|
Less we forget how important drag is in our hobby, I took
a formula for calculating drag at different airspeeds and the Hp required to
push the given frontal area along at the stated airspeed.
This is for two of our traditional GM evaporator cores
using their combined frontal area of 180 sq inch or 1.25 sq feet.
This assumes that airspeed shown represents the velocity through the
cooling core (which is not really likely to reach speeds above 80 mph if you
have any sort of ducting), but that is an assumption on my part since as Bill
keeps reminding me I have not instrumented my ducts {:>)
|
Air Speed (MPH) |
HP
|
|
40 |
0.533333 |
|
60 |
1.80 |
|
80 |
4.27 |
|
120 |
14.40 |
|
140 |
22.87 |
|
160 |
34.13 |
|
180 |
48.60 |
|
200 |
66.67 |
Clearly the faster your cruise speed the more important it
is to minimize cooling drag. Of course the airspeed the core sees should
normally not be over 10% of your cruise speed or 30% of your climb speed
(According to Horners rule of thumb). So slowing down your cooling airflow
to lessen drag is one reason for paying some attention to your ducting.
However, cooling again depends on many other variables, for instance accepting a
high velocity airflow through your core may permit you to use a smaller frontal
area core thereby offsetting to some extent the higher drag. In
fact, space constraints may force you to his configuration
regardless.
Another factor to consider is trade off between frontal
area drag and thermal transfer efficiency. A large thin radiator is
theoretical the most efficient due to that factor. However, it disturbs a
larger segment of air (resulting in higher drag) - not really important in an
auto at 60 mph but very important in a Cozy at 200+
MPH.
A thicker core with smaller frontal area disturbs
less air and while it has more skin drag that is small compared to the frontal
area drag. Tracy refers to the approach of thicker cores as
"... getting the most cooling possible for the smallest column of air
disturbed". So while theoretically the thicker core is less thermodynamic
efficient - it turns out with sufficient dynamic pressure available it provides
definite benefits in our application. The average thickness of
NASCAR radiators is 3" and up to 7" for the longer high speed tracts.
Since they operate in speed regimes close to what most of us fly - they just
might know what they are doing given the $$ they will spend for even a slight
speed advantage.
Ok, back to creating a company - boy, a lot to
learn
Ed
|