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In Mendenhall's book about the Gee Bee racers, he describes that on the Models Z, R-1, and R-2, there was a strong tendency for the engine cowl to get pulled forward and into the prop during races, requiring reinforcement of the cowl attachment points to the engine & airframe- which puzzles me to no end. Can someone please explain the pressure distribution right behind the prop that would cause this?

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    $\begingroup$ IF that's the case, then there must have been an imbalance of pressure on the forward and back side of the cowling. It might be more intuitive to think about the cowling being pushed forward from high pressure behind the cowling than being pulled. $\endgroup$
    – Jim
    Jan 26, 2023 at 6:29
  • $\begingroup$ On airplanes with sliding teardrop bubble canopies, the aerodynamic forces can sometimes push the canopy forward and if they are unlatched they just sit there. $\endgroup$
    – John K
    Jan 26, 2023 at 17:00
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    $\begingroup$ I have not done any sort of aerodynamic analysis of this but I'm speculating that another factor might be a lower-pressure region behind the center area of the propeller caused by the centrifugal force of the swirling airflow. This is likely negligible for "normal" propeller-driven aircraft but the GB racers push the envelope of performance. $\endgroup$
    – jwh20
    Jan 26, 2023 at 17:20
  • $\begingroup$ This has a lot to do with proper understanding of engine compartment air flow and should be studied further. $\endgroup$ Apr 19, 2023 at 7:13

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You are looking for 'cowl lip suction' or 'lip thrust'. This is a phenemona that also happens on jet engine cowls when they have lots of spillage.

When the cowl flaps are (nearly) closed, the engine bay approaches stagnation pressure. Many of the streamlines that would otherwise go into the front of the cowl instead are diverted around the outside -- this is a situation similar to engine spillage (when mass flow can't go through the engine, it goes around).

This air is forced to rapidly accelerate around the cowl lip -- this region of high acceleration leads to high velocities which lead to a local suction.

Here is a panel code solution demonstrating this.

The magenta represents the inside of a blocked engine -- the pressure there reaches the stagnation pressure for the flow.

The green and blue around the cowl lip represent high velocities and strong suction pressure -- some of that area has a forward-pointing outward normal vector. This means that the local suction will resolve into a thrust.

If the cowl were truncated somewhere on the constant-diameter section of this nacelle (and you can imagine the entire engine compartment is at stagnation pressure (magenta) then you can see how the cowl would have a net forward force on it.

enter image description here

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  • $\begingroup$ Rob, really appreciate your explanations, and I followed this up until the last sentence, which seems to suggest that the right shape can provide thrust for free (once whatever speed is initially attained). Does the "net forward force" exclude the rearward force on the magenta area, or is something else going on here to preclude the free lunch scenario? $\endgroup$
    – Ralph J
    Apr 19, 2023 at 4:07
  • $\begingroup$ @RalphJ "this region of high acceleration ... leads to local suction", you know, like wings and propellers? $\endgroup$ Apr 19, 2023 at 7:09

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