Small Model engines (Again - was Model IC Engine help)

Given the need to get the fuel/air mix into the cylinder, why do most designs have the carburettor venturii facing away from the direction of
travel (aero engine) so the intake is in low pressure air? Seems obvious to a beginner that you would want the carburettor intake facing into the airstream. (Four stroke singles/twins in mind).
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Mostly it's so that when you crash the plane (note 'when'), there's a better chance of the carb and manifold surviving. Plus convenience - easier to connect the throttle to the servo and less likely to shred a finger while adjusting the needle.
The pressure aspect is miniscule, even theoretically (stagnation gain = rho*v*v/2 which at model speeds is close to bugger all) Say a pretty fast model at 60 mph which is conveniently just about 30m/s, rho is a whisker over 1kg/m3 so the stagnation gain is 1*30*30/2 whcih for a quick mental calculation is a bit less than 500Pa. Consider that one atmosphere is 100,000Pa and you see the gain is less than half a percent.
However, the air disturbance due to the propellor is large (you often find in full size practice that carbs breath from a 'still air box'). Pointing the carb. forward, just beind the prop. you end up with negligible theoretical gain and considerable practical loss.
Richard On Sun, 18 Oct 2009 08:43:38 +0100, "Steve"

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wrote:

Thanks Richard
Hadn't spotted the problem with "dirty air" off the prop at all. In my head I was worrying about the air pressure in a rear facing carb being materially less than atmospheric. If I understand your calc, its looking at the pressure improvement from the forward velocity? In my case, the engine will be in a slow flying model (think Keil Kraft vintage cabin wing trainer) so this looks all a bit immaterial anyway. I'm going to build an engine and keen to be optimum it about along the way as these projects are typically taking me 1-2 years to complete (even if the actual workshop time is only 50 hours!).
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Richard Shute wrote: ... snipped

... snipped
Richard, I agree about the stagnation pressure point but I think just about every piston-engined aircraft that I have flown (probably about 80 types) has had the induction intake facing forwards. The air box is to allow an alternate non-filtered induction air source to be selected (for carburetted engines this is also heated).
Dave Caveat: I drive them rather than designing them ;-)
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wrote:

Dave, You are quite right that it was & probably still just about is the norm for 2 strokes, 'though there is a move towards remote/rear needles. A simple front induction 2 stroke is cheap and cheerful to make as well as being light in weight. Four strokes conversely are nearly rear carb'd, they generally have significant induction ducts as well. Even those with carb's near the front don't face them forward as far as I can think, almost invariably updraught. The comment about crap up the choke is also valid, 'though whether the designers/vendow consider that I'm not sure.
I think the real point is that it makes no noticable difference and there are many other aspects of the design which can have vastly greater influence on the engine power and reliability.
Certainly a carb. heat system is used on carburated piston aricraft, I was actually thinking back to cars & bikes where a still-air box was one technique used back in the days of Phil Irving or maybe it was P H Smith's book. Been a while since I read them....
Richard
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Richard Shute wrote:

I wonder if we're at cross-purposes about induction systems on aircraft (I don't fly 2 strokes (shudder)). All the Lycoming and Continental flat 4/6 cylinder engines that I have seen have updraught carbs (or whatever the correct name for the bit containing the venturi is on an injected engine) and induction systems which have a forward-facing inlet. The carb for the Lycoming R680 radial is at the back but I'm fairly sure that the air intake (on a Stearman) was forward-facing. The newer Rotax 912s in VLAs are usually also forward-facing. I can't remember where the intakes are on Tiger Moths and Chipmunks (Gypsy major) or Yaks (Vedenyev M14) but I think they're inside the engine compartment.
I can't say whether it makes a difference, but an awful lot of designers seem to choose to get their air from the front. Maybe the high pressure region behind the prop is beneficial.
Dave
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Wow!
I notice that the Gardan Minicab that Roland Beamont used as a shuttle between Samlesbury and Warton is up for sale. Sadly my flying is limited to the passenger seat :( Major shortage of roundtuit's I'm afraid.
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Steve wrote:

It helps if you get paid to fly them, rather than having to pay! ;-)

Have you looked at the NPPL route? Or even the 3-axis microlight options? You could even build (and design, if you're feeling brave) your own aircraft www.lightaircraftassociation.co.uk
Dave
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A colleague of mine put me off the microlight piece, he had a gearbox failure on an R22, fortunately got away unhurt, but he did visit the Air Investigation hangar during the investigation and he said it was wall to wall with microlights...
I only live 10 mintues from a local airport with two heli schools and at least two fixed wing schools, so I've really no excuse!
.
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Steve wrote: ... snipped

There was certainly a time when Microlights were marginal and the training regime was possibly too "relaxed" but this changed many years ago; I don't often fly microlights but the newer aircraft with 4-stroke engines are quite good. There's a new "single seat de-regulated" category that might appeal to some of those on the group; simple and cheap to build and fly.
You should get yourself down to that airfield for a 30 minute trial lesson! If they have an aerobatic aircraft it'll be slightly more expensive but worth every penny! If you want to know more we ought to go off-list to avoid irritating everyone else: snipped-for-privacy@gsocksmail.com (remove socks to reply); I'm happy to answer any questions.
Dave
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Richard Shute wrote:

Not pointing it forwards might help to keep crap out?
--
Charles Lamont