Aerodynamics Lesson

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I took the Better Half's Boxster S on a long, top down errand by the shore. Cloudy but warm and very gusty so I kept the top down through some light showers but got caught in some highway stop and go traffic when a downpour hit so I raised the top in motion (crawl speed, handbrake up just enough to light the light and allow top movement), but the top cover didn't fully retract, remaining open about 1 inch. No big deal, I figured i'd drive like this until the next exit. I do not typically do things like this, but we are talking maybe 3 miles.

Traffic cleared and I accelerated watching the cover to make sure it wasn't buffeting, which it never did. Then as I accelerated through ~70, the car started to feel very light and twitchy, completely unlike how it typically drives. The feeling increased with speed. Wasn't ight at the nose or tail, the entire car felt light. Then a crosswind gust hit that really caught my attention, enough that I slowed way down and pulled off at the next exit. The felt instability was very noticeable and, for the GA pilots here, felt very similar to the few seconds before rotation where the wing is starting to produce significant lift and the plane is light on its wheels and feels like it is skipping (it is not) down the runway. Fun in a Cessna...not so much on Rt 95.

I walked around the car with the cover still slightly raised and then it dawned on me. The Boxster has a small spoiler that raises at ~70mph and I assume that the slightly raised cover was smoothing the airflow from the top over the rear and also shrouding the rear spoiler, essentially causing the car to generate a considerable amount of lift...very effectively.

I built a wing.

I have even more respect for the engineers and aerodynamicists in Stuttgart to design a car and system like this. Less respect for me for disrupting it
grin.gif
Rather impressive demonstration though and another lesson learned.
 
Originally Posted by JLTD
Very interesting....wonder if any of the convertible top hardware was stressed or damaged.


Not at all.

The fabric top was fully up and locked, only the metal cover was raised about 1", and it and its supporting linkage is a pretty robust structure. I'm rather OCD with our fun vehicles and I was closely watching the cover the entire time. It is close behind the top, never shook, twisted or appeared to have any significant aerodynamic load on it throughout this. Apparently it doesn't take much force to significantly affect airflow....I knew this, but was surprised at how significant the effect was, on a car, at a low speed.
 
Originally Posted by AZjeff
Whew! Thought this was headed for the top coming off at first. Looking at your roll call, which do you consider not a fun vehicle?? Amazing fleet.


Thanks..they are all fun in different ways. There are a few others that are in process of being sold, the buy/fluff n buff/sell hobby supports keeping the fleet.
 
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