Re: Small Model engines (Again - was Model IC Engine help)
- From: NoSpam <nomail@xxxxxxxxxxxxxxx>
- Date: Thu, 22 Oct 2009 15:29:19 +0100
Richard Shute wrote:
On Sun, 18 Oct 2009 10:41:56 +0100, NoSpam <nomail@xxxxxxxxxxxxxxx>
wrote:
Richard Shute wrote:
... snipped
However, the air disturbance due to the propellor is large (you often... snipped
find in full size practice that carbs breath from a 'still air box').
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 ;-)
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
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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- Re: Small Model engines (Again - was Model IC Engine help)
- From: Richard Shute
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