downforce calculations!
 

I was wondering if anyone knew of how to find the down force of a flat plate. I know that us airplane guys don't normally deal with downforce, but any help would be appreciated.

RE: downforce calculations!
 

at what angle of attack bernoulii will help you need density of the air speed of the air sujrface area of flat plate

RE: downforce calculations!
 

45 degree, 5.5 cm^2, 90kmph

RE: downforce calculations!
 

I just happen to have my fluid dynamics book closeby. :D

If you really mean 5.5 square centimeters (a very small wing), at 45 degrees it is quite separated flow, of course. Also at 45 degrees, downforce will be about equal to drag. I get APPROXIMATELY .86 ounces of downforce - this will be a ballpark number.

Some assumptions are the plate is close to square shaped, and thickness is less than 10% of the edge length.

RE: downforce calculations!
 

awesome! thank you, u don't happen to have the formula with you do u? so i can plot it with barious speeds on my TI-84

RE: downforce calculations!
 

downforce approx = 0.00000802 X V^2.

Where V is in feet per second, and force will be in pounds.

Take this generated load as a ballpark value.

RE: downforce calculations!
 

Ron, where does that take into consideration the area of the plate? and it's angle? All I see is velocity squared times a constant.

RE: downforce calculations!
 

Google for Foilsim. If you reduce the wing size and speed to your model and the airfoil's thickness and camber to 0 you can calculate the "lift" for any reasonable angle of attack. You can also leave the thickness at 0 and add in some camber to see the highly positive effects you can get from just curling your flat plate a little.

Keep in mind that this is for a clean flow environment. If you're lookint at a wing for a car or boat then the local airflow will be anything but clean and results may well vary wildly. For such applications there's no way to see what the results will be other than shoving the whole thing into a wind tunnel. This is why you sometimes see wings mounted with what appears to be a zero angle. WHen that's done it's usually because the local airflow has a strong downwards component due to a raised area in front and the air is reacting and flowing downwards. At that point the apparent angle is zero but the wing may well be seeing a proper 6 or more degree angle of attack. Keeping in mind all this is reversed of course.

RE: downforce calculations!
 

very nice guys good to see people that know math! google physics of flight and you will get everything. fluid dynamics, laminar flow equations. bernoulii is your basic lift equation but only works if you are not in stall. even a flat piece of balsa has lift ( toy gliders)

RE: downforce calculations!
 

da Rock,

You are right, area, density, angle of attack, are all constant from the values he has given me. If any of those items change, then his answer will be wrong.

He asked what his downforce would be per a given velocity. My equation has all the constants included, and fixed. The actual equation is for approximating drag of flat plate, from about 12 deg to 90 deg aoa. Around 8-12 degrees aoa, seaparation is occuring, and the equation shouldn't be used.

Another assumption is that it shouldn't be used at velocities where compressibility effects raise their head.