snips...

A little turbine theory.
When the blades are stationary and the gas velocity flowing through
the blades is high torque is a max but HP is zero. When the blades are
going as fast as the gas the torque is also zero as well as the HP.

Right around 50% blade velocity to gas velocity HP is a max
and torque is half of what it is a zero blade velocity.

I had to explain this to all the phd's in charge of Formula One
R&D at the World MotorSports symposium :)

It is
in my video.
http://www.rotaryeng.net/Oxford-640-4802.wmv

Shaft seals made from Teflon would work. SS hubs will transfer
less heat to the shaft. Also the blades are self cooling as they
are hollow and pump cooling air through them. A feature of all
early GE aircraft turbo chargers starting back in WW I.

Paul Lamar


Paul,

Thank you for the clarification... its seems obvious now that you have explained it.

Some rough calculations for 350mph blade velocity at 7500rpm put
the direct drive turbine diameter at about 1.35 ft... not too
unreasonable.  do you see a problem with incorporating a second
stage turbine to recoup the an additional 25% from the exhaust flow?

Does anyone produce 'off the shelf' axial turbines  that are
suitable for this design?

I am trying to envision the concept of air flowing threw the
vanes of the turbine.  I can see how air would be forced
threw by way of centrifugal force but how does this cooling
air exit the system??

The only way I can see this at the moment is if exhaust
ported out to the atmosphere immediately after hitting
the turbine.  Would you happen to have any
detailed pictures of this type of system... perhaps one
of the earlier GE systems you made mention of?


Thanks much

No and in fact that is a good idea. That is a relatively cheap
and technical risk free way of doing it as turbine technology is off the shelf
from Barber Nichols.

  http://www.barber-nichols.com/

Here are some pictures of the R3350TC housings.

Paul Lamar




   Again thank you for the info and the pictures.  I was up into
the wee hours last night scething, drawing and coming up with
new questions. -In short, desktop experimenting.

I mention off the shelf turbines because hobbyist need to remain
as practical as possible.  Would you happen to know of a
Barber-Nichols turbine model/part number suitable for what we
are talking about?  Also would you have a ballpark idea of their
cost?

Also... something else that I can not pin down in my head...
Considering that it is being propelled by the motor, does a the
compounded turbine develop PEAK HP at 50% flow rate ?  I guess
this question comes to mind because in my visualizationson of
the system, I see the the exhaust flow 'chasingin a moving
target' thus closing/widing the speed gap between the exhaust
flow and the turbine. -or- does the fact that the exhaust flow
is perpendicular to the rotation of the turbine make this a
non-issue.............. I am starting to think that RPM should
be determined by the flow to the turbine with the engine
throttle being second.  Are my assumptions on on the right track
or am I missing something?

My mind is spinning faster then the turbine.

Rick.

Other than a R3350 turbine I know of no other parts numbers but
I am not into turbo jet engines. I'll bet there are a bunch
of used turbo jet turbines out there that will work.

Here are the pictures Rick is referring to.

It develops PEAK HP at 50% no load peak FREE to turn turbine RPM.
Not shown in the pictures.
Lets say the exhaust flow is 800 MPH. If the blades are going
800 MPH on a free turbine then there is no load on the turbine.

In the pictures the turbine is fixed to the e-shaft and cannot exceed
the RPM of the engine therefore the blade diameter is sized to go half
exhaust gas velocity.

At 50% exhaust flow rate the turbine HP would be way down.

The exhaust flow to the turbine will be max at wide open throttle
at the engine red line. In the case of a P-port rotary that
will be around 10,000 engine and turbine RPM with this configuration.

At wide open throttle at less than red line the flow to the turbine
will be a max FOR THAT ENGINE and TURBINE RPM!

This remains the easiest and lowest cost way to build a turbo compound
rotary to demonstrate the fuel burn advantages and HP increase. No high
reduction ratio gear box required. All that is required is a little
Inconel or SS sheet metal fabrication and an off the shelf turbine.

Paul Lamar

Paul,

One one point.  Load, Turbine/Engine RPM, exhuast flow output and efficecny
are all directly interdependent.  However, I suspect that thier
interdependence is not LINEAR in nature.  This being so, I can't
see one physical configuration covering the full range of engine
opperation.  This is why I am guessing that a variable 'pitch' turbine
would be a good idea.

How far off is my thinking on this, are there any other general factors
that I am not taking into account?


Rick

You are right on. The saving grace for aircraft, boats and generators
is they are all point design. A car engine on the other hand is a huge challenge
as you want a lot of HP for rapid acceleration and very low fuel burn
for low HP cruising on the freeway. An increase of 5 free HP at 70 MPH
would be a drop of almost 10% in fuel burn.

BTW almost every truck engine manufacturer in the world has a turbo compound
engine development program. Detroit Diesel is shipping a very good version
called the DD15.

Paul Lamar


Paul,

All excellent points.  My interest is in Rotary aircraft applicaiton.
While an aircraft PROP has a relatively narrow RPM operating band
(cruise to max power) this RPM band is multiplied in a Rotary by
the PSRU and even more so with a CS Prop.  Whould you happen to
know what the RPM range may be?

I was thinking of variable design that does not result in increased
exhaust back pressure.  Also. one that maintains a maxium HP transfer
efficiency out of the Exhaust flow.  I suppose that there are a lot of
variables/trade-offs to consider here but the back pressure variable is
one that one would like to keep way down no?

I was thinking of a variable pitch turbine where the blades of the turbine
change pitch similar to a CS prop. (much easier on a Axial turbine)  What
are your thoughts on this concept and has it been tried before?

Rick.


First back pressure does not hurt as much as one might guess.
It is something to be researched for sure.

Guys use 7500 engine RPM (2600 prop RPM) for take off and cruise at
6000 RPM (2100 prop RPM).
6000 sounds high to people because of the years of experience
with piston engines but the rotary is very comfortable with
6000 RPM continuously. That is where the loads on the rotor bearings
are a minimum. The inertia force on the rotor is counter balanced
by the compression forces. On a piston engine this occurs around
3600 RPM.

As an aside the compression forces are much higher
on the connecting rods than the piston inertia forces on
direct drive aircraft engines running at 2600 RPM. This is why some
direct drive aircraft engines can get away with low fatigue limit
aluminum rods. :) Also why geared aircraft piston engines run
in the 3600 RPM range.

Now if you are really cheap and you are not in
a hurry you can cruise the airplane in the drag bucket down
around 140 MPH or so at 4000 RPM. Prop RPM would be about 1500
or so. The unfortunate part about this is the BSFC rises so the
fuel burn is a combination of rising BSFC and lowering HP requirements.

This is an old BSFC map but the curves are qualitatively the same.
There are only minor improvements in the newer engines.

The best way to get a handle on all this is full scale test in
an actual airplane for MPG. The same thing happens in cars and
something around 55 MPH gives the best results.

Paul Lamar



Paul,

What sort of fuel burn rate are guys seeing at 7500 engine RPM
(2600 prop RPM) for take off and hat sort of fuel burn rate are
guys seeing at cruise at 6000 RPM (2100 prop RPM)

How much of a cruise RPM drop do you suppose one might get
with a CS prop?  Is anyone out there using a CS prop with a rotary?

Rick

My guess on the fuel burn is .55 to .65 BSFC full rich full power
7500 RPM 12.5:1 mixture ratio but that is typical of piston engines as well.

Fuel burn at cruise is about .47 leaned out to a couple of hundred
degrees below peak EGT. I would say the mixture is around 18:1  in
those conditions. The rotary has this strange ability to run leaner
that a piston engine. Probably due to the rotating combustion chamber.

Mark Steitle is running an electric varible pitch prop and others, not
on the list right at the moment, have also run electric variable pitch props.
In my C182 it only drops the prop RPM 50 or 100. I see a little improvement
in the MPG but it is not earth shaking.

Paul Lamar

Paul,

I noticed Barber-Nichols has updated their website and offered some
excellent explanations about the merits of both axial-reaction and
radial-impulse turbines. They mentioned hybrid combinations as well
which is the variable vane technology and IMHO the most suitable for our
use.

Turbo-compounding:  Time for a reality check, again:)
Large diesels engines have used both turbine types, and both of these
have been in small compact packages running a high rpms. The primary
design philosophy was  compactness, and keeping the heat radiation
inside the engine compartment low. Diesels tolerate back pressure better
than rotaries and have a cooler exhaust so their design approach is a
bit different.

Scale a turbine up, and the size, weight, and hot surface area increases
more. A direct drive turbine looks nice in an illustration but it is not
a KISS idea. It just moves complexity somewhere else.  And since
aviation applications are designed with a mind toward saving grams and
fitting components  with a  shoe horn, we have to ask ourselves a
question.

Is turbocompounding for my aircraft really needed?
How many people in this group plan to fly their plane for more than six
hours at a time repeatedly? I can raise my hand because that will be the
norm with several flights lasting fifteen hours. I am not totally
convinced I'll need turbo-compounding and I have been looking into it
for five years now.  Still I havn't dropped the idea just move it to
the lowest priority in the my R&D program.  Perhaps turbo-normalized at
high altitude will be enough.  Depends on the fuel choice.

A more realistic question to ask is. At my desired Hp. setting and plane
performance what do I need to do to  decrease drag and weight while
optimizing thrust?

To start with we have to input the cruise speed we want the airplane to
fly at. This answer will vary depending on the type of plane and the hp
needed to  fly that speed.  If you don't know go flying with someone
who has a plane like what you plan to make.

Decreasing drag can be done in many ways and is the subject of several
books.  Less drag equals less hp to fly your flight profile.  Less hp.
means less fuel per mile.
Next Improve the efficiency of the fuel burn.  Several concepts are in
the works that look promising. Pauls multi-strike ignition, and my
direct in-housing fuel injectors. More efficient burn of fuel gives a
better BSFC. Other ideas are still located in the gray matter.

Then think about altitude. Altitudes is a 'it depends' answer.  Someday
you can go high and fast with a tail winds.  Other days low and slower
to avoid the large headwind.
High days it is advantageous to have a turbo that can help to normalize
the engine.
This could increase the mpg. if you factor in air density, temp. and tail
winds. Fly only when their is a tail wind.  Don't laugh, on some long
ferry flights that is exactly the technique. Like sailing a day full of
tacking is a drag.

Turbos give you a nice option for more power on T/O or more power high
up. It's nice to be able to disable it though.

Turbo compounding=long distance records from Hawaii to NYC, or commercial
operations where the cost/performance is acceptable. IE replacing
turbines engines with four rotors.  The  C/P factors in  weight and
complexity and the smaller the engine the less attractive it will be.
For puttering around the airfield  and doing hamburger runs.
'Faggitaboutit,' unless you like to order $6,000  hamburgers.

Doug in Japan