Briggs vanguard conversions

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

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  1. Oct 9, 2019 #1161

    blane.c

    blane.c

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    I was looking to build the airframe first per "normal?" But now I want to build engine(s) first for the reasons you stated, it seems on one hand to be the cart before the horse but knowing the exact weight of the firewall forward package(s) is essential to keeping it light and making a flying lawnmower there is a lot of wonderment about the weights.
     
  2. Oct 10, 2019 #1162

    blane.c

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    Some may find this interesting. I cannot upload whole PDF to large. How I make wood Propellers by Al Schubert.

    Annotation 2019-10-10 123606.png
     
  3. Oct 10, 2019 #1163

    pictsidhe

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    That is well worth reading for anyone thinking of hacking their own propeller.
     
  4. Oct 10, 2019 #1164

    TiPi

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  5. Oct 11, 2019 #1165

    pictsidhe

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    Could you repost them, I dont have facebook.
     
  6. Oct 11, 2019 #1166

    Vigilant1

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    TiPi, thanks for the link. We've talked here before about the likilihood that the SE-33 has slighty advanced timing, via the modified flywheel, compared to a stock engine. That, plus a slightly higher CR and some mild porting would logically result in some more power (while increasing the risk of detonation--users would need good fuel).
    It normally is no challenge to get higher HP from engines by turning them faster, increasing the CR, improving induction and exhaust. But with normally aspirated air cooled 4 strokes, the reliable continuous HP is generally limited by the ability of the heads shed head. No presently fielded air cooled 4 stroke acft engine I know of (to include VWs in airplanes) has a specific continuous output above about 0.033 HP/cc ( i.e. it takes 30 cc to produce 1 reliable continuous HP in acft use). With 810 cc to work with, that would be 27 HP. Now, this isn't a law of physics and there could be some scale effects or some advantages with 2 cylinders (no rear cylinders hiding back there!), but it does seem likely that, at 33 HP we may already be pushing the thermal limit for reliable aircraft operation.
     
    Last edited: Oct 11, 2019
  7. Oct 11, 2019 #1167

    pictsidhe

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    It is not as simple as hp/cc
     
  8. Oct 11, 2019 #1168

    Vigilant1

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    Of course not. But perhaps we can explore why this engine exceeds, by that metric, any Lycoming, Continental, Franklin, or Jabiru product. Scale (more head/cyl surface area per CC at these small sizes)? Configuration? Limited field experience?
     
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  9. Oct 11, 2019 #1169

    Vigilant1

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    For starters, I think we can safely note that there's something amiss with Minisport's simultaneous claim of 33HP and fuel consumption of 1 gph at 75% power. That would give a BSFC of 6/ 24.75 = 0.24lb/HP/hr. I don't know anyone who would believe these air-cooled, fixed timing, pushrod engines are anywhere near that efficient. They burn closer to .40 lb/hp/hr (which is fine as far as I'm concerned). But it does call into question the published stats, IMO. If the fuel burn is really 1 gph, then the HP being produced is about 15. If that's really 75% power, then this is a 20 HP engine (which I also don't think is likely).
     
    Last edited: Oct 11, 2019
  10. Oct 11, 2019 #1170

    BBerson

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    It's the hp per cubic inch at same rpm and compression ratio as a Continental or Lycoming. So a O-200 has 100 hp with 200 cubic inches and 2750 rpm. Half hp per cubic inch.
    A 810 cc (49.5 cu.in.) should have 25 hp at 2750 rpm, probably less. So what would that 810 have at 3200 rpm or whatever rpm it is rated at?
     
  11. Oct 11, 2019 #1171

    Vigilant1

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    The B&S chart for their highest HP 810 (a Vanguard with EFI) is here:http://bsintek.basco.com/BriggsDocumentDisplay/default.aspx?filename=mopuuPqb733wQ9vcv4 At 3600 RPM they rate it at 28 HP, and at 2750 RPM it's about 23HP. If we accept that the SE-33 puts out 33 HP at 3600 RPM and the curve is the same as the stock B&S curve, then at 2750 RPM it will be making 27 HP (or .55 HP/cu inch).
    But I don't dispute that it is possible to make that specific power (with, say, a high CR.) I'm saying that the O-200, like an O-360, a VW, a Jabiru, any other air-cooled aircraft powerplant, is effectively limited by the heat the heads can shed. Turning the engine faster or slower isn't the issue--burning enough fuel to produce more than about .033 continuous HP/CC is the limit, in practice, for normally aspirated 4-stroke air cooled aircraft engines. I'm not saying this is a limit due to physics, it's entirely possible there's an engineering answer for going higher (heat pipes, more efficient fins, etc). But Lycoming, Continental, etc have been doing this awhile and they aren't claiming they can do it.
     
  12. Oct 11, 2019 #1172

    Hot Wings

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    Maybe not?

    Depends on how they define 75% power. Some have the mistaken idea that power, RPM and cruise speed are directly linked. Throttle back to 2700 rpm (94mph?) and the fuel flow may go to only 1 gph.......... but at what manifold pressure!? Keeping in mind the cube root rule for speed increase per Hp, their "75%" may be much closer to 60%.
     
  13. Oct 11, 2019 #1173

    pictsidhe

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    We aren't playing with lycontisaurs or an O-100. We are playing with a completely different engine. What works for one engine will be different for another one. I really don't feel like posting all the math, as nobody would read it all, anyway.
    While I could work out the achievable cooling limited power ouptut of a Briggs at X rpm, I'm going to plaster one with sensors and see what i can do with it. Experience tells me that there is some more to be had, especially if I add extral coolng capacity.
     
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  14. Oct 11, 2019 #1174

    Vigilant1

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    I suppose. But if an airplane designer (or any private pilot) doesn't know the difference between "75% RPM" and "75% power," we have a problem (and I'm not saying Igor Spacek doesn't know the difference). According to TiPi, the SD-1 requires about 17 HP to remain airborne at 70 KTS. If this engine has a BSFC of .40 (which is pretty optimistic), then 17 HP would still be more than the claimed 1 GPH.

    Hey, your success will be a win for everyone. I hope it works. It will be great if these inexpensive little engines can keep their cool at the specific power levels that we're talking about. I'm sure that the details (baffling, proper intake and exit geometry, etc) will make all the difference.
     
    Last edited: Oct 11, 2019
  15. Oct 11, 2019 #1175

    TiPi

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    The SD-1 requires about 21hp for 80kts (148km/h or 92mph) in level flight.
    The fuel burn data is for the listed speeds which are not necessarily at 75% power. Remember that these are not certified aircraft & engines and data is gathered with the available means. I can run my 18hp Briggs mower for about 3h on 5-6lt of fuel.
     
    Last edited: Oct 11, 2019
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  16. Oct 11, 2019 #1176

    BBerson

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    I see it differently. On takeoff, the engine is generally cooler and it will only put out so much power at takeoff airspeed regardless of the cylinder or oil temp.The prop limits the power to about 60% rated power. The carb on a Lycoming 320 will enrichen the mixture and dump more fuel at full throttle in takeoff to cool it for continuous rated power. I think my Lyc 320 burned 14 gallons/hour at full throttle and 9 at 75% cruise.
    A simple carb on a Briggs won't have that enrichment, so may overheat after several minutes in climb at max manifold pressure. But it should be ok in cruise at 75% power. I assume sport flyers are ok with reducing the throttle after a 4 minute climb. My Limbach (Grob) would overheat climbing through 5000 feet which takes about 10 minutes of climb.
    We are not racing at maximum continuous rated power.
     
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  17. Oct 11, 2019 #1177

    TiPi

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    OK, I have the measurements. This is going to confuse people:(
    First, the fan on my mower engine (18hp Intek 40R677) is BIGGER than the fan on the 49M977. Same dia and same number of blades but about 2mm taller. The fan has 18 blades all up, 14 blades 62mm tall and 4 blades 57mm tall (shorter over the ignition magnet and counter weight). The OD is 240mm, the ID (fan blade leading edge) is 148mm). 40R fan on left, 49M fan on right:
    upload_2019-10-11_17-57-16.jpeg

    I drilled a couple of holes into the fan cover, one at the "bottom" of the cover (opposite the cylinders) and one in a dead spot inside the cover beside the air filter:
    upload_2019-10-11_17-55-32.jpeg

    The results are:
    1,700rpm: 0.8 at the back and 0.6 at the air filter
    3,440rpm: 3.2 at the back, 2.7 at the air filter
    Units are cm of H2O, measured with a precision electronic unit.

    I'm very surprised that the 40 engine has the same fan, but that might be because originally it was a 3,000rpm engine (no-load 3,200). I bumped the engine speed up a bit.

    BTW, the Intek 40R677 has a single-barrel carburetor. There is also a model of the 49 with a single (49J677)
     

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  18. Oct 11, 2019 #1178

    TiPi

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    upload_2019-10-11_21-11-0.jpeg upload_2019-10-11_21-11-9.jpeg upload_2019-10-11_21-11-15.jpeg upload_2019-10-11_21-11-21.jpeg upload_2019-10-11_21-11-26.jpeg upload_2019-10-11_21-11-42.jpeg
     

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  19. Oct 11, 2019 #1179

    Hot Wings

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    There are plenty of pilots, and probably too many aircraft designers, that don't know the difference. The only time this is a problem is if the pilot is trying to get somewhere based on fuel burn calculated from RPM or when publishing accurate data for advertising. One is a real problem for the pilot and his passengers. The other is mostly just annoying for those of us that can calculate BSFC. ;)
     
  20. Oct 11, 2019 #1180

    TiPi

    TiPi

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    I suggested to Igor to slightly retard the ignition as the mixture burns slightly better with higher CR and better volumetric efficiency (dynamic CR will increase as well).
    This is a comparison of the industrial V-twins models from about 7 years ago. The Kohler CH750 had the highest specific power of 40hp/lt and is sort-of the benchmark. Increasing this to 42-43hp/lt is getting the max out of these engines without resorting to exotic solutions. For the Briggs, this means 35hp is achieveable with careful porting, improved carburation and attention to details.
    upload_2019-10-11_22-16-28.png
     

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