Originally posted by Badboy
I know it is tempting to use aerodynamic data, but you can't reach conclusions about deceleration by comparing Cdo values alone. The reason is that deceleration comes from two terms, the prop drag divided by the mass of the aircraft and the total aerodynamic drag divided by the mass of the aircraft. The aerodynamic drag is made up of two components, induced drag and profile drag. At high speed, where you would normally be able to assume that the induced drag was negligible, and you might therefore be able to compare the zero lift drag coefficients, you still can't, because unless the prop can be fully feathered the prop drag will have a very significant impact on deceleration. At lower speeds, this is compounded as the induced drag increases, so regardless of where you are in the envelope, a comparison of Cdo isn't a reliable indication of deceleration.
Badboy
I understand. I also understand the relationship of drag and mass to deceleration. Added weight should increase induced drag at lower speeds. If for no other reason than needing a higher AoA to maintain level flight. If that is factored into the modeling, it's completely masked (in terms of test data) by the increase in mass. You can observe the effect of added weight visually while testing. At low fuel, with Combat Trim on in Auto-Level, cut power and watch as the aircraft is trimmed ever nose high to maintain level flight. Add weight and this occurs at a higher speed and the angle of attack is eventually greater. This would certainly increase induced drag, to my thinking at least.
By increasing the fuel weight of the P-47D-40 by 1,372 lbs, it decreased the speed bleed time by almost 9%. So, a 9.5% increase in mass resulted in a 9% decrease in deceleration.
I tested the La-7 with full tanks, or 122 gallons. This added an additional 549 lbs to the plane's mass. This was roughly a 7.5% increase in mass over the 25% fuel weight of the plane. When tested, this yielded a 7.2% decrease in deceleration.
This tells me that same rules apply to both aircraft. The problem as I see it is that the La-7's baseline mass is either too high or the total drag is too low (or some combination thereof). Can we assume that the La-7's total drag is greater than that of the P-51D? I think it's probably a safe bet. Even if these were identical, there's no getting around the difference in mass. There is no doubt that the P-51D has a far greater mass (about 30% more). You don't need a full-blown analysis to see that something is unusual here. It's the old dead skunk analogy; you really don't need to see a dead skunk to know it's there.
I'm sure Hitech and Pyro will be looking at the equations to see why this exists. I'm also curious to know why the Spitfire decelerates like it's tied to a figurative tree. Must have the draggiest propeller in the history of aviation.
Seriously tho, sometimes simple testing can reveal things never thought to be unusual. In hindsight, I always thought that the La-7 retained E far better than I would have imagined prior to flying it online. I never expected it to be anything more than my own perception.
My regards,
Widewing
