Okay, I gotta ask--how did you get/find that? And if we need to move 800 CFM against that pressure, can we figure out the approx wattage/HP needed by the fan?
Briggs vanguard conversions
- Thread starter Hephaestus
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Right, my fan book shows an 8" diameter wheel at 3735 rpm would produce 4" static pressure at 349 Cfm and .41 hp.
My Limbach is rated at 4" static pressure. (thought that was around 100 mph)
pictsidhe
Well-Known Member
Coud produce. Actual pressure will be a bit lower to get a decent flow. The Briggs fans are annular, so it's lower again, but they gain a bit by being forward curved.
poormansairforce
Well-Known Member
8"π*3600÷60÷12)^2*.5*.00237÷144=.129 psi
Or you can be lazy like me:
http://forestryforum.com/members/donp/Blowercalc.htm
The hp will depend on thickness of wheel. And as pictsidhe noted there are other factors as well.
Well at some point oil or liquid/water cooling methods are cooled by air. Just moving the heat to a place it can be removed more easily or faster.
What we don't know is the resistance (pressure drop) of the ducting system on the stock 810cc engine at whatever CFM volume the engine needs for cooling (TiPi's estimate: 800 CFM). A formula with input of RPM, wheel dia, assumed efficiency may be able to tell us the maximum practical static pressure that fan can provide, but obviously the actual pressure behind the blower depends on the restrictions in the duct (e.g. if it is wide open, the developed static pressure will be zero--and the CFM will be very high).
I don't have an 810cc engine, but the blower wheel looks to be about 10" dia and about 3" thick. A blower like that turned at 3600 RPM should (according to the calculator provided at the link by poormansairforce) be able to provide up to 5.7"wc of pressure and a volume of 524 cfm at that pressure (and it will take about 0.9 HP to do it). If the ducting is less restrictive, the backpressure developed by the fan will be reduced and more CFM will result. Just a mark on the wall.
I don't have an 810cc engine, but the blower wheel looks to be about 10" dia and about 3" thick. A blower like that turned at 3600 RPM should (according to the calculator provided at the link by poormansairforce) be able to provide up to 5.7"wc of pressure and a volume of 524 cfm at that pressure (and it will take about 0.9 HP to do it). If the ducting is less restrictive, the backpressure developed by the fan will be reduced and more CFM will result. Just a mark on the wall.
FWIW, 4.0"wc (0.145 psi) dynamic pressure would be at about 91 MPH. The expected dynamic pressure (standard day, SL) at 100 MPH would be 4.9"wc.
pictsidhe
Well-Known Member
TiPi, If you have accurate RPM measurement, measure the pressure drop at max revs and about 2k, from that I could make a table of pressure and flow at any reasonable rpm.
My 40ci fan is 5 3/4" ID, 9 3/8" OD Blade height is about half at 1 3/8", half at 1 5/8". So call it 1 1/2" average.
pictsidhe
Well-Known Member
I am poking my calculator...
920cfm at 1kPa, ~.147psi or 4" water, is 434W of work. If the fan is 60% efficient, thats 720W, or about 1hp.
We could use a lower pressure drop oil cooler and save some power, but it would need its own air system if the engine uses a higher pressure drop. The engine will inhale about 40cfm, so we can trim down the air filter flushing flow. The 1% supercharge by using plenum air for the engine would be worth keeping if the ducting is not too horrible.
710cfm is 77% of the stock 920. I bet there are many baffle leaks that could be trimmed away, too.
I am thinking about blowing the air forward. It doesn't matter to the fan which way it blows and I need some heat source in the cockpit most of the year anyway. The fan intake would be the center of the pusher prop which is a dead zone anyway. Might even reduce drag.
Industrial engines are designed for a certain amount of dirt clogging. Not a problem for aviation.
A contractor told me his Honda V-twin got clogged tight with dust while grinding cracks in roads 8 hours a day.
Eventually he cut holes in the shroud so it could be blown with compressed air daily.
Industrial engines are designed for a certain amount of dirt clogging. Not a problem for aviation.
A contractor told me his Honda V-twin got clogged tight with dust while grinding cracks in roads 8 hours a day.
Eventually he cut holes in the shroud so it could be blown with compressed air daily.
pictsidhe
Well-Known Member
There is something wrong with your oil cooling numbers, TiPi.
3.3kW into 72cfm would raise the air temp 80C. That'll be tricky with 80C oil and non Siberian temperatures.
12lpm and 10C drop I get around 3kW, so that is in the ball park.
3.3kW into 72cfm would raise the air temp 80C. That'll be tricky with 80C oil and non Siberian temperatures.
12lpm and 10C drop I get around 3kW, so that is in the ball park.
I know you are planning for a low speed project, but going with the breeze does give you the "free" flow from the airspeed dynamic pressure. Also the cooling air that gets dumped out the back helps reduce the magnitude of the low pressure zone that would otherwise exist behind the blunt engine. If you are planning for a flywheel drive prop, the hot side of the jugs will be forward, of course.
With a proper inlet, you'll get a little help from the airflow, right? Similarly, on the back end you can arrange things (that exhaust/ejector . . . ) so that the pressure is slightly below ambient. Every little bit . . .
pictsidhe
Well-Known Member
Inlet will be annular behind the spinner. I might have the exit under the mufflers, stealthy, but not the best place aerodynamically. Alterntivley, a nice rearward exit thanks to a 'badly fitting cowl'! Mufflers will be external and disguised as ejector pipes. That ticks the 'some external components' box for 103 power calculation. 
An ejector system would be nice, but a major PITA to build with my twin mufflers.
The thing that looks a bit like an air scoop beind the spinner is an oil deflector for the reliably leaky Rotol prop. Walk back a few more paces.
I cut the former that is about at the end of the first ejector stub last weekend. It's the approximate firewall. My 'lil Briggs looks small sat on it.
An ejector system would be nice, but a major PITA to build with my twin mufflers.
The thing that looks a bit like an air scoop beind the spinner is an oil deflector for the reliably leaky Rotol prop. Walk back a few more paces
.I cut the former that is about at the end of the first ejector stub last weekend. It's the approximate firewall. My 'lil Briggs looks small sat on it.
I don't think scooping up dynamic pressure is really "free," compared with a streamlined nacelle and a buried fan. Especially when gliding.
That's possible, I did that some time ago and it was a rough measurement of air velocity out of the cooler (and no load on the engine). Will redo this when I have the engine running under load and properly instrumented.
you might need to add quite some prop extension to get the cylinders back far enough to clear the belly slope. Might be best to hang the Briggs upside-down, add the prop extension to the flywheel and leave the fan on there. Just reduce the weight and size of the fan shroud and use any opening in the fuselage to expel the hot air. This is going to be a beast to cool with airfow only.Inlet will be annular behind the spinner. I might have the exit under the mufflers, stealthy, but not the best place aerodynamically. Alterntivley, a nice rearward exit thanks to a 'badly fitting cowl'! Mufflers will be external and disguised as ejector pipes. That ticks the 'some external components' box for 103 power calculation.
The thing that looks a bit like an air scoop beind the spinner is an oil deflector for the reliably leaky Rotol prop. Walk back a few more paces
I cut the former that is about at the end of the first ejector stub last weekend. It's the approximate firewall. My 'lil Briggs looks small sat on it.
Keeping the flywheel will allow you to mount a light-weight 3-blade prop for authenticity.
Very true. And if you'll be gliding with the engine shut down or at low throttle, relying on the fan for airflow will also reduce the chances of shock cooling the engine.
You'll have that unused spinning shaft in front, right near the hottest side of the engine (assuming a prop on the flywheel end). The stock fan could be repurposed to go there, and then just duct your cabin heat forward from behind the engine. I'd want a reliable CO detector in the cabin.
Might be a good way to go for the rear engine on an inline twin. . .
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pictsidhe
Well-Known Member
I'll be using a redrive. My engine bay looks like it will have an echo. Lots of space. I need to know the pressure drop before I can determine what airflow aids are needed.
"Tiny engine package!" was what I thought when I saw the drawing Hot Wings posted (below):
Of course, it will get a bit more crowded under the cowling once we have a carb, air cleaner, induction and exhaust, engine mount, etc. But still, for those of us with some flexibility in the airframe layout, having a comparatively compact chunk of metal that weighs about 70 lbs makes it tempting to hold off on the final fabrication of the cowling and engine mount to put that engine just where it needs to be for CG purposes. It sure beats adding ballast, 8' battery cables, etc.
