What Al, said George with a heavy dose of
what Ernest Said.
As Al said, you really need to start
knowing approx how much heat you will need to remove under the worst
conditions. If you don’t know that then all else is probably worst than
guess work.
For my book, take off is the worst
of all cooling conditions. You are producing maximum heat and have
minimal air mass flow for cooling. Yes, you can add things like core water
sprayers that can temporarily help for a limited amount of time. But, in
my opinion, the best approach is to first determine you cooling requirements as
in how many BTU/min you need to get rid of under the worst conditions unaided
through your cores.
The most efficient way is to transfer the
heat directly from coolant/oil to the air. However, space and other
considerations may force you to consider oil/coolant heat exchangers for
example – but, you get the greatest heat transfer efficiency going from
liquid medium straight to the air medium in one step – any intermediate
steps in heat transfer are going to cost you some efficiency. But, like I
said - at times there may be reasons to do otherwise.
Yes, you will have to make some
assumptions, like what will be the delta T across the core – I think
80-100F seems to be achievable across a coolant core and perhaps 40-60F across
your oil cooler. But, until you determine the size of core you that
will meet your requirement, its premature to start getting concern about the
inlet size.
Now if for some reason (design of the
aircraft perhaps), you are really constrained to inlet size, then you may need
to work it from maximum inlet size to the core size you need.
From what I have read it appears that an
inlet area ranging from 25 – 40 % of your core surface area is in the
ball park. At least you can start there, then calculated your mass flow
needed to get rid of the requirement amount of heat through your core.
Then determine which size inlet will give you 10% of your cruise airspeed or
30% of your climb airspeed after being slowed down by the diffuser. But,
all of this is just back of the envelope calculations to get you in the ball
park. As Ernest pointed out, some folks will heat the nail on the head
the first time – but, most of us have to work at it a bit.
Ed
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Al Gietzen
Sent: Monday, January 25, 2010
7:52 PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re:
Wedge/Oblique Duct
George;
I haven’t been following this
thread, and stepping in here may reflect that; but I don’t really
understand your question. The size of the inlet opening is determined by
the amount of heat you need to reject, best taken as corresponding to something
close to the climb power. That; with the density and specific heat of
air, gives you the cfm required. When you know that, and the approximate
climb speed of your application, you can calculate the required inlet opening.
Having done all those calcs, I (or ED,
I’m sure) could scale those numbers for you fairly readily to your power
level and speed.
Other things follow from there;
including the face area of the rad. Optimally, you’d like to have a way
of adjusting inlet and exit areas for absolute minimum drag; but that’s
very complicated. If you can do one of those, adjust the exit area. If
both are fixed and you want least drag, you can size the inlet for something
closer to cruise speed, and add the complexity of the spray-bar idea for added
cooling during high power regimes.
It’s good to have some sort of
control over the amount of cooling to maintain engine temps within some
reasonable range. If the cooling system is sized for climb on a hot day,
a thermostat is a way to maintain temps for other regimes. There is the rare
possibility of a thermostat sticking closed, but that is something that
hopefully would be noted before takeoff.
Not that I advocate this approach, but I
have a setup that doesn’t require any pilot intervention and serves quite
well. I have 2 coolant radiators, one which can handle the cooling for
normal cruise (both coolant and some of the oil heat), and one (connected in
parallel) that can handle maybe 30-40% more (in the wing root with presumably a
fairly low drag inlet/outlet. The second has an in-line thermostat, so only
comes into service during high-power operation, or very hot days. I have
an oil/air cooler that handles normal cruise power, and small oil/water
exchanger (in parallel) that keeps the oil temps at a safe level for high power
by transferring heat to the coolant. Yeah; the plumbing is a little more
complicated, but I like the way it works.
(Sorry, it seems what starts out to be a
simple answer always turn into more complicated discussions).
Al G
-----Original Message-----
From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of George Lendich
Sent: Monday, January 25, 2010
2:31 PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re:
Wedge/Oblique Duct
I have been thinking about this, if
I use A1V1=A2V2 to solve for the inlet opening size, how do I know if the speed
of air through the duct is correct i.e. 10% for cruise and 30% for climb.
Also If I use the Cessna 171 speed
of approx 100 K for cruise = 10 K through core, 70 knot climb and approx 20 K
through core. Do I then use the climb speed to calculate inlet air openings and
attach an adjustable louver exit OR I suppose I could do as Tracy does and calculate
for cruise and attach a spray bar.
I'm still a little confused on the
best approach.
BTW it's Australia Day today -
arrival of the first fleet.
George, For the bell
shaped duct, what I have read in K & W it appears they say the inlet
area should be between 0.25 – 0.40 of the core area. The more
slowing of the air velocity that needs to be done by the diffuser before the
core that is needed appears to call for lower area ratio values of Ai/Ab (area
inlet/area core). In the example plotted of an streamline duct in K&W
the chart shows the area ratio was 0.40.
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of George Lendich
Sent: Wednesday, January 20, 2010
6:16 PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re:
Wedge/Oblique Duct
If Ai is .33 Ab and .44 Ab for the
wedge ducts, what's it for the bell shaped duct - is there any similar
drawing?
----- Original Message -----
Sent: Wednesday,
January 20, 2010 3:39 AM
Subject: [FlyRotary]
Wedge/Oblique Duct
George, here are two figures from
K&W – one could be described as a “Wedge” figure 12-6 and
the other one a “streamline” wedge figure 12-12.
Always dangerous for me to try
to interpret what K&W are really saying, but from what I get from these two
figures, it would appear that the “streamline” wedge permits you to
use a smaller inlet (Ai = 0.30 Ab) for the same performance (same losses) that
you get with a larger opening Ai = 0.44 Ab for the pure Wedge. But, that
is just my take on it.
The slight bump (where the 64 deg
angle is annotated) of the Streamline wedge may be to increase the boundary
layer velocity to delay separation of the boundary layer in that far
corner where duct meets core.
Ed
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
http://www.andersonee.com
http://www.dmack.net/mazda/index.html
http://www.flyrotary.com/
http://members.cox.net/rogersda/rotary/configs.htm#N494BW
http://www.rotaryaviation.com/Rotorhead%20Truth.htm
__________ Information from ESET NOD32 Antivirus, version of virus signature
database 3267 (20080714) __________
The message was checked by ESET NOD32 Antivirus.
http://www.eset.com
--
Homepage: http://www.flyrotary.com/
Archive and UnSub:
http://mail.lancaironline.net:81/lists/flyrotary/List.html
__________ Information from ESET NOD32 Antivirus, version of virus signature
database 3267 (20080714) __________
The message was checked by ESET NOD32 Antivirus.
http://www.eset.com
__________ Information from ESET NOD32 Antivirus, version of virus signature
database 3267 (20080714) __________
The message was checked by ESET NOD32 Antivirus.
http://www.eset.com