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View Full Version : How much mass do I need to keep a Briggs 18HP flat twin engine from jumping around?



winchman
07-04-2008, 06:15 PM
I don't want to have to haul my lawn tractor around when I go to fly my sailplane, so I need to make my new winch more portable. I'd like to keep it as light as possible, but I don't want it walking around when it's running.

Is there some sort of guideline for engine HP vs the weight of what it's attached to?

The 100 pound engine runs smoothly installed on the tractor, which probably weighs about 350 including the engine. The winch adds about 15. I'll need a starting battery for another 15, and the frame will add about twenty.

That's at least of 150 pounds for a portable unit. Will that be enough to keep it steady when it's running? Or, is there no way to tell other than trying it, and adding more as needed?

Roger

Swarf&Sparks
07-04-2008, 06:35 PM
errm
how about you bolt it to a drum
(44 gal type)
add/remove water til you're happy
you now have a mass established
you can either fill it with concrete, or leave it empty to port
just fill with same mass of water when you get to the airfield

doctor demo
07-04-2008, 09:24 PM
Hi Roger, I'm no expert when it comes to calculating anything that has to do with mass...but I think footprint would have a greater affect on stability than mass. Maybe some sort of retractable outrigers incorporated in your frame design, kinda like on a truck crane.
Steve

Evan
07-04-2008, 11:48 PM
Is this an opposed flat twin? If so it should be very smooth and well balanced.

oldtiffie
07-05-2008, 03:43 AM
I'd agree with Evan as regards the flat-twin motor being evenly balanced and probably as smooth as any other.

Is the question related to having the motor-winch assembly placed on the ground with sufficient resistance to being displaced by "pull" when the sail-plane comes "on-load" on the winch cable?

Any chance of posting some representative pics? If too many pics (say more than four) then use links.

It sure would reduce the number of guesses - from here at least.

winchman
07-05-2008, 06:55 AM
Yeah, I lucked out on getting a smooth-running flat twin. It's just that I've never seen any small engine run when it wasn't attached to something a lot heavier than the engine itself.

Here's the engine and winch still on the tractor:

http://img.photobucket.com/albums/1003/winchman/Briggs%20Winch/Briggswinch006.jpg

I've got a to add the drum brake assembly, but that won't add much to the weight or size.

The engine/winch package will be about 28" long, 22" wide, and 24" tall. If I take it off the tractor, I'll need to have a lawn tractor battery, fuel tank, and control panel for the keyswitch, throttle, and choke.

I like Steve's suggestion of using outriggers. I could put the winch on a skid in the truck, slide it to the edge of the tailgate, and put the outriggers down to the ground to raise the skid off the tailgate. The forward end of the skid would still be in the bed of the truck, so there wouldn't be any load on the tailgate when the winch was running. It would be easy enough to secure the forward end to the truckbed tiedowns, and the weight of the engine would be mostly on the outriggers. I'll need to reroute the exhaust, but that's easy enough.

Roger

Evan
07-05-2008, 09:59 AM
You don't need to take vibration into account then. There won't be much. The opposed flat twin is one of the few inherently balanced engine designs. That's a cool engine. I didn't know Briggs made such a beast.

A.K. Boomer
07-05-2008, 11:53 AM
Horizontally opposed engines are very good for balance BUT as the # of cylinders go down the balance becomes offset and a twin is of course the worst,
The reason being is the offset that has to be incorporated into the crank to allow for both journals to reach TDC at the same time as this is a part of their ingenious balance both mechanically and in power impulses, With a twin however it starts to rear its ugly head in the crank actually pivoting the block back and forth at TDC and BDC -- anotherwords, standing on top of the engine it will actually make the entire block pivot CCW and then CW and on and on...

The best possible way to take care of this effect with the least amount of weight is to Jut out from one of the horizontal ends of the engine with some structural length and have the weight mounted there, or keep the entire engine mounted to a larger mass.

The only way around this with an opposed twin is in engine design --- the crank can be kept equal and an offset to one side avoided - however - it takes one main crank journal to run one rod and piston and two smaller journals with a "Y - Yoke rod" to run the opposing, the result is the best balance in a twin that you can achieve -- there are some examples of these engines --- but there also not all out race stuff as they have inherent design flaws with RPM's due to the nature of having such a funky connecting rod set-up with the one side, the cylinders however are perfectly across from one another, and the crank goes through ZERO pivoting at TDC and BDC. But you trade one inherent design flaw for another,


EDIT; If you look at Winchmans briggs in the photo you can immediately tell that its the offset design just in the way that one head leads the other to the PTO side, Even the highly successful BMW boxer twin is of this design, they however have a flywheel on them the size of a small hit and miss engine, Once again, be it on the crank or on the block, Jut out from a horizontal plane with something heavy ;>}

Bill Pace
07-05-2008, 01:21 PM
I agree with the rest of the majority, I dont think it'll be a problem--- think generator sets... they got every kinda configuration imaginable mounted on the flimsiest of frames (I know, the armature is acting as a flywhwheel, but still .. some of those frame works ....!!) Pumps are something else that can have the dinkiest of framework, and theres not a lot of mass there...

With a little care constructing a frame with some outrigger support ---- oughta work. I would probably do a temporary mount, say bolted on a piece of plywood clamped down, and run it thru its range of RPM's and get a feel of what it does.