Prop Selection

[nichzimmerman]

A lot of salvage shops appear to have separated prop hubs from blades and sell them separately. Since it's difficult to find a specific McCauley, is it conceivable to buy a hub from one place, blades from another, then assemble them and have a prop shop balance it? I've never worked with props before so have no idea what is involved in this area.

[Bdflies]

I can't think of why that wouldn't work. If I were pursuing that route, I'd ask around some local airplane guys about a reputable prop shop. Contact the shop and tell them what you have in mind. If they absolutely swear never to assemble such, you might take pause. On the other hand, I could imagine the shop might be very helpful in sourcing the parts you need.
I had some work done at Jordan Propellor service (http://jordanpropellerserviceinc.com). Great folks ro work with!

Bill

[Gerhard Rieger]

I have a IO 540 C4B5 and a MTV14 76'' MT 4blade prop. The engine speed will be ajusted for the recommended 2700rpm for the prop

[rodsmith]

Wow! How did you decide on a 4 blade?

[Gerhard Rieger]

Friend of mine had it hanging from his roof and offered it to me, I made contact with MT and they said if I take the RPM up from 2575 to 2700 it will work on my 540. I need to get it serviced as it was last used in 99. if you enlarge my photo you can see it hanging from my roof now. not to long from now it will hang on the Bearhawk

[Erik van Veenen]

I must say, that as a Maule owner and re-builder, the Bearhawk Forum is my favorite site for all sorts of Technical info, also pertinent to the Maule.
Although I work with the certified Maules only , the Bearhawk Forum is a valuable reference source for me. Some pretty unique topics posted by the Bearhawk afficionados .!
As holder of 3 different McCauley Propeller STC's for the 235hp Maule ( you can look my name up on the MaCauley STC holder list ) I am willing to offer any Bearhawk builder looking for a MaCauley Prop a factory new Prop at 100.- dollar over factory Invoce . Plus shipping anywhere in the world.
you have the choice between the 3 Blade 80", 2 Blade 86" & 2 Blade 81".
I,d like to invite any Bearhawk builder / owner to visit my FB Page : " Life is good Flying Maule Airplanes" to look at some of my Maule technical projects which are equally applicable to the Bearhawk. For example my newly redesigned Maule Instrument Panel with Shock Mounted, removable Six-Pack. ( inspired by the Aviat Husky ) i can also be reached via email: classic.carriers@gmail.com
i am also an official dealer for TRIG @ Aspen Avionics,, JPI, and offer the same unbeatable discounts on these products . ( as a small company I have minimal overhead and can afford giving the discounts )

[Chris In Milwaukee]

Thanks for the kind offer Erik! We can learn a lot from your cool Maule mods here, too. Your flap handle update would be a great Bearhawk update as well.

I follow your projects (as you know ), and appreciate the things we have to offer each other!

[Rollie]

My prop selection was pretty simple. I knew I wanted constant speed so I wouldn't have a big compromise for either short takeoff or good cruise. I talked to Bob, who was testing out the Trailblazer at the time, and he had good things to say about it. The short version was that the Trailblazer performed significantly better than the metal CS Hartzell and that on the Patrol, I would have cg issues with a metal constant speed. I was originally thinking that maybe I could go with a used metal prop and save a bit of money, but Bob's recommendation against that and the fact that both Bob and Mark are running the Trailblazer and are happy with them was enough for me. And I'm happy with it too. I've only got 5 takeoffs and 3 hours of flying in my plane so far but that prop seems to be doing its job well. It performs great in all phases of flight and its smooth.

[schu]

Well, at this risk of boring avid readers, I will re-write-up my prop selection process highlights here.

We ended up with a Hartzell Trailblazer 83" carbon fibre two blade prop. We really like it so far.
Following a lot of research, talking to as many people as possible who have composite props, we decided to try the newest offering on the market. I chose this over the major competitors (MT, Whirlwind) for a range of reasons, including Bearhawk relationship with Hartzell.

Combining the research and our subsequent experience operating the prop, we like the:
- testing by Hartzell which shows it produces 18% more thrust than their standard metal prop offering for a Bearhawk,
- sharp price offered through Bearhawk Aircraft,
- reduced vibration compared to a metal prop,
- lighter weight compared to metal,
- reduced inertia compared to a metal prop, meaning it responds a lot faster,
- shape and width of the blade aesthetically,
- reduced risk of engine damage in the event of prop strike.

We felt the biggest risks with a composite prop were largely the same for all manufacturers:
- stone damage to the carbon fibre; although carbon can be repaired unlike metal, a stone does a lot more damage to carbon,
- chips to the nickel leading edge from stones (same for aluminium really),
- higher cost of replacement blades,
- lower knowledge base in the maintenance shops compared to metal.

Overall we are really happy with the prop so far, but we haven't picked up a stone chip yet....



Now for the negative feedback.

Originally we wanted to get another MacCauley, but wow what a difficult company to deal with. I loved their props, but their "customer service" was bad enough to put me off. They literally would not sell me a factory new prop to mount onto an experimental Lycoming IO-540-D4A5 engine. That, and they were going to charge me 50% MORE for an inferior metal prop, than the Trailblazer cost me.

Most surprisingly, over a dozen people contacted me after my post on BCP about experience with composite props. All the people were unhappy with their Whirlwind or MT props (sorry rodsmith!!). I also had lots of public posts in support of MT. Most people with composite props on Lyc/TCM engines have MTs. A popular and wide-spread prop will always have some issues and detractors. The common complaint was damage to the leading edge causing a blade write-off and need to a whole new balanced set of 2 or 3 blades. Expensive returns to Deutschland for repairs, instability of the composite material itself (warping) which MT said was just visual (not structural), and aesthetic finish breakdown (paint cracking) also came up.

Battson can you tell me the speed differences you noticed between the 84 Hartzell metal prop and the 83 Hartzell trailblazer? 18% more thrust says it will probably work much better off of the ground, but what about speed?

Also, it seems the 3 blade would keep the CG in about the right place, but I suspect the 3 blade is slower, and you need to give up a vital organ to pay for it.

Thanks,
schu

[AZBearhawk272]

All the positives you hear about the 3 blade Trailblazer are true.
Weight is about the same as a metal 2 blade and lighter than a metal 3 blade.
Change in cruise speed is not noticeable.
3 blade metal Hartzell vs 3 blade composite Hartzell, composite is sooooooo much more efficient that it does not exhibit tip vortices sucking ground debris up into the prop.
I could not find anyone willing to make use of my vital organ so I opted for a prop that put a smile on my face every time it was cranked up, and in the end hope to get cash back. : )
Every time I slide the hangar door open I want to hug and kiss that big ole composite prop, never felt that way about metal.

Kevin D #272

KCHD

[Nev]

Is yours flying now Kev ?

[AZBearhawk272]

Not quite, spending too much time hugging the prop.
Have fuel in it…..

Kevin

[Battson]

Battson can you tell me the speed differences you noticed between the 84 Hartzell metal prop and the 83 Hartzell trailblazer? 18% more thrust says it will probably work much better off of the ground, but what about speed?

Also, it seems the 3 blade would keep the CG in about the right place, but I suspect the 3 blade is slower, and you need to give up a vital organ to pay for it.

Thanks,
schu

We changed wingtips at the same time so I can't give you a conclusive answer. We increased wingspan with Hoerner tips and were definitely faster afterwards, if anything - but not by much - maybe a knot or two. I agree they Hartzell guys are almost certainly talking about static thrust.

We've definitely seen that 3 blade props are slower on a Bearhawk, all other things being equal. But again - only a knot or two at worst.

[Nev]

We've definitely seen that 3 blade props are slower on a Bearhawk, all other things being equal.

Hartzell point out HERE : "In general, 2-blade propellers are slightly more efficient. However, efficiency doesn’t propel an airplane, thrust does".

There's a lot of variables to test this - same airframe model, same wing, same engine, same atmospheric conditions, known airspeed position error, (importantly) same manifold pressure and RPM, same aircraft weight, same tire size, etc etc.

When you say "definitely", has this actually been tested, or just anecdotal?

[Battson]

Hartzell point out HERE : "In general, 2-blade propellers are slightly more efficient. However, efficiency doesn’t propel an airplane, thrust does".

There's a lot of variables to test this - same airframe model, same wing, same engine, same atmospheric conditions, known airspeed position error, (importantly) same manifold pressure and RPM, same aircraft weight, same tire size, etc etc.

When you say "definitely", has this actually been tested, or just anecdotal?

The test was as scientific as possible given each Bearhawk as slight variation. We had two practically identical Bearhawks (from an aerodynamic and power plant perspective) flying in formation with the same energy input, i.e. equal fuel burn rate and power settings. The two blade ship was usually between 0.5 kts and 1 kt faster indicated, and that's what was observed - it slowly pulled ahead of the other aircraft. We flew for around an hour on several different tracks. At times the two blade machine was as much as 2 or 3 knots faster - such as descent. Climbs seemed to erode the benefit. While such a small difference could be explained by a whole range of factors, the most obvious and major difference was the different propellors fitted to each plane. It definitely appeared that the two blade machine was quicker.

Anecdotally, I think it's a fairly widely accepted phenomena in our aircraft configuration, from what I have heard others say about it. The added drag from more blades is obvious in the fact that an identical aircraft with a three blade prop has more drag in landing approach flight phase, which a bush pilot can use to his advantage by using the prop as a handbrake if required. This is pretty well discussed by a lot of pilots.

It seems to make sense. The third blade clearly gives more static thrust and thrust at low airspeed - while there is little drag on the plane. This advantage is easy to notice as a pilot or observer on the ground. Once in cruise flight, airspeed and drag increases. It seems natural that the extra drag created by the three blade prop now becomes a factor in the maximum airspeed which can be achieved, because a fixed input of power is now overcoming more drag and that power is being used less efficiently, i.e. more prop losses, so the maximum airspeed is probably affected because the thrust to drag balance now favours the two blade machine slightly, in the case of our airframe design and powerplant.

It's a complex thing I am sure it's a whole science of it's own, so I can only do practical tests and apply theoretical thinking. Of course we can only talk about one airframe / engine / prop combo at a time. Once the powerplant and/or airframe changes, ground clearance and/or tip speed of the blades limits the prop design and selection choices. That sometimes means more blades are required to transfer the power effectively because the prop cannot get any larger, such as in some aerobatic aircraft or some turbine aircraft which need 3, 4, 5 or more blades. At that point, I believe one has to accept lower efficiency in favour of raw thrust - probably what Hartzell is talking about in the link you've posted above.

[Mark Goldberg]

My understanding from talking with Hartzell over the years corresponds to what Jonathan reports. A three blade will give more take off thrust up to around 30 mph on takeoff. Then the two blade gets closer to the thrust of the three blade. And 1-2 kts slower cruise for a three blade is what I have been told to expect from Hartzell engineers.

When Hartzell sent me the first three blade Trailblazer to try - they said it should have 20% more thrust than the aluminum two blade that was on my plane. We confirmed that more or less. We found the three blade TB reduced take off roll a LOT. And Hartzell told me the two blade would be an 18% better thrust than the aluminum two blade. Only 2% difference in thrust two blade TB compared with three blade TB. Mark

[nborer]

This is a fun topic, and I've enjoyed reading it (and seeing it brought to the forefront). I had to chuckle at "efficiency doesn't propel an airplane, thrust does." Propeller efficiency = thrust x velocity / power (though admittedly this doesn't work for low speeds). So for a given power and airspeed, a more efficient propeller will generate more thrust. Usually, if someone is quoting metrics like "20% more thrust," I would presume they mean static (~zero airspeed). If you're talking about constant-speed propellers, there are a few factors at work here.

In flight, a constant-speed propeller is continually adjusting blade pitch to maintain a target RPM (how that occurs depends on a variety of control mechanisms I won't belabor). Unless it's a very poorly-designed propeller, the blade won't be stalled during just about any flight condition. In this case, fewer blades is generally better.... they are more efficient, meaning that for a given input power, they will produce more thrust (see the formula above). It's similar in the comparison of monoplanes to biplanes, in that fewer propellers (rotary wings!) will be better (yes, there are tandem-wing designs that buck this trend, just as we can talk about coaxial propellers but that's another topic for another day). In fact, a single-bladed propeller is theoretically more efficient - it's just a pain to manufacture, balance, maintain, etc.

The reasons you may add more blades for cruise flight generally come about if your tip speed starts to approach the speed of sound, which starts to happen as you increase the power of the engine. Power is directly related to the product of torque and shaft speed, and torque is what swings the blade through the air. The tip speed of the propeller is geometric sum of airspeed and rotational speed, so sqrt(airspeed^2 + {pi*(rev/[unit time])*(prop diameter)}^2). Just like when a wing goes supersonic, unless it's designed specifically for it, you pick up a lot of drag (in a propeller, looks like "need more torque") and lose a lot of lift (in a propeller, "lose thrust"). You see people do some things to delay shock-induced separation, like sweeping tips, etc. which tends to be easier with composite blades than with aluminum blades from a manufacturing perspective. However, at some point, you have too much power to absorb from two blades without turning them into paddlewheel blades, so it starts to make sense from an efficiency standpoint to add blades. Look at the propellers on a C-130H vs. a C-130J, for example.

Most GA airplanes fly pretty slow (under 300 knots, or much less than Mach 0.5), so it's really just the prop shaft speed that sets the tip speed limit. Additionally, most GA piston engines don't use gearboxes, so you're already stuck in a compromise between the engine speed that produces the best power and/or torque and the propeller speed that produces the most (or most efficient) thrust. The two rarely match. I haven't independently verified it, but I've been told that most direct-drive GA airplanes spin their props too fast for best propeller efficiency. For most piston engines, you need a high engine speed to generate the torque needed to swing a prop, and the more prop diameter you have, the more torque you need. In addition, if your prop diameter gets too large, you run into tip speed issues (see paragraph above) or ground clearance issues (why Whee wants to have longer lander gear!). Finally, prop noise goes with the fifth power of tip speed, so even small reductions in tip speed can reduce your noise pretty substantially. (The acoustics folks I work with hate it when I quote that relationship, because there are a million other factors, but it works fairly well in the range that we operate GA props.)

Okay, so why do three-bladed props pull so hard statically vs two-blades ("20% more thrust")? Unless you have a reverse-thrust option for your constant-speed prop, most propellers have a low ("fine") pitch limit that will be active during low speeds. In static conditions, even at full power, the blades are either at or very close to the fine pitch limit, which means they act like fixed-pitch props (and why throttle will control RPM while you're taxiing with the prop lever full forward). That means that at low airspeeds in particular, the inner section of the propeller is likely stalled - producing less lift (less thrust) and a lot of drag (requiring more torque to maintain a given RPM). You can usually hear (and feel) when the prop governor starts moving the blade pitch during the takeoff run. Even then, the blade twist distribution will be designed as a compromise between absorbing maximum power in climb vs. best efficiency in cruise - for many propellers, the top of climb condition is a driver. You want the blade to be efficient in cruise but be able to absorb all of the available power at top of climb. So, at low speeds, the inner portion of the blade will be stalled, and the net/net effect is that more blades will probably pull more (the outer portion produces most of the thrust anyway), and you're just increasing that by the added number of blades (even though you're also overcoming the additional torque of the stalled inner portion). Your static RPM will probably drop just a touch for a given power, which reduces thrust per blade, but the net effect is that you'll produce more thrust.

I have a 4-place Model B, and my current plans are for a four-cylinder engine, so 170-210hp. I suspect I'll end up going with a constant-speed two-blade prop, mostly due to cost, but also because I'm not planning on dominating STOL competitions or flying into (or, really, out of) places that I really need the extra oomph (or noise restrictions). But no doubt, if I were going the STOL route, I'd think hard about a big engine and three-bladed prop. If you were made of nothing but money, go with something light that gives you amazing torque and add blades - a geared piston engine, a turboshaft (which will already be geared), or even electric. There's a reason that an electric Extra 500 holds the time-to-climb record.

[Nev]

A very good read Nick, thanks.

[DBeaulieu]

Geat discussion...I learn something just about everytime I get on the forum!