hevansrv7a
Well Known Member
Introduction
I'm working with Jack Norris ( www.propellersexplained.com) on the numbers for my 7A with a SJ cowl. See Appendix "SSSS" in the book. The short version is that the air that the inboard portion of the prop encounters is not at the TAS. Rather, it is slowed down by the push-back from the cowl. The slowdown is greatest nearest the prop, declines as you go aft. The angle of attack of the blade is the result of the forward vector (slowed) and the rotational speed (greater as you move outboard on the blade). Therefore the design of the prop must compensate for the cowl in order to be entirely correct.
The Measurement
So, with that background, it is necessary to measure the square inches of cowl at a number of stations from front to back. That is then converted into an equivalent "body of rotation" which is sort of bullet shaped and has, at each station, the same area as the cowl, but converted into a circle. In the book you can see the measurements for an RV-8. Those were the numbers used for the Whirlwind RV prop.
The Problem
The problem is the air that enters the cowl for cooling and for combustion. That air is not pushed back as hard/as much. So, to correct the numbers, it would be very helpful to know either the velocity of the air entering the cooling inlets or the static pressure inside the cowl, above the engine. If you have such data, please help. Thanks in advance.
I'm working with Jack Norris ( www.propellersexplained.com) on the numbers for my 7A with a SJ cowl. See Appendix "SSSS" in the book. The short version is that the air that the inboard portion of the prop encounters is not at the TAS. Rather, it is slowed down by the push-back from the cowl. The slowdown is greatest nearest the prop, declines as you go aft. The angle of attack of the blade is the result of the forward vector (slowed) and the rotational speed (greater as you move outboard on the blade). Therefore the design of the prop must compensate for the cowl in order to be entirely correct.
The Measurement
So, with that background, it is necessary to measure the square inches of cowl at a number of stations from front to back. That is then converted into an equivalent "body of rotation" which is sort of bullet shaped and has, at each station, the same area as the cowl, but converted into a circle. In the book you can see the measurements for an RV-8. Those were the numbers used for the Whirlwind RV prop.
The Problem
The problem is the air that enters the cowl for cooling and for combustion. That air is not pushed back as hard/as much. So, to correct the numbers, it would be very helpful to know either the velocity of the air entering the cooling inlets or the static pressure inside the cowl, above the engine. If you have such data, please help. Thanks in advance.
