Briggs vanguard conversions

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

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  1. Oct 11, 2019 #1181

    BBerson

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    Was that fan pressure measured with the hose pointed direct into the fan wind?
    Normally a mower isn't mowing at maximum rated power. The mower will bog down below governed rpm when cutting tall grass. That simulates a prop in climb which doesn't need a governor because it is bogged down through out the climb.
    I suspect the fan cooling rate is about the same as free air cooling at say 60 mph. At 90 mph dynamic pressure in a pressure cowl would exceed the fan cooling. Using both fan and ram air pressure cowl would cool even more for those running high power and PSRU.
     
    Last edited: Oct 11, 2019
  2. Oct 11, 2019 #1182

    blane.c

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    I look at these engines as 25hp METO based on 1/2hp per cubic inch. 810CC POWER CHART.png This is @ about 3,040RPM on the stock curve. Any power above that is take-off, initial climb, or emergency power. Until more data is in on the ability to shed heat at higher power for "X" length of time. Because the SD-1 has 33hp doesn't mean you should consider it METO that is crazy talk. Nobody that I know runs around at METO anyway except possibly G. Gordon Liddy who stated he liked to fly and I paraphrase at full throttle full rich. So "I think" these engines will be best run around the 20hp figure plus or minus depending, in normal climb or cruise. Having 33hp for take-off and initial climb is a luxury that should be used if necessary in regards flight conditions for example - max gross weight, high density altitude, short runway, and imminent terrain or other flight hazard. Power should be reduced from Maximum as prudent if conditions allow. I know that ain't the way it is taught until you fly professionally but it is then, trust me your boss doesn't want you yammering his engines unnecessarily or wasting his fuel, you shouldn't either. For example If you use max power at high density altitude and everything is "A-OK" and another day everything else is the same except lower density altitude why wouldn't everything be "A-OK" at a reduced power that approximates the power you had on the high density altitude day? You will need a little program or a chart. Engine longevity is tied to hp used. Nobody wants longevity to disappear prematurely.

    Many improvements have been made in engine monitoring and design that allow a lower specific fuel consumption (sfc) @ cruising power. It is entirely likely that these engines will cruise around 1/3lb per hp hr + or - I see no evidence they would not. Of course that would be EFI, carbureted the sfc may be slightly higher but still under .36lbs to .38lbs is likely, although a well engineered carburation system could match EFI. Higher power settings will use considerably more fuel the additional fuel is to help cool the combustion chamber and aid in the prevention of detonation.

    The reason I posted the propeller info yesterday is because of the "WIDE RPM BAND" these engines will likely be operating at (possibly around 1,100RPM) and how are we going to keep the manifold pressure at a usable level in regards the propeller being able to have actual thrust? I believe this is a valid concern that is not being adequately addressed in the discussion. Presently the idea to have the propeller efficient at max power and RPM necessary for single engine operation regards take-off and climb leaves the operator having to cruise at higher RPM for propeller efficiency? Or how low can manifold pressure go vs RPM to remain efficient? Some "X" amount of pressure is necessary on the top of the piston? There are some reference to a "flexible propeller" aka like the "Prince" propeller, and a flexible propeller may aid in this regard? Anyway does anyone else see manifold pressure/propeller usage being a problem?
     
  3. Oct 11, 2019 #1183

    BBerson

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    Blane, I looked up METO power https://aviationglossary.com/meto-horsepower/. It's a term for supercharged engines, I think. You won't (can't) get 33hp in climb without a controllable prop or a prsu or a turbo or supercharger.
    So takeoff and climb is just 18-22hp without those. And that's what I have been saying.

    Unless you want to cruise around flat out full throttle at sea level like G.Gordon Liddy 33 rated hp is irrelevant.
     
  4. Oct 11, 2019 #1184

    blane.c

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    OK, take a Lycolming O-290 D2 (an engine I am somewhat familiar with) It is a 135hp engine direct drive rated for 140hp for (if memory serves) 2 minutes. I say it is 135hp METO, what do you say?
     
  5. Oct 11, 2019 #1185

    pictsidhe

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  6. Oct 11, 2019 #1186

    blane.c

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    Max
    Maximum-Except-Take-Off
     
  7. Oct 11, 2019 #1187

    blane.c

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    The propeller RPM/power/manifold pressure … ain't making sense to you either?
     
  8. Oct 11, 2019 #1188

    Vigilant1

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    We could have 33hp in climb with a fixed pitch prop, but it would need to be very low pitch (to allow the engine to reach 3600 rpm at climb airspeed). That's not what I'd like to do, but if you have a situation where you need all the ponies for climb, it be apwould be apropriate (e.g maybe a heavy motorglider where the engine is to be used primarily for self launch or to climb and then glide back home).
     
  9. Oct 11, 2019 #1189

    BBerson

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    Well, you are using this term METO that GA pilots don't use, but old transport guys do use.
    I think the 0-290 is rated 135hp at say 2700 rpm. You can't get 2700 rpm in climb with a normal climb prop so sure, you have 135hp Max except takeoff. But that is just confusing this thread throwing in DC-6 terms.
     
  10. Oct 11, 2019 #1190

    blane.c

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    For single engine it is likely that the propeller will be set up to be most efficient at max power at least for many (most), this bodes a problem manifold pressure and useful cruise yada yada. Especially considering how wide the RPM band is.

    The micromaster is going to need the propellers set up for max power in case of one engine loss of power?

    Setting the propeller(s) to max at around 3,000RPM to 3,100RPM around 25hp each (maybe a horse or so more with mods) and cruise between 2,300RPM and 2,400RPM would be easier regarding propeller efficiency? But not likely to achieve power goals for take-off, and asking a fixed prop to be efficient from 3,600RPM or more down to 2,300RPM or ? So manifold pressure becomes an issue vs propeller efficiency? So shedding heat and higher cruise power settings and speed may be the easiest answer … so may be speed brakes on the aircraft "aka" spoilers or a split rudder in regards not being able to reduce propeller RPM for descent?
     
  11. Oct 11, 2019 #1191

    BBerson

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    Yes.
    But Briggs rates it around 27hp at 3600 rpm. So how do we get the 33hp at 3600rpm?
     
  12. Oct 11, 2019 #1192

    Vigilant1

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    We're obviously mixing a lot of things here, and it makes things confusing. So, hoping I don't make it worse:
    1) There is one RPM at which the engine will make maximum (continuous) power--in our case, let's say it is 3600 RPM. We can have that power at any airspeed we choose by selecting the right prop pitch. That max engne power will only occur at one airspeed, though.
    2) In general, propeller efficiencies are higher at higher airspeeds.
    3) In general, thrust for a given amount of HP declines with airspeed (even though Rule 2 above is true).

    At our anticipated burn rate (1.5-3 GPH), I'm not going worry about trying to get highest engine efficiency. Since the MicroMaster was brought up, I anticipate that single engine climb will be the most challenging design case, so I may have to choose a fixed pitch prop that will give best thrust at climb speeds (approx 70 MPH), and just take whatever I can get at normal cruise airspeeds from two engines (they will be underpitched, so will be loafing at low MP and fuel burn). We'll see.

    I don't see the situation with these engines as being any different than with other normally aspirated engines using fixed pitch props. You choose a prop that gives you what you value most.
     
    Last edited: Oct 11, 2019
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  13. Oct 11, 2019 #1193

    Vigilant1

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    It looks like the approach taken by TiPi and/or MiniSport is:

    1) Improved induction (smoothing out the airflow through the carb, probably a less restrictive air cleaner, etc). Very thorough air filtration/cyclonic air cleaning, etc is key to long life in a zero turn radius mower were the engine spends its life right in the middle of a cloud of grass clippings and debris. That filtration can be expected to reduce airflow. In our airplanes, we can use less thorough filtration with no problem.
    2) Slightly higher CR (we can probably assume our airplane owner will be willing to use higher octane fuel than our mower operator)
    3) Adjusting the timing
     
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  14. Oct 11, 2019 #1194

    blane.c

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    Letting the propeller drive the engine in descent is known to cause problems in some engines. Some gears are in "reverse mesh" to start off with, insufficient pressure in the combustion chamber may allow the rings to "float" and others that do not come readily to mind.
     
  15. Oct 11, 2019 #1195

    BBerson

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    You probably will cruise about the same 3000 rpm as climb, for slow cruise. After leveling off in cruise, the throttle is reduced to reduce manifold pressure to less than what it was in the climb. Fast cruise would be around 3200 rpm and full power cruise 3600 rpm or more if you want to exceed the factory limit.

    The Briggs 1800 rpm idle is a concern. My Honda Gx670 is 1200 rpm idle. I would consider installing Honda coils. My Honda can be hand flip started. Can the Briggs be hand started?
     
  16. Oct 11, 2019 #1196

    Vigilant1

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    Sorry, I should have said earlier: The .55 HP / cubic inch at 3600 sounds entirely reasonable to me--the racers who turn their O-200s at high speed probably see more than that specific power at high RPMs. But they need to use extraordinary measures to even allow their heads/valves to finish a season: antidetonation injection (which cools the heads), very rich mixtures, sodium exhaust valves, etc. I think the 810cc engine is in the same situation--it can make much more than the B&S rated HP, but for how long? It will al hinge on cooling. We've seen B&S CHT numbers for their 810cc engines, but there are too many unknowns to draw conclusions (measured where? How long was the engine at max load? Was it at max load, or just max RPM? That's with a dedicated blower--can we do the same with ambient airflow? etc). I'd guess someone in the SD-1 world knows the answers. TiPi appears to be working through the issue methodically, and he's shooting for limits like those used by Jabiru and others: 180C (356F) max for continuous ops, 180-200C max (356F-392F) for 5 minutes. Depending on where that's measured, those limits are generally in line (or a bit lower) than what is seen in the VW aviation world.
     
  17. Oct 11, 2019 #1197

    BBerson

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    That .55hp / cubic inch might be reasonable. The higher compression Limbach 2000 (122 cu) claims 80hp, so .65hp/cu.in. at 3500 rpm for 5 minutes. I don't believe it is quite that much.

    Only get average 250 hours per head life, however.
     
  18. Oct 11, 2019 #1198

    Vigilant1

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    Thanks for doing the measurements.
    I suppose there's probably a way to use the data you've collected as wel as the physical dimension of the fan to figure out the resistance to flow (pressure drop) of the baffling at both of those RPMs and, at the same time, the CFM throughput. But, even without that, it's good to know that, if we have a baffling setup that is as good as the stock one, and we see approx 2" wc of pressure on the intake side, then we'll have enough air to keep the engine cool. At 65 kts (climb speed?) we have 3"wc of total dynamic pressure available. If 2"wc of pressure remains in the front of the ducting, then it is "leaking" the equivalent of 1"wc of airflow through the system (maybe more if we can produce "negative pressure" on the exit side using a lip, fan, exhaust ejector, etc).

    Wow, it just shows the nature of this business I guess. B&S saved maybe $20 in costs vs a 2 barrel carb? And the result is reduced power and fuel efficiency for the life of the engine. But, if that's what the market wants, the successful manufacturers will do it. I know when I bought a mower last time I just looked at the power claim onthe sticker and didn't delve into an examination of the innards.
     
  19. Oct 11, 2019 #1199

    blane.c

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    Nope, not cruising at 3,000RPM more like 2,400ishRPM.

    1,800 RPM is high for idle, is there something about the engine that would suggest it is not capable of idling lower? Sheesh my cub would take off with 1700RPM no problem, empty of course. Something under 1,000RPM would be better.
     
  20. Oct 11, 2019 #1200

    Vigilant1

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    I think the high idle speed is a function of the fixed advanced timing and (in the stock engines) the limited rotating mass of the engine (incl flywheel).
    For most single-engine planes that would use this engine, idle at 1800 RPM probably isn't a huge problem. They won't stay airborne on that much RPM/power, and the biggest issues would be more brake wear during taxi and a higher-than-desired approach speed at idle. If it isn't practical to get the idle speed lower, then a plane designed for this engine could just add another notch of flaps. Now, for a tiny seaplane, it would be a problem!
     

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