Subject: [FlyRotary] Re: radiator


>
>
> Great post, Al.
>
>  Just a comment on one of the "assumptions" we all start with when
> discussing estimation of cooling system sizes for hot weather:  How much
hp
> is being developed, i.e., how much heat is to be rejected?
>    - I wonder if everyone is "assuming" that they must calculate radiator
> areas based on heat being generated/to be rejected while generating 100%
> power, i.e., 160 or up to 180-205 for a n/a rotary, depending on what you
> think you will be getting?
SNIP

> So, these become "constants" to be applied at the outset of setting up
> (calculating) our "heat to be rejected" value, which value is the starting
> basis of all subsequent calcualtions of radiator size, pressure drops,
> velocity changes, etc, etc, that the equations work on.
>     -  In other words, if I have a 160hp engine, I should NOT design my
> cooling system to reject 160hp worth of heat on a hot summer day climb
out.
> I should be using some lower value that was objectively arrived at as
> discussed above.
>
> David Carter
>


 So good points, David.  While I agree with you (in theory {:>) the down
side is - if you are betting your cooling on implementing a "near perfect"
cooling system with no margin for that 160HP on a Hot day, and  if it turns
out your cooling system capability is less than you plan for (for any number
or reasons) OR you ARE producing 160HP on that hot day, then I think you may
find yourself with over temp problems.

The one thing that I have learned is that all the theory does is get you
closer to an approximate answer.  There is still that semi mysterious world
of imperfect implementation that can (will?) result in less that your
planned performance in a cooling system.

If you find yourself with a bit "too Much" cooling, the worst is you are
going to loose a bit of top end speed (I don't like the cost of running at
top speed - but others may not mind the fuel burn). On the other hand, if
you plan your cooling system "too close" and you are off on the inadequate
side - then you could have a major redo and additional expenses.

You will likely find that if you do plan your cooling system for cruise then
you are likely to overheat on climbout. My cooling system has 50% more
cooling capacity at cruise than I need, but my coolant temp will still get
up past 210F on a hot day climb out.  Remember, you almost always have to
pick some performance point for  the design of your cooling system - be it
take off , climb or cruise.  Whatever point you pick -  if you are not
operating on that point for some regime of flight, you could  either have
excess cooling capacity (not a disaster) or inadequate cooling capacity
(that could hurt). True, fans and cowl flaps can aid in minimizing excess
cooling capacity, but you can encounter less than expected in that arena as
well.

Keep in mind that the rotary reportedly does not withstand overheating as
well as the old V8s etct.  While I do not think it is as delicate in this
area as I once did (and they have improved the design of the castings over
the years), I know you can overheat them without really trying hard(don't
ask me how I know).

From my decade of listening to folks with problems getting the rotary
powered aircraft into the air,  inadequate cooling initially has been a
problem for at least 80-90% of them (including Tracy Crook, yours truly and
a number of others). Yes, you can assume our knowledge of what it takes to
cool a rotary in real world was inadequate at that point, but it wasn't
because we didn't attempt to use the best information available as well as
our best educated guesses {:>).  I have noticed that more recent launches do
seem to be doing better in the cooling area which is good to see.

 One reason for the "cooling issue" that it is difficult to optimize for all
flight regimes as there is the wide range of power you may produce.  At take
off, most of us want all the power we can get (say 160HP) at which I am
producing approx 5000 btu of heat that needs to be rejected by the
radiators.  On the other hand at fast cruise I am producing apporx 80HP at
7.5 GPH that produces 2500 BTU of heat for the radiators to get rid of.  So
there is a 2 fold difference in cooling capacity needed.  If I optimized for
the 2500 BTU cruise figure at 170 MPH TAS at 81 HP, I could get by with one
evaporator core and still have a 17% reserve cooling capacity at cruise.
However, if I go to take off with the one core, I will be overheating until
I reach an airspeed of approx 145 MPH TAS.  The question is - would I feel
comfortable overheating (and by quite a bit) until I reach 145 MPH TAS and
the answer in my case is - NO.  You on the other hand, might be willing to
accept that.

Not to discourage you from using every tool in planning for your optimum
cooling system, but I think you need to be careful about trying to cut it
"too fine" with standards.  Additionally each homebuilt and its engine
installation is different.  Even the Lycoming guys (and you would certainly
think they would have settled on a standard decades ago) still have problems
(in some cases) getting their cooling sorted out.

But, this is  just based on my experience,  Perhaps we are to the point that
"point design" for a cooling system will be successful with smarter more
experience folks who will bring it off.  So best of luck in your cooling
design.

FWIW

Best Regard

Ed Anderson