Originally Posted by Marc Bourget
If pressure recovery ranges between .6 and .85 and 100 mph gives us 4.94" H2O, we're down to 3" with a poor cowl installation. So, what can we count on ??
Let's polish a few points.
4.94" available Q would be at sea level, so at any realistic altitude (or high density altitude), theory says we don't even have that much.
The stated velocity of 100 MPH (87 knots) is somewhat slower than normal RV climb speed. It does illustrate how difficult the cooling issue can become with STOL aircraft, IF they are expected to maintain high AOA and full power for an extended period. It's not a problem for either if we're just doing a quick climb to clear the trees.
A subtle detail...velocity for Q calculations assumes true
airspeed, not indicated airspeed. Readers doing pressure measurements must convert to TAS when calculating available Q, or the pressure recovery ratios will appear better than they really are.
On the positive side, we usually assume steady state level flight when calculated available Q. With an RV, the given 87 KTAS in level flight would be at a very low power setting, thus there would be little velocity in the propeller outflow. A climbing RV with a big motor at 27/2700 and 87 KTAS would have quite a lot of prop outflow. The trick is to harvest that outflow velocity as increased upper plenum pressure. So far, it appears there is a lot of variation in this area. RVs with inboard inlet area (closer to the spinner) and prop blades with inefficient blade root airfoils (like the round roots seen on some composite CS props) are not going to get much increased Q due to prop outflow. Efficient inboard blade sections and good outboard inlets can harvest quite a lot. The 390/Hartzell BA/outboard low ratio inlet combination is picking up about 33% compared to Q due to aircraft velocity alone.
CR3405, prop outflow as a function of blade radius: