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Charlie,
<... Or have I completely missed the point & the goal is actually to tune for air
temperature? ...>
Actually, if I'm not badly mistaken, that's exactly what we're tuning for. I
haven't done any of the studying that Ed has, and there's one hell of a lot I
don't understand about how rotarys breathe. But a lot of the talk here turns on
"resonance" and "wave pulses" and what not. Resonance is sound. The pulses
we're discussing are sound. Sound travels at the speed of sound. We're "tuning"
the intake. DIE will occur at an rpm (frequency) that matches the time it takes
a pulse to travel the length of the intake runner. That time is a function of
the speed of sound, which is a function of temperature. It follows that at a
given rpm (which is easily variable) and temperature (which has been ignored
heretofore) you will have A (as in singular) tube length that is resonant. If
you can't control the charge temperature and want a particular rpm, the only
thing you can vary is the runner length.
Suppose all we wanted is resonance. Suppose that was our goal. If that were the
case, why couldn't we compute the runner length for the rpm we wanted to cruise
at and the intake temperature we were most likely to encounter at the altitude we
were most likely to cruise at, and build it that way. At this juncture, we have
two choices: We can vary the rpm so as to achieve resonance at the intake
temperatures we are "tuned" for, or we can adjust the intake temperature to
resonate at the rpm and runner length we are carrying. If we design for
relatively mild temperatures, say 60F, we can cruise comfortably at 8k' - 10k'
during the summer. Just climb until the intake charge matches the design intake
temperature and cruise there. In winter, we can turbocharge, adjusting the
wastegate to achieve the desired intake temps. We might have to cruise at 15k'
in winter, adjusting boost to achieve the intake temps we need for resonance.
Or we could build runners for the temps we expect and just accept departures from
optimum temp as one of those little things we have to live with in this uncertain
world. Or hang a CS prop on the beast and vary rpm at WOT to match the intake
temp required for max power. Runner length is not the only variable. Perhaps
there's some advantage to looking at them all
Or perhaps this whole thing is just a remarkably extended brain fart... Jim S.
Charlie & Tupper England wrote:
>
> >
> Hi Ed,
>
> I was trying to ask about the Q, or 'peakiness' of the tuning technique.
> My point about the 5k to 6k rpm is that with the 2.17 gearbox, rpms
> outside that range aren't of any real interest in a fixed pitch a/c
> engine application. Even with a fairly high 'Q', if you tune for your
> expected cruise rpm (somewhere between 5k & 6k?) how much will you give
> up if you don't have in-flight adjustable tube length? My assumption is
> that with a fixed pitch prop on a fast clean plane like the RV-class
> planes, you will turn at least 5k when you begin the takeoff roll & not
> exceed 6k or you will run into problems with the prop tips approaching
> the sound barrier. Is the peak so sharp that it will require varying the
> tube length over a 1k rpm band?
>
> I realize that the Lemans racer was using the variable helmholtz tuning
> method, but the variable length intake pipes were intended to move the
> torque peak over a several thousand rpm range to allow accelerating out
> of 20 mph curves & still pull well at 240 mph. My confusion is why
> tuning over a several thousand rpm range is needed in an a/c (Unless of
> course the Eddie effect is EXTREMELY high Q requiring tuning over a 1k
> range).
>
> Or have I completely missed the point & the goal is actually to tune for
> air temperature?
>
> (now even my fingers hurt.)
>
> Charlie
>
> >> Homepage: http://www.flyrotary.com/
> >> Archive: http://lancaironline.net/lists/flyrotary/List.html
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