salinas didn't give you the idea that a three bladed prop could be installed on a stock engine.

Scroll down and look at a post with similar heading. There is an stc for the Lycoming engine.

Its gonna be heavy. The new MTV 15 prop will probably be stc'd soon, and would be substantially lighter and perform at least as well.

Mike Vivion

Just to add a thought or two to this thread...
For aerodynamic/engineering reasons the most efficient prop is a ONE bladed prop. It's just hard to balance.
The next most efficient is a TWO bladed prop. They give better performance and higher cruise speeds than comparable multi-bladed props.
After the two bladed prop....the multiblade props are less efficient.
So why do people want them? Looks for some. Ground clearance for others. (To absorb the same amount of horsepower, more blades may be shorter. But adding more blades of similar or greater length will load an engine down where it will not develop hp, and will add unnecessary weight to the airplane.) Multiple bladed props are also more expensive to buy and maintain, and overhaul.
Why do mfr's install them on original equipment installations? In order to absorb excess hp of some engine combinations, or for ground clearance, or both.
A 3 blade prop is not a good thing on a stock 170, and is a questionable mod on a re-engined 170 in my opinion. It would be a very special set of circumstances before it would have advantage, and I doubt the investment would ever be recouped except possibly as a new engine/prop STC combination.

George,

I couldn't agree with you more in reference to the comments on the 170. I also agree with the notion that all other things being equal, a one bladed prop would be most efficient.

That said, you used to hear from the 180/185/206 guys how their two bladed props significantly outperformed the three bladed props, and why would any rational individual ever order a three bladed prop from the factory? You don't hear that much any more, and if you do, I'll tell you that that individual hasn't flown any of the new generation of three bladed props from Hartzell or McCauley.

This isn't inconsistent with your assertion, though, as long as you stick to the "all other things being equal" like blade design.

Fortunately for us all, propeller manufacturers have made some pretty good advances in blade technology in the last few years, and the newer props can be significantly more efficient than older design props.

So, this is one of those "never say never" kinds of things. I can show you clearly that a specific three bladed prop on a specific airplane is much more efficient, and delivers more thrust than an older design two bladed prop, even though the diameter of the two bladed prop is significantly longer.

But, a three bladed prop on an O-300? Doubt it. On an O-360 Lycoming? Not in my experience, and I have a little bit, with a Scout wearing a three blade prop.

But, on the bigger engines, there is no doubt that some three bladed props are much better than the old two bladed noise makers.

I'm running an MT composite two blade prop on a Husky with an O-360, and so far, I'm pretty impressed. Time will tell.

Mike

One of the complaints against the Lycoming conversions is that they are not as smooth as the 145. Of course, this complaint is usually coming from the guy who does not have the Lycoming conversion, as one of the reasons why he doesn't. So I don't know if they run rougher or not, I do know that they sometimes look like they're trying to jump right off the front of the airplane when they start up.
I would guess that, if nothing else, a 3-blade prop would increase smoothness with one of them 4 cylinder shakers.
There was a 180-horse 170 featured in Private Pilot magazine aboput 6 years ago which had a 74" McCauley 3 blade. Whose prop is this new STC for- Mac or Hartzell? And what diameter? What blade design (scimitar,etc)?

Eric

For some fun reading, see:

http://www.pponk.com/HTML%20PAGES/propellers.html

For my money (if I owned a 180, I agree a 3-bladed club
on a 170 is a waste of time), I'd go with the 88" seaplane
prop. It pulls plenty strong (per the chart) but costs
substantially less than the 3-bladed clubs on that list.

Strangely enough, I dont' see any cruise speed comparisons
connected with that chart. Practical experience (addmittedly
not a great deal of same) says the 88" seaplane prop is
faster in cruise by a good 4-5 mph (we "played" with an
airplane that had a 3-blade McCauley Black Mac on it, and we
threw a spare 88" seaplane 2-blade prop on it just to see what it
would do).

My "dream" airplane (should I win the lottery and get to keep
my 170 + buy another airplane) would be an early 180 ('59 or
earlier) with an R or K engine (or a -50 since we won the lottery!)
with an 88" seaplane prop. Lotsa fun when you push the
big black knob forward....

One practical advantage (not performance) of a 3 blade prop might be increased ground or water clearance versus a 2 blade, due to the smaller diameter. This could be a factor with a seaplane or with a nosedragger that's gonna be used "off-road".
Also, besides smoothness, a 3 blade should be quieter than a two blade, again due to the smaller diameter. At higher rpm, a 2 blade would go supersonic at the tips while the smaller-diameter 3 blade at the same rpm would not.
A guy at my airport has a Glastar with a 320 and a 3 blade c/s composite prop. It really sounds good taking off-- quiet and smoooth, almost a turbiney sorta sound to it. It might be quieter inside the cabin too, that's be nice for those of us who don't have ANR headsets.

Eric

It wasn't mentioned if this prop was a variable pitch or not. If so, it would outperform a fixed pitch two bladed prop.

When I had my 206 with an IO-520 in it and a 2-blade McCauley prop...and my neighbor had a 210 with the same engine and a 3-blade McCauely on it...everyone at the airport said HIS 3 blade was MUCH noisier than my 2 blade.
2-blades don't go "supersonic" and make more noise. Their diameters are limited and their pitch-governors are set to prevent their going supersonic because any blade which does so loses efficiency rapidly. (We've already discussed the fact that 2-blades are more efficient than 3.)
The pponk link listed previously by Bela demonstrates the "pull" of the props during a static run-up. The relationship between the props does not remain constant. As the aircraft gains forward speed the 2 bladed props gain on the 3 bladed props and at typical cruise speeds surpass them. (This only applies when apples are compared to apples. Change models and one frequently finds that blade shapes are also changed, as Mike pointed out.)

It's also interesting to note that McCauley now has an 88 inch diameter three bladed prop approved on 180's and 185's. Same diameter as the two bladed "seaplane" prop.

My experience with a 185, going from an 88 inch two bladed prop to one of the new generation McCauley 84 inch diameter three bladed props was that the airplane got out of the water noticeably quicker with the three blade.

It is interesting to note that Wilbur and Orville's first prop design on the 1903 flyer was very nearly as efficient as any prop today. Not very practical with our engines, but very efficient.

There is no doubt that 3 bladed props are smoother, but dynamic balancing is the key to smoothness in any case, with a 2 or 3 bladed prop. EVERY engine/prop should be dynamic balanced, in my book.

The four cylinder Lycomings are shakers, for sure, but with a well balanced prop, a good dynamic balance and some care, they can be pretty smooth.

Mike Vivion

I just bought a 170 with a Lyc O360. It was the first 170/180 I had ever flown. My first comment was "I thought these things were suppose to be rough!". I have owned other stock 170's and find this plane to be just as smooth as the stock birds. Now I will say that if you don't follow Lycoming's shut down procedure, it will shake like a dog passing a peach pit (to borrow another member's expression).

I have also put a McCauley three blade prop on my stock 182. It is smoother, I would say it's quiter, and it looks REAL good, but other than that about all I can say is; well, it was a pretty expensive face lift.

I would consider putting one on my 170/180 IF I thought it would give me any performance gains. However, based on my 182 experience, I'd have to say the jury is still out.

What prop? I have heard of some props with a "harmonic balance" or some such device installed?
A friend is converting his Swift to an IO-360 Lyc. His doesn't have it, but according to him another guy's Swift has an IO-360 Lyc with counterweights, like on the C-145/O-300 cranks.
I'm sure that would help things immensely.
A couple other guys I know have O-320 powered airplanes (an RV & a Citabria), they both just installed Lord mounts & claimed it made a world of difference. One guy paid about $400 for the set, the other guy about $275. Pretty cheap, if it makes the airplane that much smoother.

Eric

I can't say anything about the 3 bladed prop, as I haven't flown one, but we are just finishing up on our '53 C170B with a Lyc 0360, and a Hartzell 80" c/s prop and dampener. We have put 7 hours on it so far and are delighted with the conversion. The Lyc runs very smooth. The smaller 76" props don't require the dampener, but the 80" and the 82" do. We have the dynafocal mount and it seems very smooth, like I said before. The penalty you pay is in the weight, but it really isn't that much. I put in a Javelin tank and moved the battery back, (which isn't neccesary), and the licensed empty weight C/G is within .05 inches of when it came out of the factory. The other day with most of the interior out, 1/2 tanks, and myself on board, I was showing a hair over 1500fpm climb out at 80 mph, (sea level at 75 degrees). The prop is a little noisier, but we all wear headphones, right!? I don't have any #s on fuel consumption etc, as we are just breaking the engine in, (just 7 hrs on the rebuild so far). Will have some other data later as we get more hours. Jon

Jon, what engine conversion STC are you using? Is the dynafocal mount required by the STC spec's, optional, or what? This "dampener" you mention, is this part of the propeller assembly?
For some reason, I thought (at least most of) the 180 Lyc conversion STC's spec'd out a 76" prop,either fixed or c/s. I wasn't aware that an 80" or 82" prop was acceptable.

Eric