Briggs vanguard conversions

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

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  1. Oct 7, 2019 #1121

    blane.c

    blane.c

    blane.c

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    If a pressure cowling, can't you direct the air to convolute in any direction you desire before exit?
     
  2. Oct 7, 2019 #1122

    blane.c

    blane.c

    blane.c

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    In some applications these engines may keep or have retrofitted sufficient electric power to run an electric fan for ground operations? Which may be heavier or may be lighter depending on the design? It may be easier to add an electric fan for some?
     
  3. Oct 7, 2019 #1123

    BBerson

    BBerson

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    The stock plastic fan is about 6 ounces.
     
  4. Oct 7, 2019 #1124

    blane.c

    blane.c

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    The total weight after remounting/retrofitting for an aircraft engine?

    I am dubious it wouldn't depend on the application.
     
  5. Oct 7, 2019 #1125

    Vigilant1

    Vigilant1

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    Yes, but not for free. Everywhere along its path the air has to flow from higher pressure to lower pressure. So, the quantity of air we can move through the ducting depends on two things:
    1) The pressure drop across the whole system. In the natural state, we have about 14 PSI at the front and at the back, so no flow. We can increase this in two ways:
    a) Increase pressure at the front (ram air from airspeed, from prop blast, from a fan, etc). A pusher has a handicap here, due to no prop blast, and it's the main reason pusher engines are harder to cool.
    b) Reduce pressure at the exit (a cowl flap or "lip"on the exit, an ejector using the exhaust, a fan at the back, etc).
    2) Resistance to flow within the ducting itself. Every time we turn the air, this resistance increases, and that resistance reduces the amount of air that will flow by the heads and therefore reduces the amount of cooling we get. So, more twists and turns in a ducting system (all else being equal) means reduced cooling airflow.

    I'd sure resist the addition of an electric fan. For one, we don't >know< there's a problem yet. Electric fan = more weight and complexity, and if we need it for climb, we've added another flight-critical part. And, due to conversion losses, it will actually take more engine HP to run it than a mechanical fan. In fact, with the little alternator only making 16 amps (and we sure don't want to run it at that all the time), we've only got limited juice to power a fan (less than 1/4 HP--that's not a big fan). The only "plusses" are flexibility in where it/they goes and the potential for running it/them only when we need it, which might allow a tiny bit of additional power for cruise.
    Yep. I think a plastic high-solidity axial fan (like one used on electric car radiator fan units) might weigh about the same and could be used to pull air through a pusher plenum (mount the fan directly to the prop shaft or to the flywheel, if it is there). But it will require a tirght shroud and duct to translate that negative pressure at the back into meaningful airflow across the heads and oil cooler.

    Also, the fins on the oil cooler have (presumably) been optimized to work best with the higher static pressures in the plenum available from that stock centrifugal blower. I'd think the available static pressure differentials available from a practical "pull through" system are going to be smaller, and result in less flow through the oil cooler if the fins are especially closely spaced. Folks run into this same problem when attempting to use automobile heater and A/C cores in cooling systems for liquid-cooled aircraft engines. The fins on these cores are tight (designed to work with the relatively high static pressures developed by the car's centrifugal blower), and at the small air pressure differentials available in the aircraft's cooling plenum, very little air flows through them if it can instead go somewhere else.
     
    Last edited: Oct 7, 2019
  6. Oct 7, 2019 #1126

    BBerson

    BBerson

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    But the car blower "blows", it doesn't pull. I have no real data, but I think pushing the air works better.
    Molt Taylor did use an axial pulling fan ahead of the pusher prop on his "Coot".
     
  7. Oct 7, 2019 #1127

    Vigilant1

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    At least in theory, pushing >can< work better because the amount of pressure differential you can build up is unlimited. When "pulling", even if you draw a perfect vacuum, you are only going to have 14 PSI of pressure differential, because that's all you've got on the input side.
    Thanks, so that's another example (like the Cessna 336/337) where a designer of a pusher elected/needed to use more than the prop to get the pressure low enough on the exit side of the ductwork to keep the engine cool. I think the Vari-Ezes and their cousins primarily depend on the prop, but many builders also make an ejector for their exhaust to draw through a little more air.

    A nice exit fan shroud and a plenum for the 810cc engine (drawing air over both heads and through the oil cooler, also making sure the magnetron coils get some) would probably be a good project for a composite layup, using an epoxy that could stand the heat (esp the heat soaking after shutdown with no airflow). It might be faster to bend one out of aluminum bits, but it could be made closer to airtight and also smoother/less draggy with composites. As pictsidhe points out, at least an axial exhaust fan will produce a >little< thrust in the bargain.
     
  8. Oct 7, 2019 #1128

    BBerson

    BBerson

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    A nice shroud is required if pulling the air. That shroud complicates inspection and spark plug access. An axial fan on the engine front could blow on an open air engine. Seems to work on my ground testing.
     
  9. Oct 7, 2019 #1129

    Vigilant1

    Vigilant1

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    How are things set up? (Pusher prop on the flywheel end, axial fan at the front mounted on the PTO shaft?). If the exhaust ports are at the back, I'd think a shroud and some sort of ducting to turn the airflow and keep it against the fins would be needed. How are you handling the oil cooler, just out in the breeze?
     
  10. Oct 7, 2019 #1130

    BBerson

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    I don't have the pusher engine cooling or prop mount planned yet.
    Building airframe parts first.

    But I think open air might have merit because heat can freely radiate outward. A shroud contains the radiation and only gets heat out with convection. My wood stove would get hotter with a shroud, even with a fan, I think.
     
  11. Oct 7, 2019 #1131

    Vigilant1

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    Heat exchange due to forced convection will be a lot more effective in this case than radiation. If we just consider the fins, they contribute virtually zero to radiative heat exchange (since what little they radiate goes into another fin). And the oil cooler won't shed significant heat via radiation.
    I think you'll find forced air is what you'll need: plenty of it, and kept close to the hot bits.
     
    Last edited: Oct 7, 2019
  12. Oct 8, 2019 #1132

    TiPi

    TiPi

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    The rule-of-thumb that I found for the industrial V-twins: cfm of free-air flow is the same as the displacement in cm3 eg for the 49, around 800 cfm of free-air flow to keep the temps in check. This is from the formula when enclosing an engine, for air entry requirements. Outflow is bigger.
    This equates to an area of 148cm2 at 50kts, 113cm2 at 65kts or 87cm2 at 85kts (equivalent to the free-air column of 800cf or 380litres).

    To put the heat to be removed into perspective:
    Heat to be removed by cooling system (cylinders & oil): approx 30kW
    The same heat generated by a "normal" hair dryer of 2,000W: running 15 hair dryers at full speed.

    Every installation will have its own challenges to meet these basic cooling flow requirements. Some might need help with additional fan power.
    A Trike with pusher set-up would be best with the OEM cooling setup, reduce weight where you can.
    A pusher aircraft might be ok with free air cooling if speeds are reasonably high and air intakes get non-turbulent air.

    One option that hasn't been mentioned yet is a small radial fan (like the OEM fans) between the cowling and the prop to assist in expelling the hot air (reverse to the cooling fan). The dia is pretty much the same as a spinner, so it will look like a spinner extension.
     
  13. Oct 8, 2019 #1133

    pictsidhe

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    As a first approximation, maximum cooling load is about the same as maximum power output.
    Intake area has a small effect on flow rate. Outlet area has a very large effect on flow rate. A guess at the inlet area and a tweakable or adjustable outlet is the way to go if using free air.
    Intakes are most efficient at stagnation points.
     
  14. Oct 8, 2019 #1134

    BBerson

    BBerson

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    Airspeed (dynamic pressure) has the most effect on air flow. Many airplane engines are designed for the dynamic pressure around 100 mph and tend to overheat gradually in a full power climb at less airspeed.
    It would help to know what the standard fan pressure is on these engines.
     
  15. Oct 8, 2019 #1135

    TiPi

    TiPi

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    Sorry, forgot to mention that the above is the air volume that needs to be scooped up at those speeds to meet the OEM spec assuming 0% losses
     
  16. Oct 8, 2019 #1136

    pictsidhe

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    I've been wanting to measure that. Perhaps I should put the engine back together in single cylinder mode...

    The OEM baffling is quite leaky and could be substantially improved. Since I want to increase power, I'm going to assume that I'll need a similar airflow to standard.
     
  17. Oct 8, 2019 #1137

    Vigilant1

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    Thanks, and that is in close agreement with the breakdown you gave in an earlier post (if we subtract out the induction air and filter flushing air)
    But if we need approx 800 CFM to actually pass through the ducting and near the cylinder heads/through the oil cooler, the trick is to get it to actually pass through the system (rather than just pile up outside the inlet and go around the airplane). For perspective, 800 CFM is as much as the airflow used by a 2-ton home air conditioner. Yes, 65 kts is 6580 FPM, and if our duct opening is 113cm2 = .12 sq ft, the we could see that it would "gulp" a column of air of about 800 cubic feet per minute, but that will only be true if it goes into/through a frictionless duct of exactly that same cross section. Things change a lot once we start turning it, asking it to go through the slots between the fins or the tiny fins of the oil cooler, etc. At the inlet face the dynamic pressure (q) at 65 kts is just .11 psi (at sea level, reducing as we go up), so it's not much to work with.
    Clever!
    I'll bet it is more than .11 PSI. If we want to play around with this--the static pressure at 65 kts = .11 psi = about 3" of water column. So, if a water manometer tube introduced to the inside of the fan housing (not facing the airflow, though) shows a rise of 3", then we could expect that the dynamic pressure of air at 65kts should perform as well as the stock fan in pushing air through the stock baffling.
    FWIW, 3" of water column is >huge< pressure by home HVAC standards. The total delta P typically required of a furnace blower is less than .5" wc. Yes, that will move 800 CFM of air throughout a house using just a 1/3HP blower, but the starting duct areas are typically 200 sq inches, not the 18 sq inches (113 sq cm) we are talking about here.
     
    Last edited: Oct 8, 2019
  18. Oct 8, 2019 #1138

    TiPi

    TiPi

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    I'll try to measure that on my mower (18hp 40-series) this weekend
     
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  19. Oct 8, 2019 #1139

    poormansairforce

    poormansairforce

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    An 8" diameter wheel at 3600 rpm would produce roughly 3.6" water column or .13 psi.
     
  20. Oct 8, 2019 #1140

    Vigilant1

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    Be careful out there.:)
     

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