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charles.harbert@fmglobal.com wrote:
Paul, as you know, turbocharging is a simple concept, but complicated
application. An excellent basic information handbook is Hugh McInness'
Turbocharging (HP Books) for about $10. They even cover aircraft engines and
altitude compensation. I read Jeff's plan and having covered this same ground
recently, although without the same level of technical knowledge, I found out
several things that Jeff and others might be interested in:
1. It can be a problem limiting the boost using only the throttle because you
must be careful to not overboost during takeoff. I have a large wastegate set at
5 psi (20 psia) with the large turbocharger (T-04) that can easily overboost
above 4,000 rpm on a 20B. I'd estimate that this same size turbocharger on a
13B will provide 20 psi of boost (way too much) at sea level w/o a wastegate. If
you have too big a turbo, you'll get pumping loss (hot air, too) at lower
altitudes, until you get to higher altitudes. If the turbo is too small, you
lose a lot of hp at higher altitudes. I talked to Ola at Turbonetics and he was
very helpful in sizing the turbo for my needs.
A wastegate reduces the overboost problem and allows you fly the plane during
takeoff. I know from my high spd taxiing that I'm able to get over 10 psi at
4,000 rpm due to the higher mass flow (actual controlling factor of turbo boost)
at high load conditions. I put a 5 psi spring in the wastegate so I didn't have
to worry about overboosting. At higher altitudes, I can restrict the pressure to
the wastegate to increase turbo boost to keep sea level absolute pressure.
2. I know Jeff knows this, but for others, you need to know that most A/C
turbocharged engines begin to lose sea level hp at 12-15K' (turbo normalized to
a certain altitude), but because of the reduced air friction of the higher
atmosphere the planes are able to operate up to 30K' with the available
remaining hp. The real problem you will face is turbine overspeed to destruction
because of the higher compressor ratio. At 30K' you have less than a fifth of
sea level air pressure, so to have sea level hp, you need a 5:1 compressor
ratio. Most turbos will overspeed above 3:1 (usually 125K rpm-refer to
compressor map)(Turbonetics makes a special ball bearing turbo that will take
160K, but its 3 times the cost ($1,800 vs $600)).
3. I tried to designed my plane (Velocity) to fly up to 25K', but there's a
whole bunch of problems to deal with when you do this. One of the biggest is the
outside air temp which is way below zero (minus 20-30 deg F). You really have to
seal all openings and have a very good heater. I added a water heater core
inside the cabin that is connected to the normal auto heater connections. Also,
because of the thin air, you can overheat the engine because of the lack of
cooling mass flow. Another problem is that your prop loses efficiency at higher
altitudes, so you need a bigger blade and it needs to be adjustable to increase
the pitch. Above 25K' (really 20K'), you must wear an oxygen mask as a cannula
is not effective, plus you will use a lot of oxygen (see A/S ads). You may want
to design for a lower cruising altitude to not have such intense conditions to
deal with.
4. If you're going with a SS exhaust manifold, I suggest that you make it thick
(.125+") enough to handle the high temps (>1,600 degF). I understand SS is
better than mild steel which can burn out relatively quickly in unturbocharged
engines. The Turbonetics turbo that Paul recently mentioned is a must IMHO as it
can handle the heat and costs very little additionally. I use a SS turbo exhaust
pipe. There is a tremendous amount of heat generated around the turbo (red hot),
so plan on extensive sheilding and cool air ducting, especially between intake
manifold, cowling, and coolant/fuel/oil hoses.
I hope I'm not sounding like an expert, because I'm not. I just have spent a lot
of time and talked to lot of people regarding this project over the last 3 yrs.
I told Paul when he was up to see me last month, that if I had to do it over
again, I'd go with a turbo 13B. It's a great combination.
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