propeller theory

A place to discuss different propellers and their pros & cons.
Reheat
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propeller theory

Post by Reheat »

Food for thought...
We all know that constant speed propellers provide more power available during takeoff and climb. They are also heavier than fixed pitch props, and approximately 5 times more expensive.

A fixed pitch prop is simple, light, cheap, and basically optimized for a given speed regime. The Sensenich appears to be set up to cruise around 190-200 mph and will give less climb rate than the constant speed prop, assuming equal WEIGHT. Don't forget that climb rate is directly proportional to gross weight. So, if you can save 100# on a 1000# airplane, your climb rate will improve by 10%.

Maintenance is less on the fixed pitch.

The MT type props seem attractive given constant speed and light weight.
They are pricey.

I like wood, but the rain thing is a problem...

I think if you keep your machine simple and light, a fixed pitch will service your needs nicely. If you want full IFR and all the bells, whistles and 5 coats of paint, then maybe the constant speed will be necessary to get a decent climb rate.

Just my thoughts... most of my flying has been jet-powered so I don't have a lot of experience with propellers to fall back on. Welcome all comments and thoughts on the matter.

Cheers and happy holidays!
8QB -fus

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728GD
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Post by 728GD »

Keep one thing in mind, we are talking about RV's. Even a poor climb rate in an RV is better than almost all spam cans! I have a constant speed prop on my 6, I cruse about 10 to 20 mph faster than most fixed props. I climb out a little faster. The biggest advantage is when flying in high density alt and fuel management.

These airframes are so clean, unless you operate in high density alts all the time, a fixed pitch will work just fine. I was lucky and got a good deal on my prop otherwise I would have used a fixed pitch.

Either way your a winner when it is stuck on the nose of an RV!!
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Post by Spike »

Constant speed all of the way. Well, at least thats what I thought until lately. After looking at prices and reading some of Dick V's writings I think I have changed my mind. The performance on a FP prop is nothing short of awesome and it seems to me that I dont ever plan on being in a situation where the difference between a CS prop and a FP prop will matter. I guess in the end its more of a cost benefit analysis for me. In that light the FP prop wins every time.


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728GD
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Post by 728GD »

I hear you John. If you are interested though, I have a contact with a prop shop out on your end of the Country. This guy will build up a non-certified prop, usally the blade roots are out of tolorance by a few thousands so they don't make certification on an overhaul. Anyway, you can get a perfectly good CS for almost half the price of a new one.
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Post by Spike »

Hmm, now that sounds interesting ......
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l & d lewis
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Post by l & d lewis »

John,

What did you read from Dick V, and where can it be found? According to his literature his performance figures are with a CS prop. It would be interesting to see some comparisons. Cost vs. performance.

tnx....Larry
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Post by Spike »

Hmm, basically what I read was a bunch of comments and comparisons between props in the various articles in the "18 years of Rviator" collection. There were some performance comparisons between wood, metal, CS, etc, yadda yadda yadds. It really seems to me that the biggest gain is in the climb performance and top speed. What I asked myself was if gaining 200 FPM over an already 1500+ FPM climb rate and another few knots justified the cost, weight, and maintenance expenses. For me I have been coming up with the answer of "no".

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l & d lewis
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Post by l & d lewis »

Fair enough, I thought maybe he had some specific insite into his airplanes, and their performance. I guess its back to what kind of performance you want to squeeze out of the plane, and how much are you willing (read able) to spend. Same goes for the 180hp vs. 200. Thanks.....Larry
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jim_geo
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Post by jim_geo »

Well actually Van does cover this subject pretty well in the pre purchase planes set that we're all supposed to have read before starting. Pretty much what I gathered is that he thinks CS props are unesessary. Also what I have found from my own on line reserch is that if you intend to do airbatics a constant speed prop is not recommended because of gyroscopic and vibrational wear. That bit of info is straight from Hartzell.

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Post by Snap »

I would like to put an MT three pronged thingy on the front but you just can't go past the dollar/performance of the Sensenich (no AD's). The just seem to be 'IT' for dollar value.
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Post by Spike »

Exactly my thoughts snap.
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hngrflyr
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Post by hngrflyr »

I chose a Catto propeller for my RV-6. The propeller is light and durable and is not harmed by flying in the rain, and importantly does not have the RPM restriction of the Sensenich metal prop. It doesn't make sense to me to neuter the engine by restricting it to lower than rated RPM. At full throttle at low altitude, the plane is 10 mph faster than it was with the wood propeller The Catto Propeller replaced.

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props

Post by Reheat »

I've heard good things about the Catto. I assume you are running a 0-320 on your 6 since you mentioned the rpm restriction. I did not realize that the Catto was "rain-proof". Did you get the 2 or 3 blade model?
You also seem happy with the pitch. Did Catto get it right on the first rry, or did you send it back for tweaking?
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l & d lewis
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Post by l & d lewis »

Jim you must be reading a different set of perview plans than I am. In mine Van wrote "If you want the ultimate performance from your RV, there is no substitute for a constant speed propeller". In the aerobatic section he talks about the braking action of CS during the dive portion of maneuvers and the "slow speed thrust advantages" of CS. Granted there is more stress on the crankshaft due to inertia. My query is simply if anyone has real-world experience/information that might be helpful in selection for an all round -8 that I can cruise one day and tear up the sky the next? Tnx.............Larry
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hngrflyr
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Post by hngrflyr »

For a good description of the Catto Propeller's construction, see:
www.cattoprops.com.

Our RV-6 has a 160 hp 0-320.

Mine is a two blade propeller. I had Craig Catto build the propeller assuming I would be running electronic ignition. I am still firing the plugs with Slick Magnetos, so my RPM is still a little lower than I would target. At full throttle at 3000' I show 191mph IAS. The engine turns about 2675 RPM. After installation of the electronic ignition, I plan to tweak the prop, if needed, to get around 2800 RPM.
A 4 way run checking with GPS gave me a solid 153 KTS average @ 22.5" MP at 3000'. RPM was slightly under 2400. At the present time my climb performance is about equal to the climb performance with the wood propeller. That should also improve with the electronic ignition.

I couldn't be more pleased with this propeller.

Opinions are very subjective, but for me, the flying characteristics of our RV-6 are so delightful, I'm not sure I would want a heavier prop up front.
With two people aboard and full fuel, the trim tab flies flush with the elevator. The airplane seems rigged very well in pitch. I like the CG where it is.
Another consideration is, if you're into aerobatics, gyroscopic loads increase with propeller weight. This effects the front main bearing in the engine as well as flight maneuvers.

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Post by maverick »

I believe that one argument for a heavy CS prop would be to move CG forward. I was reading the RVator book last night. It has several articles about RVs being susceptible to aft CG problems, especially tandem RV-4 (which I'm now a proud owner of) and maybe -8(?), but also side-by-side planes.

One way to move weight forward is a C/S hub/prop combo. Of course the counterargument is added weight, which Van cautions against. Avation -- everything's a compromise.

Max

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Post by hngrflyr »

Yer right. There's no free lunch.

Trading this for that, is one of the things that makes this so much fun.

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Post by jim_geo »

I & D lewis, I do need to keep in mind that you are all building several different models of the RV line and that I at this point am only reading about building one of them. I apologize to everyone for the curt sounding remark I made about reading the preview plans. It wasn’t meant the way it came off. My only excuse is that I was deep into a lousy cold and felt like hell anyway. Surely wasn’t as diplomatic as I could have been. My preview plan set is for an RV7 and while it’s been awhile since I read that part it seems that what I was expressing was accurate for an RV7. I will go back and do some review. Although it may be that there is enough difference between an 8 and a 7 and the expected respective uses that that C/S prop would be more appropriate on an 8 Therefore a difference, of course, in the plans set info. Personally I am trying to have as many of decisions about engine, prop, canopy, landing gear, primer, tools etc. done by the time I order the first kit. The other thing I mentioned was that Hartzell doesn’t recommend C/S props for aerobatics. That is from their web site. What I failed to mention, also from their web site, is that they do build custom C/S props for aerobatic planes.

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Post by 728GD »

I think the question of CS vs. FP and the issues of aerobatics with a CS vs. FP are summed up in one statement. Which ever one you like the best is the best for you. As far as the issues with aerobatics with a CS. Hartzell says it is fine if you ensure you have an aerobatic hub on your prop. They do not recommend aerobatics utilizing the "standard" hub as those found on most certified airplanes. Just walk through a airshow sometime, I can not remember the last time I saw an aerobatic pilot who did not have a CS prop on the plane, if only for the ability to control overspeeding the airplane in vertical dives.

As far as the CS vs FP you can get the same arguments when asking what is better side by side or in line seating? RV6 vs RV6A, sorry old builder, RV7 vs RV7A, how about the 7 vs 8, 8 vs 8A, what about the 9 or 10..... Bottom line, each has benifits and each has downsides. You will never convince a pilot who "loves" their CS that a FP is a better option. Just as you will never convice a FP driver that a CS is worth the weight and cost increase. Each has value.

As I said before, what is most important is we are all sticking them on the nose of our RV's. One of the best airplanes ever designed (for the cost)!!

There, now that statement should start another series of debates!!

Happy Holidays all.

Dale
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l & d lewis
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Post by l & d lewis »

No offense taken Jim, I'm just a resident of the state of confusion. Actually I was just hoping someone who read the post might have experience in both configurations, and have an opinion. Its easy to build a machine for a single purpose, but trying to get a balance (between cruise and sport) is a different story. I currently have a Varga Kachina that suffers from some of the same ailments as the RV4 i.e. aft CG two-up and baggage in the back. The weight of the CS would help, although the forward baggage compartment in the -8 should help. The varga is limited to 50 lbs bagage, during the winter most of that is taken up by the survival kit required here in Alaska. I have one last commitment to attend too, before ordering the empennage, but probably won't decide on an engine and prop combination until well into the fuselage.

Thanks for the comments folks, happy building/flying.....Larry
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