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Mcculloch 4318A prop size

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Iain

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Jun 2, 2020
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I'm looking for suggestions as to what prop size would be appropriate for a Mculloch 4318a 72 HP motor. I live in Whitehorse Yukon and picked this motor up recently with the intention of building a small air boat type design that can run on SNOW. I have experience with RC airplanes in the remote past and owned a 1962 182 for about 12 years with 650-ish hours total time. I know from past experience that higher diameter, lower pitch props perform better with acceleration and that lower diameter higher pitch props give better top end speed. I think that given the application, I'm looking for a higher diameter, lower pitch prop as snow is all about getting going from a standing start, and I'm not really interested in travelling at high speed in a quasi-controllable prop driven sled. Does anyone have any suggestions as to what prop size works with a 72 HP Mcculloch 4318A drone engine? The only internet references I can find to prop diameter are to a 45 x 24 and a 48 x 20 prop. Presumably the 48 x 20 prop would be better for my application. Any real world suggestions? Thanks in advance.

Iain
 

BJC

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Iain:

Welcome to HBA. I can’t help you with your question, but when you get your air sled running, put an N number on it and post a photograph here, along with a photo of the C182.

Good luck.


BJC
 

Armilite

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Sep 5, 2011
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Location
AMES, IA USA
I'm looking for suggestions as to what prop size would be appropriate for a Mculloch 4318a 72 HP motor. I live in Whitehorse Yukon and picked this motor up recently with the intention of building a small air boat type design that can run on SNOW. I have experience with RC airplanes in the remote past and owned a 1962 182 for about 12 years with 650-ish hours total time. I know from past experience that higher diameter, lower pitch props perform better with acceleration and that lower diameter higher pitch props give better top end speed. I think that given the application, I'm looking for a higher diameter, lower pitch prop as snow is all about getting going from a standing start, and I'm not really interested in travelling at high speed in a quasi-controllable prop driven sled. Does anyone have any suggestions as to what prop size works with a 72 HP Mcculloch 4318A drone engine? The only internet references I can find to prop diameter are to a 45 x 24 and a 48 x 20 prop. Presumably the 48 x 20 prop would be better for my application. Any real world suggestions? Thanks in advance.

Iain
=======================================

The McCulloch Model 4318 (Military designation: O-100) was originally used as a drone engine.
Two-stroke, 4-cylinder opposed, 72 HP @ 4,100 rpm (some versions 84 HP @ 4,100 rpm),
3.1875" (81 mm) bore, 3.125" (79.4 mm) stroke, 99.7 in³ (1,635 cc) displacement, weight approximately 78 lbs (35.4 kg)
Chrome-plated aluminum bore, pistons probably deflector type (as opposed to Schnurle loop-scavenged), single ignition
Cool little engine! 72 HP, 75 pounds. Bensen Gyrocopters were designed around those engines. Subaru"s have taken over since the drone engines have become pretty rare.
They are 2-strokes, so they really wail. Sound like a V8! Great for snowplanes and airboats even if no longer airworthy. Pretty thirsty, though, and not environmentally friendly with gas-oil mix.

You're only limited by what you want to Spend for a 2, 3, 4 Blade Prop, and the Max length you can use. That 1635cc is a lot of CC to work with. It probably used just a Muffler with a Low CR vs a Tuned Pipe with a High CR. 1635cc / 7cc = 233.5 hp @ 6500rpm.

With 11.5cr, Proper Size Carbs, and a Good Tuned Pipe.
1635cc at 6500rpm at 93% VE = 233hp
1635cc at 6000rpm at 93% VE = 215hp
1635cc at 5500rpm at 93% VE = 197hp
1635cc at 5000rpm at 93% VE = 179hp
1635cc at 4500rpm at 93% VE = 161hp
1635cc at 4000rpm at 93% VE = 143hp
1635cc at 3500rpm at 93% VE = 125hp
1635cc at 3000rpm at 93% VE = 107hp

Check your Numbers with this Calc.

AirBoats are Draggy and usually need lots of HP. A Snow Glider doesn't need as much HP account of less Drag.

These White Snow Gliders were built for an Antarctic Expedition, originally with BMW 100hp engine, but they change to a Rotax 912 100hp Engines.
SNOW GLIDER 8.jpgSNOW GLIDER 7.jpg

Nice idea. I always wondered if you couldn't take a Chevy or VW Van and strip out all the Drive Train and any other none essential Weight, and replace the Glass with Lexan and use Aluminium Skies. Even a VW Bug could work, all depends on how many Seats and how much you want to haul.
SNOW GLIDER 4.jpg

These yellow one and two-seat Snow Gliders were made in Russia. The Yellow two-seater with a Rotax 447UL (40hp) would do 80 mph.
SNOW GLIDER 2.jpgSNOW GLIDER 1.jpg
 

n3puppy

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Jun 25, 2019
Messages
148
I'm looking for suggestions as to what prop size would be appropriate for a Mculloch 4318a 72 HP motor.
Iain..
There is a company in Texas that rebuilds the McCulloch drone engines.
They might be able to give you some actual prop data based on their customer experiences.
R&D Aeronautical Engineering Company, Inc. - Home

They can probably also give you a real world assessment of the problems you will encounter tripling the Hp of the 72hp engine to 233hp as suggested in a previous post.
The simple boost from the 72hp to the 90 hp version required upgrading things like bearings and pistons since they were not suitable for the 18hp increase. 233hp likely requires major changes.
Here is a good read on the 4 banger and changes made to create the 90hp version
https://www.rd-eng.com/docs/history_of_the_mcculloch_engine.pdf

Another resource from R&D is the overhaul manual. It will help you maintain your engine down the road especially if you increase the hp, (reducing the TBO)
https://www.rd-eng.com/docs/overhaul_manual.pdf
 
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Armilite

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Location
AMES, IA USA
For such engine speeds and ground use, you need to use a gearbox.
==============================

That example was to show what a 1635cc 2 Stroke is capable of doing. Most 2 Strokes used on Planes use Max 6500rpm for takeoff 2 min then throttle to 75% Power! Just like a Plane you have to figure what is the Ideal Power to Weight Ratio needed. It's obvious this is a Direct Drive Air Cooled Engine as is, and I haven't heard of any Reduction Drives for them, doesn't mean one can't be designed for it, so you're going to be limited to a lower than 6500rpm yet to be determined by what you actually do to the Engine. The PROP Size & Number of Blades used is going to Dictate the RPM & HP made. it doesn't matter if it's an AirPlane, Boat, or Air Boat in his case, you can go to a Full Power Setting for a short time to get it to move, to get a Boat on plane, then throttled back to the Safe Continuous Rpm. The Lazair with Twin Rotax 185UL were turned 5000-5200rpm in Direct Drive mode. The Faster you turn an Engine the Smaller the prop you can use. I have seen Air Boats with 2 to 12 Blades. On Warp Drives site their Air Boat Props seem to range from 72" to 82" and 3 to 12 Blades. For most people, it usually boils down to what they can afford, not always the Best Prop.

The McCulloch Model 4318 (Military designation: O-100) Two-stroke, 4-cylinder opposed, 72 HP @ 4,100 rpm (some versions 84 HP @ 4,100 rpm),

Example: Cheapest Warp Drive, Air Boat Prop is $1,275.00, (3) Blade, 72". Using 77F for Air Temp and .9 for CF. Dialing in a (3) Blade Prop for the Stock 4100rpm.

72" x 10 4100rpm says Danger, Supersonic blade perimeter speed!

At Stock 4100rpm Direct Drive, a 58" (3) Blade is all you can use.

58" x 11 at 4100rpm makes 642.67 lbs and needs 73.441 hp
58" x 10 at 4100rpm makes 642.67 lbs and needs 66.764 hp

Now let's see if we can make that 72" (3) Blade Prop work by lowering the rpm.
72" x 9 at 3400rpm makes 1049.53 lbs and needs 81.375 hp. Danger! Supersonic Blade Perimeter Speed!

72" x 9 at 3300rpm makes 988.70 lbs and needs 74.404 hp.
72" x 8 at 3300rpm makes 988.70 lbs and needs 66.137 hp.
It wouldn't take much work and upgrades with 1635cc to make this 66-74hp@3300rpm.

As I showed you, a 2 Stroke at 1635cc/7cc = 233.5 hp@6500rpm is a lot of CC to work with. With 11.5cr, Proper Size Carbs, and a Good Tuned Pipe vs a Muffler! Rotax Ricks 670 is 669cc and 92hp@6350rpm. 669cc x 2 = 1338cc for 184 hp

1635cc at 6500rpm at 93% VE = 233hp
1635cc at 6000rpm at 93% VE = 215hp
1635cc at 5500rpm at 93% VE = 197hp
1635cc at 5000rpm at 93% VE = 179hp
1635cc at 4500rpm at 93% VE = 161hp
1635cc at 4000rpm at 93% VE = 143hp
1635cc at 3500rpm at 93% VE = 125hp
1635cc at 3000rpm at 93% VE = 107hp

Check your Numbers with this Calc.
Static Thrust Calculator - STRC
WARP DRIVE AIR BOAT PROPS.jpg
 

Armilite

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Joined
Sep 5, 2011
Messages
3,306
Location
AMES, IA USA
Iain..
There is a company in Texas that rebuilds the McCulloch drone engines.
They might be able to give you some actual prop data based on their customer experiences.
R&D Aeronautical Engineering Company, Inc. - Home

They can probably also give you a real world assessment of the problems you will encounter tripling the Hp of the 72hp engine to 233hp as suggested in a previous post.
The simple boost from the 72hp to the 90 hp version required upgrading things like bearings and pistons since they were not suitable for the 18hp increase. 233hp likely requires major changes.
Here is a good read on the 4 banger and changes made to create the 90hp version
https://www.rd-eng.com/docs/history_of_the_mcculloch_engine.pdf

Another resource from R&D is the overhaul manual. It will help you maintain your engine down the road especially if you increase the hp, (reducing the TBO)
https://www.rd-eng.com/docs/overhaul_manual.pdf
=================================

I didn't say for him to Tripple his HP for his application, I showed what, a 2 Stroke at 1635cc/7cc = 233.5 hp@6500rpm is cable of making. That's also WHY I gave approximate HP all the way from 6500rpm down to 3000rpm so he would have an idea of what it can make if upgraded. The Prop Lenght and Number of Blades he uses is going to dictate How Fast he can really turn it. All Rotax 2 Strokes are turned 6500rpm for 2 minutes then turned usually at 75% Power continuous.

More than likely they just changed the CR with different Pistons and Exhaust or Carb Size used between the 72hp to 90hp Engines.

Just like Rotax Ricks 670 which is Water Cooled at different Power Settings, it can Crusie all day at 5700 RPM = 65 HP, so this Direct Drive Air Cooled motor can probably be turned Avg 20% less than a Water-Cooled all day long. 5700rpm - 20% = 4560rpm! But the Prop used in Direct-Drive is going to set the Max Rpm it can be used at, which will probably be even lower.

As usual, you take things out of context to make your remarks!
 

n3puppy

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Jun 25, 2019
Messages
148
Any real world suggestions? Thanks in advance.
Iain
Iain,
One thing to remember about these drone engines is that they were designed to fly for a few minutes and then get shot out of the sky

Since you are asking about real world experience -Gyrocopter flyers are your best bet.
They have learned the weak points and developed various preventive measures to handle some of the more common failures associated with the McCulloch. These failure modes included piston seizures, carb float failures, crank breakage, wrist pin failure, fuel pump failure and weak magnetos.

I'm not sure if it is OK for me to mention other forums here, but if you google Benson gyrocopter you will find a couple places on the web that have guys that regularly fly Mcculloch motors. Besides theory, they can give you the "real world suggestions" you are seeking for your engine.
What reliability modifications they made, what props they use, etc.

My twin cylinder O-45-35 drone engines had the same reliability problems when they bumped the HP from 22 to 35hp. They also have tiny bearings and weak cranks.
When the 35hp twin motor could not be pushed any further without unacceptable failure rates, McCulloch created the 4 cylinder family (60, 72, and 90hp)

If you plan ANY upgrades your 72HP engines horsepower remember,....
Jumping from 72 to 90 hp required upgrades to Pistons, bearings, rods, etc. You may need those parts too.
90 hp is the most those upgraded engines could sustain. When the military needed more hp - they could not "Hot Rod" the 4 cylinders to any higher numbers and came out with the 110hp six cylinder O-150-1 engine. (120hp turbocharged)

Just for fun, here is a test run of a 72hp version of the four banger -
Comment section says they got 535 lbs of thrust with the prop they were using.
No muffler - just like the original drone application. Gyro guys could help you find an exhaust system if you need to cut the noise level in your area. (Some still fly without)

 
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n3puppy

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Jun 25, 2019
Messages
148
I live in Whitehorse Yukon and picked this motor up recently with the intention of building a small air boat type design that can run on SNOW.
Ian,
Not sure what you are building ice boat wise but thought this might be of interest to you.

In the Mid 80's a bunch of plane guys in the Oshkosh area got together and made snow scooters out of old non-airworthy craft. Used them on Lake Winnebago, couple even took them out on the snowmobile trails.

Pardon my artwork, (no photos available) The concept and build process were quite simple.
Take off the wings, put a pusher prop on the engine. Put Main gear skis on backwards, and adapt a ski to where the rear wheel would go on a taildragger.
C-150's were the most popular. (Training accidents) Turn the seats around and drive looking out the back window.

Eventually the DNR shut them down because there was no way to tax them - not boats, not snowmobiles, not ATV's

Having spent some time in Alaska, I did notice a fair number of "Bent" tubs due to bush operations. Maybe something near you.....20754679-9E46-490B-B413-3E5C3159D575.jpeg
 

Armilite

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Sep 5, 2011
Messages
3,306
Location
AMES, IA USA
Now that's a nice idea N3, only I would cut off the Tail about 2-3 ft behind the rear window to make it shorter, have a tighter turning radius. Luggage Rack on Top and Storage in the Tail, now Nose area. Mount some Lights on Luggage Rack. A 40hp Rotax 447UL made the two-seat Snow Glider do 80 mph, so it won't take much hp.
 

n3puppy

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Joined
Jun 25, 2019
Messages
148
These guys didn't change engines - Used the original firewall forward (rearward?) set-up with a pusher prop. They did put one of those airboat looking prop guards on for safety.

Went pretty good on the ice - full length fuselage with long turn radius may have helped stability at the speeds they were capable of with full power motors.
 

Iain

New Member
Joined
Jun 2, 2020
Messages
2
Thanks for all the feedback and suggestions. I like the idea of a bigger prop as it would encounter "cleaner" air further away from the vehicle body, but as illustrated above, there is a limit to how fast a prop tip can travel through the air before running into problems with cavitation. From the little bit of reading I've done about the motor, it sounds like it runs with a fair amount of vibration just due to its design. While some sort of gearbox for the crankshaft would allow me to spin a bigger prop at reduced prop RPM I don't know how well a gearbox would tolerate higher vibration. That plus the fact that I'm an M.D., not a millwright leads me to the conclusion that the smart call here is likely to look for a direct drive prop that I can spin at mid-range RPMs i.e 3000-3500 for instance so as to not run the motor at max power settings all the time and still get reasonable performance. I think I'll see if I can get an idea as to what kind of horsepower the motor can make at various RPMs from the guys out there who flew them or are perhaps still flying them and then look for a decent diameter prop that I can turn at reasonable revs to make a decent amount of power without pushing the motor too far. I'll probably look for a prop with variable pitch settings if possible as although that will likely be more expensive up front, it will give me a lot more options for fine tuning the performance once installed as I doubt there is much accurate data out there on what kind of power the motor produces at various revs. I might be wrong about that, but we'll see. I suppose the other option would be to look for a motor with known performance and more availability of parts, etc such as a gearbox to reduce output RPM, but for the time being, I'll continue to pursue collecting ideas about the McCulloch. That being said, in relation to swapping out the bearings, does anyone have any suggestions as to what kind of bearings would make sense to put in? I don't mind tearing down a motor, but if I'm going to do it, I may as well spend the money and put in parts that will last. Thanks for all of your input and ideas; there's a lot of interesting info out there.

Iain
 
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