On Nov 21, 2007 12:51 AM, David Leonard < wdleonard@gmail.com> wrote:
This brings me to my final point (promise I won't make anymore points). Use the widest-thinnest radiator you can reasonably fit under the cowl and make appropriate ducting. I guarantee that you don't want to use the thickest-narrowest rad you can fit. That would not cool well at all.
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David Leonard
Hey, wait a minute! Why didn't you take your own advice? I'm sure you could have found a radiator thinner than 3". Did you mean to say widest & thickest? That would be the combination to use to insure the best chance of cooling adequately (but not necessarily the lowest drag or weight). Bernie Kerr did use the widest & thinnest approach on his RV-9 and it had lousy cooling.
Tracy (never promises not to make more points : )
Ok, I have sworn of making any more points here, but I am just answering the question. I did take my own advice. That radiator is about as wide as I can possibly fit without deforming the cowl into the slipstream. When I decided I wanted to beat down my cooling problems with a hammer, I made the rad thicker as the only way to add volume. - but I still contend that is more drag than I would have added if I could have made the rad wider instead.
Still, you must admit that your example does not prove the 'thin is better' theory. My example of Bernie's installation does support my 'thick is better' approach (or more accurately, 'thin is worse').
We ether rad, the goal is to let through just enough air so cooling is adequate. In all this discussion, I have never seen an argument that convinces me that the thick rad would need less air. That I why I keep insisting that we set the initial condition of the same amount of air. It is true that with the same inlets and outlets the thicker rad will flow less air (and have less drag). But the thin rad needs smaller inlets and outlets. That is how it achieves matched airflow and reduced drag.
I assume you do not buy the NACA studies that Ed has posted? I think I might see how you arrive at the thinner rad needing smaller inlet & outlet (in theory) but again, I think you underestimate the real world factors that make this unachievable. Real world rads (even thin ones) that we can buy are going to eat up that energy you are hoping to recover.
Theoretical discussion aside, I have not achieved the goal of optimum duct design (which does become more difficult as the rad get thinner) and I am currently letting too much air through my rad. So I have excessive drag. But maybe someday... :-)
I agree, but I think you underestimate the difficulty of the optimum duct for the thin rad or conversely, the advantage of the thick rad when it comes to duct design.
I do not believe that useful pressure recovery is a pipe-dream. Some report that the P-51 even gained thrust from the cooling exit (I don't think they mean net thrust from the system thought). Also, both Bill Eslick and I have the same experience that our cowl flaps want to pull themselves wide open by the pressure available at the cooling exit (well into the slip stream).
Oops, sorry, couldn't help myself.
That's OK! And it brings up a good point about the exit. I agree the exit is a key component. All my remarks are in the frame of reference of our typical 'under the cowl' cooling systems which make it much harder to do exit ducts on par with the P-51. To get any thrust from the exit you must have an exit velocity higher than the free stream air. The real world measurements on the best example of RVs (Dave Anders, Rich Jankowsky & others) show exit velocities in the range of 1/2 or less compared to free stream. This makes them a source of drag rather than thrust. And until you get the exit air at a higher velocity, the less air coming out means less drag (all else being equal).
Ignoring real world factors for a moment, let's look at the theoretical side.
Pushing the limits of my own understanding, there is another factor in recovering that energy added to the cooling air stream by the heat exchangers (which the P-51 did to some extent). This is how a jet engine works. As we know, head pressure generated by the compressor stages is absolutely necessary to the operation of the turbo jet. The diffuser is what functions as a compressor in the cooling system that generates thrust (or at least reduced drag). A thin & wide rad can't have as much head pressure as a thick & narrow rad flowing the same volume. Without head pressure, you can't recover the energy of the expanded air volume.
As I think about this, to get the thin rad to work in this scenario, most of the head pressure in the system would have to come from the exit duct which I think is where Dave is going with his argument. This argument is not without it's own merit. I just don't think the system that takes advantage of it could be built with off the shelf components and installed within the confines of my cowl.
Tracy
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