Engineers: I'd like input on two downforce measuring ideas

[304065]

Jackster, I'm going to port-tack the fleet here and suggest that what you want to actually measure is not mechanical movement but pressure. Air pressure, specifically, from a differential pressure transducer with a port on the bottom and on the top of your airfoil.

Check out the Motorola MPX4115AP. There's also a differential with two pipes on it, I dont' have the part number handy.

[Chuck Moreland]

With Peter's confige, 4 syringes may be strong enough. But I'd do some testing.

It will be a challenge to secure them to the wing and the uprights, solidly.

A breakaway would not be pretty.

Really you only need to test one side of the wing. Being symmetrical, you could just afix the opposite side to the upright and double your readings (eg. 2 syringes)

[Garen]

Interesting (and complicated) procedures. Do you think that a purely visual reference would do the trick? How about mounting a video camera to a spot on the chassis, and point it to a measuring stick (ruler?) on the suspension. There would also have to be a pointer that is affixed to the chassis. I know it won't be able to feed data to your dataloger, but you can "log" video, and compare to different wing combos. It might be easy to just see the differences between bumps, and the consistent ride height reduction that you would associate with downforce.

Forgive me now for possibly sounding too green to this physics stuff...!

[vlocci]

Jack,

In theory I like the syringe idea, but in practice I think your asking for trouble. Aside from cheap, they have little going for them. You have no extension stop (i.e. with enough movement plunger leaves syringe body and wing leaves uprights), the plunger is not designed to deal with off axis forces and the "stiction" of the o-ring is not repeatable.

Aside from all the other stuff I spouted about, if your stuck on cheap spring for a pair of small air cylinders. These will give you a mechanical stop, have better seals, and can deal with the forces a little easier. They also have mounting provisions which will likely work with your wing.

For cheap, check out surpluscenter.com under the pneumatics tab. Many cylinder choices for under $6.00. Plan on using a fluid like water in the cylinders.

Pick up a guage from them for another 3 or 4 bucks. Use a nonliquid filled cheapie, since you don't want to dampen the needle movement. Depending on your mounting of the cylinder you may need a vacuum gauge instead of a pressure gauge. Scrounge some cheap tubing to connect.

The math behind this is free. P=FA. An 1 1/8 bore gets you an almost 1:1 relationship. 1PSI on the gauge is 1lb force.

Your at $10.00 per "load cell". You could get away with 2 at a minimum. Think of this as the Burrito and a beer solution.

Vin

[Jack Olsen]

I'll look into the cylinders.

But the syringes would not play a structural role in holding the wing to the uprights. They would function like a spring that is compressed in a single direction. The wing itself would be secured in the normal fashion, except with a small amount of vertical play.

[Peter Bull]

Originally posted by Jack Olsen
But the springs and a slipring will only show me a maximum reading, which could be the result of gusts or externally-generated turbulance.

That is true, but if you do a series of test runs you should be able to get a reasonable average. Preferably on a stretch of road that can be driven in both directions so that you counter the effect of the wind.

/Peter

[Rob Channell]

I think you could measure the drag or at least the change in drag by measuring the deceleration of the car (speed over time) while coasting from a high speed with the wing in different configurations.

http://auto.howstuffworks.com/question497.htm

[newbe911]

I believe john_cramer is correct, that is to measure air pressure differences. Verticle movement of the chassis may be down if you actually have down force and up depending on how much "inverted lift" the wing is providing. The actual movement may be so small as to get lost in the road noise.
This measurement could be done by making numerous manometers with clear plastic tubing and water. The tubing could be taped to the body where ever you want to take a measurement. The only hard part would be recording the position of the water in the tubing. It may be possible to do this with a camera.

[Jack Olsen]

What would be an inexpensive source for the Motorola MPX4115AP?

[304065]

Jack, they are available from Digikey. They are priced in the mid-teens, El Gordo Burrito territory. You would use a simple 5V voltage regulator on the input side and feed the output to an analog input of the DL90. Of course the signal will chop around a bunch but the DL can filter pretty well.

http://dkc3.digikey.com/PDF/T062/1775.pdf

http://rocky.digikey.com/WebLib/Motorola/Web%20Data/MPX4115A,%20MPXA4115A%20SERIES.pdf

They also offer a two-port "differential" version that has a whole bunch of interesting uses. The classical use of a differential gauge is for a pitot tube: ram air pressure in one port, static pressure in the other, the difference is airspeed.

You could measure on the top and bottom of the wing by actually drilling holes in the wing surface during construction and connecting some plastic tubing to the holes. Then set the wing in the middle of the AOA range and drive at a constant speed, note the pressure reading. Then crank up the alpha and drive at the same speed, note the pressure. When the pressure falls off you're stalled.

This same technology is used for differential-pressure based angle of attack sensing for aircraft. http://advanced-control-systems.com/Products/AOA/aoa.html


While this approach isn't as easy to calibrate as the load cell (support car with a jack and put weights on the wing, note the resistance reading that corresponds to the weight you add and don't snap the wing in half while you're at it) it might help to tell you what the wing is actually doing.

[98GTW]

Jack,
Digi-Key has a series of Motorola pressure transducers, not all of which are currently in stock (including the one you specifically mentioned.)
On their site, I searched MPX, then selected the "Pressure" link under Transducers and filtered the choices from there.
Maybe one of them will work.

Dave

Originally posted by Jack Olsen
What would be an inexpensive source for the Motorola MPX4115AP?

[randywebb]

I am not comfortable with comparing active semiconductor devices to burittos.

Can we compare them to the different colored plates at sushi bars instead?

[smdubovsky]

You can't use pressure sensors to measure lift. If you remember the CFD pics someone pasted in the other thread (randy?) the pressure is't even from front to back (or even side to side due to the endplates) on the surface of the wing. If the COP even moves slightly w/ AOA, you could generate far more downforce, but measure less (and visa-versa). You would need to map the entire surface front to back on both surfaces. Now, add in the fact that the orientation of the tube will affect pressure (air flowing perpendicular to a tube will generate vacuum), not to mention that the tube itself will disturb airflow (unless you put them all inside), and I think it will add up to an inconclusive set of results.

SMD

[Jack Olsen]

My restless, unfocused mind was wondering if there was a scenario where it would make sense to permanently install the pressure measuring devices into the wing as part of an 'active wing' system. But the conditions behind the car should be fairly consistent, once you've measured the wing at different angles, you probably don't need to reassess how it's doing. I guess a speed sensor, a steering position sensor and a couple of accelerometers would be the more-useful components.

But that's another project.

[Wil Ferch]

Thought I'd throw-in this upcoming SAE seminar if anyone is interested...
- Wil
===========================

New! Introduction to Vehicle Aerodynamics Seminar will introduce you to basic aerodynamic principles and tools to evaluate aerodynamic features including wind tunnel testing, flow visualization, track testing, and computational methods. Testing of scale models, measurement methods, and data interpretation will also be covered. Next offering: September 18-20, 2006
http://www.sae.org/servlets/newsletter?LINK=NWAU200608a036&PID=9853210

[randywebb]

Nice - potential attendees should find out if they assume you took an undergraduate fluid dynamics course already.

[Ned nyna 11]

Jack,
Disconnect the engine cover latch and then rig a couple of air cylinders under the lip of the cover. Connect to an air pump, accelerate to 100mph and then have your technical assistant increase the pressure to lift the cover against the downforce.

The assistant should record the pressure when the cover lifts and he can then figure the force.

I know that the devil is in the details.

Ned

[unclebilly]

Jack-

One way to do this would be to buy 4 dilitometers and mount them to your shocks. They look like the nitorgen charged shocks used to hold up the wing. Omega sells them - look at www.omega.com under position measurement. These will work very accurately but do not tolerate being over extended (or over compressed). You would then just have to multiply their displacement by the lever ratio from where the shock mounts to the end of the swing arm to get your actual dispalcement on each corner.

The trick will be recording the data as I seem to recall that we used a digital to analog convertor ad a lap top to do this in the past.

This may not be the cheapest solution but it will be the most accurate and will give you other inforation such as body roll and squat under acceleration.

Another thought would be using one of those laser distance finders that Home depot sells for around $30. If you mount it in the back and point it down (maybe through a drain hole in the floor) you could see the display as you drive. I'm not sure how well the laser will reflect back off of the asphault when you are moving tho...

[randywebb]

typo - dilatometers

in case you have trouble finding it

Omega is a class outfit, BTW

[sammyg2]

Aw geez forget the downforce and just add more turbo boost