06-26-2013, 10:38 PM | #1 |
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aerodynamics
Does anyone have figures on the comparison of coefficient of drag on both a '30 town sedan and a '31 with slant windshield (and no wind-catching visor)?
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06-26-2013, 10:53 PM | #2 |
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Re: aerodynamics
Does anyone have figures or calculations indicating the HP required to push say a '30 model A coupe at 35 mph to 70mph in mph increments of 5? Assume conditions at sea level, 70 deg F., 70% RH with 35 psi in 475-500x19's and 600 wt gear lube at operating temp, no drag in brakes, etc.
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06-26-2013, 11:15 PM | #3 | |
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Re: aerodynamics
Quote:
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06-27-2013, 12:09 AM | #4 |
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Re: aerodynamics
WHY ??
Paul in CT |
06-27-2013, 12:13 AM | #5 |
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Re: aerodynamics
It may be related to his driving a speedster. just a hunch
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06-27-2013, 03:49 AM | #6 |
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Re: aerodynamics
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06-27-2013, 09:38 AM | #7 |
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Re: aerodynamics
that's really getting inovative!
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06-27-2013, 10:37 AM | #8 |
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Re: aerodynamics
Jim may have a valid point. How many of us have tried to open up the windshield while tooling along at only 35 MPH? It's almost impossible, at least with one hand! I was very surprised how much wind force is against that windshield. The slant sedans, sans visor and canted w/s frame may provide some streamlining, but I still have to think only a bit, maybe 5%, taking the whole front end in consideration.
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06-27-2013, 09:19 PM | #9 |
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Re: aerodynamics
Even though I build speedsters pretty much full time, I don't have one for myself right now, as I was behind on delivery and sold my demonstrator to a waiting customer. Also the deer are thick here so I feel safer in my CCPU with a fairly stout wind shield and posts. Chances of surviving a deer collision with no windshield are slim. But anyway, it takes noticeably more HP to push the truck at 60 to 65 than the speedster. The speedster, with less HP, would just keep accelerating right up to the point (about 75 mph) when I remembered I was winding up a nearly stock lower end and would back down to about 55. But it didn't seem to need a lot of HP to cruise above 60 or so. In my ccpu, you can feel the "barrier" at about 63 mph and its no fun to mash the gas just to prove how fast the truck can go. With a "97" on it, it wasn't noticeable, but I'm back to the sweet running single B carb and I think more sensitivity to strain on the engine.
So I'm thinking now about fuel mileage, or actually fuel wasted and unnecessary strain on the engine to push a frontal profile that has been compared to the side of a barn through the wind at 45-60 mph. I'd like to have precise measurements rather than my own rampant speculation, but here is some speculation anyway. I can't push the windshield out at 20 or so mph which makes me wonder how much HP it uses up at various speeds. Of course, it is only a small portion of the overall drag situation I did some aerodynamic study and experimenting, with measurements, when we were racing sports cars in the 70's (and also made some horrible and embarrasing blunders in this area, that I could tell you about but not right now). So I have some ability to almost see what air is doing as it moves past various shapes. And I can almost see the air slamming against the windshield, then deflecting up to the visor where it curls around forward and around and hits the WS again. I punched the visor full of louvers (slats, for you pilots, as the idea was to relieve trapped pressure and promote laminar flow over the apex), knowing it wouldn't make any measureable difference, but just couldn't stand the idea of trapped air and extreme turbulence there. You would be horrified to know that I've made a couple of '29 slant windshields with no header and no visor out of tudors just to see if it makes a difference. One, I slanted so far back that the windshield had to be opened to get clearance between the glass and the steering wheel. Well, now that you're thoroughly disgusted with me, tell me what you've done to significantly improve your A mileage and what mpg you're getting. |
06-27-2013, 09:27 PM | #10 |
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Re: aerodynamics
Craig and Eddie!!
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06-27-2013, 09:54 PM | #11 | |
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Re: aerodynamics
Quote:
i think with the A efficiency is going to depend on tire PSI, how efficient the engine is running (like advance/reatrd exactly spot on for peak power output), driver style, and elevation and how exact the fuel air mix is to a given elevation and temperature. i feel aerodynamics for an A are about as good as a garden shed and not a whole lot you can do to it short of making your own body like a dome/jellybean (think geo metro or small hatchback shape)
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06-28-2013, 03:52 AM | #12 |
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Re: aerodynamics
If you looking at reducing drag on a Model A, reducing the frontal area is what you want to do! Those posts above, talking about the windscreen etc are spot on.
You not going to be able to reduce the Cd much, but if you can reduce the frontal area, the HP you have will be more effective at moving the vehicle. For economy, you'd have to go slower :-) The increase in HP required to increase speed is very exponential. A slight tangent: I find it hilarious how car makers spout off about the Cd of the cars they make. The thing is, without knowing the frontal area, the Cd is just a number. You can't use it as an absolute comparison car to car. The only way to compare is to know the frontal area, and the Cd: Example: 1990 Range Rover Wagon Cd= 0.45 Frontal Area=29.1 sq feet Cdxsq ft=13.1 1990 BMW 325i Convertible Cd= 0.45 Frontal Area=20.9 sq feet Cdxsq ft=9.41 The BMW is still a LOT easier to push through the air!
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06-28-2013, 08:46 AM | #13 |
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Re: aerodynamics
I'm liking these responses. Any practical measures or mods that were doable by 1931 or at least no later than 1945 would be interesting to me. I would like to know what fuel mileage some of you are getting and how you're doing it. I can tell you (you already know) that an A engine with larger valves, ported, port-matched, 7:1, etc. is very sensitive to ign. timing and very sensitive to any variation in timing cyl-cyl (slightly inaccurate dist cam or worn dist)
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06-28-2013, 08:55 AM | #14 |
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Re: aerodynamics
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06-28-2013, 09:11 AM | #15 |
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Re: aerodynamics
HP v RPM and RPM v SPEED.jpg
Also see "The Ford Model 'A' Engine" on page 123 of Volume 7 of "How To Restore Your Model A" which provides horsepower and torque vs rpm (corrected to 29.2" Hg and 60 F).
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06-28-2013, 08:15 PM | #16 |
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Re: aerodynamics
I seem to recall that the selling point of the Chrysler Airflow was that most car of the time were more aerodynamic going backwards.
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06-29-2013, 10:58 PM | #17 |
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Re: aerodynamics
On my 30 coupe with OD,The original style w-s wiper arm assy would get ripped off by turbulence at 60,but at normal cruising speed of 45,it was fine.I now leave the wiper arm off & use Rainex.--Problem solved!
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06-29-2013, 11:49 PM | #18 | |
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Re: aerodynamics
Quote:
We know from talking to many people that a model A with a slightly modified engine (so called touring) will go approx. 75 mph. These engines usually develop around 60 to 70 hp. With a couple of data points you can guestimate a curve. I can tell you that the 450 for 150 is fairly close because the SBC in one of my A's (stock body 2 door) pushed a 38 Chev coupe 154 at Bonneville and it will go 140 by GPS on the stretch of road I have to test on.(still accelerating) The wind noise and buffeting at 140 is quite loud even through a helmet and ear plugs. I think the easiest way to get the data you want is to get one of your cars on a chassis dyno and get hp at a given mph. Then you can figure in the frontal area/Cd. This will get you practical data but a wind tunnel would be the last word. |
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07-02-2013, 12:31 PM | #19 |
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Re: aerodynamics
Here is some front end airflow for you.
I spotted this in front of a repair shop in Brainerd, Minnesota. |
07-02-2013, 02:36 PM | #20 |
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Re: aerodynamics
And how about the fenders scooping air at higher speed - poor man's power steering.
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