Shortround6
Major General
Part of the problem with the co-efficient of drag numbers is that they vary depending on who was doing the test or where.
Unless done in the same wind tunnel under the same conditions you are going to get a lot of variation even in the same airplane. You can get some variation between wind tunnel test/s and actual flight tests.
Conditions are important as very few of these secondary sources really give the conditions. One chapter in AHT simply says they are the zero lift drag co-efficient at moderate speeds.
Same moderate speed for all US aircraft? Same moderate speed for all allied aircraft? Same moderate speed for allied and axis aircraft?
Surface prep of aircraft in question?
When it comes to the drag of air cooled radials vs Liquid cooled in-lines I would really only consider data from air-frames that used both engines like the P-36/P-40, Hawker Tempest, Kawasaki Ki-61/Ki 100, Lagg-3/La-5? and the like. Even planes like the Macchi 200--202 have differences in the fuselage that would alter the results.
Both liquid cooled and air cooled engines also changed radiator set-ups or engine cowls/baffling on the same aircraft at times making comparisons a bit more difficult.
Unless done in the same wind tunnel under the same conditions you are going to get a lot of variation even in the same airplane. You can get some variation between wind tunnel test/s and actual flight tests.
Conditions are important as very few of these secondary sources really give the conditions. One chapter in AHT simply says they are the zero lift drag co-efficient at moderate speeds.
Same moderate speed for all US aircraft? Same moderate speed for all allied aircraft? Same moderate speed for allied and axis aircraft?
Surface prep of aircraft in question?
When it comes to the drag of air cooled radials vs Liquid cooled in-lines I would really only consider data from air-frames that used both engines like the P-36/P-40, Hawker Tempest, Kawasaki Ki-61/Ki 100, Lagg-3/La-5? and the like. Even planes like the Macchi 200--202 have differences in the fuselage that would alter the results.
Both liquid cooled and air cooled engines also changed radiator set-ups or engine cowls/baffling on the same aircraft at times making comparisons a bit more difficult.