Briggs vanguard conversions

Discussion in 'Firewall Forward / Props / Fuel system' started by Hephaestus, May 12, 2019.

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  1. Oct 11, 2019 #1201

    BBerson

    BBerson

    BBerson

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    See here:B&S 49-series (810cm3/49ci) - TiPi's conversion for aircraft use

    Might get the idle lower with prop mass, but risk it will quit if too low.
     
  2. Oct 11, 2019 #1202

    TiPi

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    I installed a 4mm rivet from the inside of the fan shroud on the indicated locations with the hose connected on the outside, so the opening is like a static pressure port in the shroud surface.
     
  3. Oct 11, 2019 #1203

    TiPi

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    with an engine like the Briggs, you are in Jabiru-territory with regards to engine rpm. The Jabs are rated at 3,300rpm (redline as well) and most will cruise between 2,700 and 2,900, some take it up to 3,100 for fast cruise. The Briggs needs to turn about 3,300 on take-off to give you the best available power for take-off and climb without getting too close to redline in level flight and WOT. Valve float doesn't happen till about 4,500rpm, so plenty of margin.
    What governs low idle speed is rotating mass (inertia) and ignition timing (to some degree). The original setup has the low idle through the governor and is set a bit above 1,800 (my 40R677 idles at 1,700rpm). The governor gets removed anyway so you can set your idle stop on the throttle shaft wherever you want. BUT the engine will display some strong vibration (shaking) when going too low for the setup (prop inertia, flywheel inertia, ignition timing, mixture quality etc). This is where a heavier prop might be beneficial as it will have much higher inertia compared to a flywheel (if the weight is away from the hub).
     
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  4. Oct 12, 2019 #1204

    BBerson

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    Then that was static pressure. The fins are wide spaced, so not much restriction to make the static pressure. A pitot tube in the flow would measure airspeed or dynamic pressure. Could be much more, I think.
     
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  5. Oct 12, 2019 #1205

    TiPi

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    Question is where do you measure it? The cross section of the fan shroud is not constant and the air outlets are not symetric. The dynamic pressure would have little use without mapping it in quite a few different locations. When I test run my 49 with the test club and fully instrumented, I'll take some more flow readings as well and will also have the CHT, oil temps etc under load.
     
  6. Oct 12, 2019 #1206

    BBerson

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    The fan could be tested alone with no engine restriction. That would give the fan outlet airspeed and from that we would know the required airspeed needed to use ram air instead of the fan, if desired.
     
  7. Oct 12, 2019 #1207

    karmarepair

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    Just saw this today https://betteraircraftfabric.com/photo-galleries/more02/more0209.jpg Looks like a commercial Horizontal Shaft v-twin virtually right out of the box, complete with the stock muffler and possibly fan cooled. Seems to have a reduction drive, as the thrust line looks well below the flywheel center of rotation. I've asked the US Oratex distributor if he knows anything more about this airframe.
     
  8. Oct 12, 2019 #1208

    TiPi

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    a bit hard to tell but I think it is a big-block Briggs engine (intake manifold), fan has been removed (flywheel still in place) and has a drop-down re-drive on the PTO end.
     
  9. Oct 13, 2019 #1209

    Vigilant1

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    Thanks, TiPi.
    Okay, here's something for folks to throw rocks at:
    Does the higher ratio of surface area-to-volume of smaller air-cooled cylinders allow them to burn proportionally more fuel/produce more HP while staying within CHT limits?

    TiPi's chart above shows the relatively high power density (HP/swept volume) these industrial engines produce (even in stock form) without overheating. They do have the advantage of fan-forced air, but the amount of air moved, and the static pressure available to move it, does not appear to be much different than the conditions aero engine ducting typically provides at 60 kts. Moreover, some of the industrial engines are powering aircraft now, without the stock fans, and not overheating despite power densities that are higher than aircraft engines. Their fins are >not< numerous, big, fine, or designed with the attention to airflow as we see in a Lycoming or Continental jug, but they seem to work and CHTs are reasonable.

    Is this because the cylinders are small?

    As a cylinder increases in size, the volume increases as a function of the square of the radius, while the surface area only increases linearly with the radius. So, bigger cylinders have proportionally less surface area, per unit volume, than small cylinders. If HP (and therefore, waste combustion heat gained by the cylinder) is a function of displacement, but cooling is a function of the surface area of the cylinder, cylinder head, and the underside of the piston, it would make sense that smaller cylinders can burn more fuel (and produce more power) per unit volume while staying within CHT limits.

    Some examples:

    .....................................Swept Volume....Surface area...HP per....Surface area...power density
    Engine.........bore....stroke.... per cyl ............per cyl........cylinder...per volume........(HP/cc)
    Lyc O-360....130mm...111mm...1473cc............453cm2.........45..........0.31...............0.031Lyc O-235C1......111mm...98.4mm...952cc............343cm2.........29..........0.36...............0.030
    Cont O-200..103mm....98mm.....823cc............319cm2.........25..........0.39...............0.030
    VW 2180cc....92mm....82mm.....544cc............237cm2.........19..........0.44...............0.034
    B&S 810cc.....85mm...73mm.....412cc.............195cm2.........16*.........0.47...............0.039

    * HP claimed for SE-33 engine
    The trend is: smaller cylinders appear to allow higher power densities.
    But:
    1) The big change in power densities also coincides with the "certified" vs "noncertified" change. So, it could result from certified engine makers rating their products more conservatively.
    2) The same increased surface area-to-volume ratio that allows small cylinders to get better cooling per unit of fuel burned, would presumably also work the other way, allowing the cylinder to take on more heat from the hot combustion gasses. I don't think we can have one without the other.

    I'm sure nothing here is new, but I'm not a heat engine expert (obviously!:) )
     
    Last edited: Oct 13, 2019
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  10. Oct 13, 2019 #1210

    Vigilant1

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    Trivia note: I was surprised to do the math and find that our beloved "810cc" engine has a swept volume of 825cc.
    From B&S: Bore: 3.33" (84.58mm), Stroke: 2.89" (73.41mm), swept volume would be : 412.46 cm2 per cylinder.

    Edited to add: Disregard all above. The correct bore is 3.30." TiPi caught my error in his Post 1214.
     
    Last edited: Oct 13, 2019
  11. Oct 13, 2019 #1211

    BBerson

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    Regarding post 1209:
    All true, but note the VW and B&S have higher rpm, so higher power density.
    On the other hand, the larger the cylinder displacement the higher the power to weight ratio. This favors fewer cylinders for low weight, but of course fewer cylinders create torsional vibration problems.
    All engines can have more power with better breathing or higher compression ratio or supercharging or exotic fuel.

    source: A History of Aircraft Piston Engines, Herschel Smith
     
  12. Oct 13, 2019 #1212

    Vigilant1

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    I suppose I should have stated my assumption that the power density for continuous HP is limited by heat rejection/CHT. The racing Lycosauruses can and do produce higher specific power for racing, etc by boosting CRs and RPMs, etc, but they would soon exceed CHT limits if they don't use extraordinary means to keep cool.

    In theory, the larger cylinders also favor fuel economy. Due to the higher volume to surface area ratio, a larger cylinder loses less heat to the engine surfaces and can convert some of that heat into additional mechanical work.
     
    Last edited: Oct 13, 2019
  13. Oct 13, 2019 #1213

    BBerson

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    The big bore aviation VW have doubled the displacement while still using the same old head fins. So naturally the result is some VW head overheating issues. But we are not doubling the B&S cylinder, so it's a non-issue in my view. (exception pushers or high rpm)
     
  14. Oct 13, 2019 #1214

    TiPi

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    you need to get some new glasses:)

    upload_2019-10-13_13-46-1.png
    upload_2019-10-13_13-51-34.png
     

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  15. Oct 13, 2019 #1215

    Vigilant1

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    :oops: Dang--you're right. My poor penmanship strikes again!
     
  16. Oct 13, 2019 #1216

    TiPi

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    The table that I published works as all engines are rated at 3,600rpm, you can't mix them with engines that are rated at different rpm. It is also not quite right to normalise that to a single rpm as the specific power potential increases with rpm (dynamics of intake & exhaust flows, higher dynamic compression).
    The SAE standard for small engines will have a lot more leeway than the standard for aircraft engines.
     
  17. Oct 13, 2019 #1217

    Vigilant1

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    Does anyone believe that our Lycoming or Continental wouldn't overheat if we ran it (RPM, CR) so that it produced .65 HP per cubic inch? That's what the SE-33 is reportedly doing. It's not a question we have to answer, but I wonder what is apparently allowing the 810cc head/cylinders to shed heat more effectively than the aircraft powerplants? It doesn't look like the answer is in the fin area/volume.
     
  18. Oct 13, 2019 #1218

    BBerson

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    The Lycoming numbers are continuous. I don't know if Briggs and Spacek hp numbers are continuous.
     
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  19. Oct 13, 2019 #1219

    BBerson

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  20. Oct 13, 2019 #1220

    karmarepair

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    I e-mailed Lars, the Oratex US Distributor who had this to say
    I did some further web searches and came up with nothing. "Saurierflug" seems to mean "Pterosaur" in German. ULF is the name of a quite different series of ultralight sailplanes, as well as a cartoon character.
     

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