Tri-Mower Design

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Vigilant1

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BBerson said:
Three 212cc engines is 636cc. and about 100 pounds. (guessing 30 each)
A single Briggs is 810cc and about 60 pounds.
But the Briggs can not direct drive the equivalent 5 foot diameter prop.

. . . . Low power is ok if high propulsive efficiency is included.
Unless we are looking at very low speed, there's not a huge difference in efficiency of the two choices.

At 60 MPH:
B&S 810cc 27 HP with 48" prop, 3600 RPM: 61% efficiency, expected thrust = 103 lbs
Tillotson 212cc, 9 HP with 36" prop, 4500 RPM: 65% efficiency, expected thrust - 37 lbs. Three engines = 111 lbs.

At lower airspeeds (takeoff run, ultralight climb, etc), the combined 21 sq ft of the three 9HP engines would have an increasing efficiency advantage over the single 27 HP engine (12.6 sq ft). The efficiency advantage of the three fans diminishes as we go faster.

If we are turning our engine at 3600 RPM, high tip speeds begin to degrade efficiency (and neighborhood goodwill) at some point. At 60" diameter, we're at 950 FPM and .85 mach.
 

BBerson

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Good to know. What pitch and diameter and static thrust/rpm.
I don't remember the pitch. The diameter was 60" and 2900 rpm. I cut it to 58" and now it gets 3000 static and 115 pounds thrust. I should have reduced the pitch instead of cutting it down. It still needs reduced pitch for static at 3200rpm.

The 212cc engine appears to be a Honda 200 clone.The Honda is about 5.5hp at 3600 rpm. I trust Honda numbers over racer numbers.
 

REVAN

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The 212cc engine appears to be a Honda 200 clone.The Honda is about 5.5hp at 3600 rpm. I trust Honda numbers over racer numbers.
Yes, it is a Honda derived engine. It has a redesigned head with better flow and a different cam, but most importantly, it is governed at 4250 rpm instead of 3600 rpm. Tillotson calls it a 10 HP engine. The actual dyno chart I have been able to get my hands on shows the power peaking at 9.1 HP. I don't know what density altitude conditions that test was conducted under, but until I have better data, I'm calling it a 9 HP engine. A better flowing air filter and muffler still has the potential to improve power. The rpm governor speed can be changed, but unless the flow through the engine is improved, there is no reason to raise it. In stock form, the power output is fairly flat at 9 HP from 4000 to 4500 rpm, after which it begins to fall off.
 

lr27

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I'm pretty sure a Briggs at 3,000 rpm is making considerably less power than at 3,600. OTOH, what really matters is how much power you get, at what efficiency, when climbing. Presumably the rpm's go up quite a bit in flight.
 

lr27

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I'm pretty sure a Briggs at 3,000 rpm is making considerably less power than at 3,600. OTOH, what really matters is how much power you get, at what efficiency, when climbing.
 

blane.c

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4250rpm 10hp sea level (most hp numbers are corrected to sea level) hp drops off 3% per thousand feet approx. 10hp sea level but at 3,000ft is 3 x .03 = .09 x 10 = 0.9 so 9.1hp @ 3,000ft density alt. is 10hp sea level … especially if you are selling engines.

The propeller is going to limit the rpm. So 10hp sea level 4250rpm but 9.1hp 3,000ft 4165rpm. Or if 9.1hp 3,000ft at 4250rpm then 10hp sea level 4337rpm. The propeller isn't going to turn as fast with less hp.
 

blane.c

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If you climb around 85% power then about 95% rpm so 3600rpm x .95 = 3420rpm. 83% power would be around 93 1/2% rpm so 3600rpm x .935 = 3366rpm. Cruise at 75% power then .9085 x 3600 = 3270rpm.
 

blane.c

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If you had an engine that turned 4250rpm at sea level and you were turning say 3800rpm flying then 3800/4250 = 89.4%rpm so around 74% power so 7.4hp approx.
 

Vigilant1

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It would be interesting to know the CHT of that 212cc engine after it has made 9 HP for about 5 minutes.

At 0.7 HP per cubic inch. . . how long would we expect an O-200 to last if we asked it to make 140 HP?
 

blane.c

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How much heat is a 13ci engine going to have to dissipate? It may be able to make a little more hp by scale to a larger one regards it just isn't that much heat in relation. But I would say it ain't going to do it all day. I wouldn't make it do it for more than a minute or two. Take-off lower the nose a tad gain some airspeed and ease back the power a bit and climb.
 

lr27

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blane.c:
Do your calculations allow for the reduced torque required to spin the prop when the air is thinner? Intuitively, I'd expect the two effects to cancel out. At least approximately. I imagine Reynolds numbers and viscosity would make it a little more complicated.
 

blane.c

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In practice I liked to cruise my little Cessna 140 at around 7500ft (it had a 0-235 lyc. conversion) at that altitude it was full throttle (most efficient) and 75% power approx. I don't remember the exact numbers anymore but at the time I knew the rpm was correct for 75% power. It is a common practice, 7500ft eastbound and 8500ft westbound, the ground looks like the VFR chart and the engine is happy and plenty of gliding room if you have a problem.
 

REVAN

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I bought one of these Tillotson 212cc engines last December. I've got it mounted to an engine test stand for experimentation. I recently bought an Xoar 36x12 propeller to test with it. It should take 9 HP at 4050 RPM and 10 HP at 4200 RPM, but I need to figure out how I'm going to mount the propeller before I can proceed with any testing to verify any of this.

My first attempt was to draw up in CAD a custom propeller interface to mount to the shaft. However, having the parts made as I drew them is going to cost a bit more than the engine, so I'm looking at different designs now and other alternatives. If anyone has good advice on how to inexpensively add a propeller interface to a 3/4" straight keyed engine shaft, I'd sure like to know about it.

I figure there is about a 50/50 chance this engine investigation is a wild goose chase, but maybe it will bare fruit in the long run. Couped with the right design, it has the potential to be part of a very affordable ultralight.
 

REVAN

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For a single seat design, I'm told that I don't need a multi engine rating, or even a private pilot certificate. I can get a student pilot certificate and get to the point where I'm signed off to solo the plane. I can then solo it as a student pilot, indefinitely.

If it is an ultralight, then I don't even need to do the student pilot certificate and the craft doesn't need registration or have any taxes. Obviously, this is the most desired if it is possible to meet the requirements. If not, the alternative is not that bad. It seems the single seat really simplifies things.
 

blane.c

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I bought one of these Tillotson 212cc engines last December. I've got it mounted to an engine test stand for experimentation. I recently bought an Xoar 36x12 propeller to test with it. It should take 9 HP at 4050 RPM and 10 HP at 4200 RPM, but I need to figure out how I'm going to mount the propeller before I can proceed with any testing to verify any of this.

My first attempt was to draw up in CAD a custom propeller interface to mount to the shaft. However, having the parts made as I drew them is going to cost a bit more than the engine, so I'm looking at different designs now and other alternatives. If anyone has good advice on how to inexpensively add a propeller interface to a 3/4" straight keyed engine shaft, I'd sure like to know about it.

I figure there is about a 50/50 chance this engine investigation is a wild goose chase, but maybe it will bare fruit in the long run. Couped with the right design, it has the potential to be a part of a very affordable ultralight.
You will likely need to make the hole in the propeller flange & short shaft smaller in diameter than the engine crankshaft. Then you can heat the propeller flange & short shaft in the oven it and the hole will get larger (the hole will get bigger in diameter as it heats up). Then it gets fun, it helps if you pack ice around the engine shaft or an engine that small put the whole thing in a freezer to get the shaft diameter as small as possible and give yourself a second or two before things normalize to the same temperature and you can't move them. Have everything prepared and ready and "tada" it is like magic. I would predrill a hole in the end of the shaft and tap it (lengthwise) so after you mate the two together you can put a bolt on the end, lol, so that it won't come apart somehow. If you figure out the right allowance to make the hole smaller when it cools down on the shaft they will be almost like one. You will be able to get it back off if you have to with a press or you can tap a larger size bolt hole in the flange when you make it, larger than the hole in the end of the crankshaft so you can just run a bolt into the flange end and as it pushes against the end of the crankshaft they will come apart. Use good material like normalized 4330 or something along those lines and don't heat it to hot and destroy it's temper. But a normal kitchen oven probably won't get it that hot anyway.
 

blane.c

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For a single seat design, I'm told that I don't need a multi engine rating, or even a private pilot certificate. I can get a student pilot certificate and get to the point where I'm signed off to solo the plane. I can then solo it as a student pilot, indefinitely.

If it is an ultralight, then I don't even need to do the student pilot certificate and the craft doesn't need registration or have any taxes. Obviously, this is the most desired if it is possible to meet the requirements. If not, the alternative is not that bad. It seems the single seat really simplifies things.
Get some instruction. Some people live when they fly without instruction, some don't.
 
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