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Dominic, Gary, Lorn, et al,
I am by no means a fluids or engine expert, but let me add the following to the discussion.
The inlets for cooling and induction air differ in one way that causes me to treat them very differently. The difference lies in what is downstream of the inlet. For cooling air it is simply a flow restriction, cooling fins etc. In the case of the induction system on the other hand, you essentially have a positive displacement pump that is going to keep volume flow rate tied to engine speed. For the induction system, changes in airspeed do not change the volume flow rate, but will affect the static pressure in the system. To capture ram pressure you need a delta between the free stream velocity and your inlet velocity which is being dictated by the engine. If the free stream and inlet velocities are the same you end up with no ram recovery. On one extreme is your pitot tube where the ratio is 0 and what you read is 100% ram pressure on the airspeed indicator. You can go in the opposite direction as well and end up pulling a vacuum by having a ratio greater than one. Something less than 1 is needed for ram recovery. Any losses in the induction system will show up as a reduction in MP.
For the cooling system, changing inlet size will also change the inlet velocity ratio but not as predictably as with the induction system. If you increase the inlet area to lower the ratio, you increase the volume flow rate, not just pressures. This is, of course, to extract more heat from the engine. Unfortunately the cooling system is littered with potential areas of losses and leakage. If these losses are excessive, increasing inlet size will result in more flow to the cooling fins, but at a high cost due to all the energy lost to inefficiencies which have now also increased. A similar principle applies to an aircraft as a whole. Want to go faster? Add lots of power, or reduce the drag just a little. I consider the first option the brute force method. In any case, if you minimize the losses in the system, a smaller inlet can do the same amount of work for you at the cooling fins. It is unfortunately very difficult to predict what a change in inlet area will yield in terms of temperatures because of all the unknowns. My current inlet ratio is about 0.25 for the induction system. For the cooling system it is closer to 0.7, but I cannot verify that without testing. Analytical methods are still being worked out. My induction inlet is diameter is 2.87 inches and the cooling inlets are now exactly 3.5 inches. There is a great NASA report covering cooling system losses and inlet design. From memory, I think it is CR3405. It should be in an old post somewhere. This report is very interesting reading. I highly recommend it. It is available from the NASA website. Dominic, I have attached a pdf file covering my recent cooling system remodel.
Marv, I can send you a pdf of the NASA report next Tuesday if you would like to post it on the web site.
Keep cool and fly fast,
Chris Zavatson
N91CZ
360 std
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