So why did my hood suck up?

[Carl Johansson]

So Jack and others tell me that the highest pressure on the car is at the base of the window -  and I believe them.  All of you know that on the C-4 body style of corvette the hood is essentially a 1 piece front cowling that opens from the rear.

so last weekend at 133 - 134 mph the hood starts magically levitating.  Ends up I'm straining to get taller in the seat so I can see over the hood.  It ends up opening up a 6 inch gap .  as soon as I pull the chute it gently settles back in. 

A failed latch is the cause -  no big deal -  and we have learned that we need to pin the hood -  no problem. 

But now I'm trying to learn from the event, and turn it to my favor.

so if high pressure is at the base of the window, how does the hood rise up? 

As far as my feeble brain can figure -  pressure inside the engine compartment pushed it up.

If thats the case -  wouldn't that argue that I ought to reduce the air flowing into the engine compartment (through the radiator?)  I never got coolant over 160 degrees - so cooling is not a concern -  should I block off some of the airflow into the engine bay?

carl  "wish I could rise on command" Johansson

[JackD]

The air takes a longer path over the top of the car than it does under, and like a wing design it produces lift.
At speed. you will find the front suspension unloads as it lifts also.
It is not air pressure under the hood but the suck on the top.
Look at roof rails and where they are placed also.
They are designed to spoil the tendency of the air to lift the car when it is sideways.
You get lift on the rear window also, that is why some run weight to compensate .
My Camaro and Buick both had about 1200 lbs. lift at 200mph. on the rear window without a spoiler.
.

[hotrod]

You have two issues going on. The higher pressure at the base of the windshield only extends forward about 1/3 of the length of the hood. The lowest pressure on the top of the hood is only 18 inches or so farther forward about 1/3 of the way back from the front of the cars nose. Due to that you have strong lift on the front third of the nose, and moderate pressure on the rear of the hood.

You also have air packing under the hood at high speed, which raises the pressure in the engine compartment. All the air that gets pushed through the grill needs to find its way back out of the engine compartment. Usually that is a long and high drag path past the engine, headers, steering gear etc and out the bottom into the slipstream under the car. At some high forward speed, the air coming into the engine compartment is more than the air that can get out, and you pressurize the under side of the hood with all that air waiting its turn  to exit. The combination of positive pressure under most of the hood due to air packing and low pressure over the front 2 ft or so of the hood results in a very substantial lift on the hood.

If you look at some of the better automotive aerodynamics books they discuss the problems of "internal drag" A good example of the effect is how your ears pop when you roll up the windows at speed and the air flow through the cowel vent pressurizes the cars interior slightly.

If the low pressure on the front of the hood is only .5 psi below ambient air pressure over an area of 3 ft wide and 2 ft long, you have 6 square feet of hood with 864 sq inches and .5 lbs per square inch that aerodynamic lift is 432#. Now add another .5 psi positive pressure under the same area of the hood and you have 864 lbs of force trying to open your hood.

The under hood pressure and high pressure area at the rear of the hood will mostly cancel out as they will both approach the ram air pressure.

If you put a water manometer under the hood and ran the car at speed you might be surprised how many inches of water pressure you develop even at highway speed if you don't close off a large fraction of the grill opening.

Larry

[JackD]

Well sorta.
The trick is to not let the high pressure air that is normally available through the front cooling system, headers, and other stuff go in the motor because it has picked up a lot of heat .
Body angle and minimum ground clearance in the front will help create a low pressure area to carry the heated bad air away and reduce the push up on the hood.

[Carl Johansson]

The air takes a longer path over the top of the car than it does under, and like a wing design it produces lift.
At speed. you will find the front suspension unloads as it lifts also.
It is not air pressure under the hood but the suck on the top.
Look at roof rails and where they are placed also.
They are designed to spoil the tendency of the air to lift the car when it is sideways.
You get lift on the rear window also, that is why some run weight to compensate .
My Camaro and Buick both had about 1200 lbs. lift at 200mph. on the rear window without a spoiler.
.

alright i am lost here.  I understand lift and venturi effect.  My first thought was that the lifting of the hood came from lower pressure over the hood creating lift.  but i don't see how this fits with the high pressure region at the base of the window.  that region extending over the back 1/3 of the hood should work to keep the hood down right -  remember it's a rear lift hood -  opens from the base of the window and flops up and forward.

The air flowing through the engine compartment is designed by chevy to be exhausted out of the gills on the lower panels at the rear of the engine compartment.  Lowering the front end to keep air out from under the car makes alot of sense to me -  and I am working on that -  but remember I'm running GT -  so I can't mess with the OEM aeropackage -  and the car is pretty low right now.

Carl "man am I confused" Johansson

[Stainless1]

Carl "lay speak" Johansson,
The down force on the base of the windshield part of the hood was not as great as the lift force on the forward part. 
Hope this helps clear it up.  Not really sure the high pressure extends as far forward as mentioned and the farther forward you go, the less pressure there will be.  Design, slope and shapes all affect the pressure area.
Add Weight and hood pins, have fun and make sure the back window is secured now that you know the GM latch is inadequate you may find other weak spots. 

[JackD]

Hey !
I bin robbed !!!!
I did a whole thing on that while Stainless did his Readers Digest version and when I went to admire my work the puter ate it.
I hate it when it does that and to add salt to the wound, he said most of the same stuff.
Now I am pissed and not in the mood.  LOL

[Sumner]

Hey !
I bin robbed !!!!
I did a whole thing on that while Stainless did his Readers Digest version and when I went to admire my work the puter ate it.
I hate it when it does that and to add salt to the wound, he said most of the same stuff.
Now I am pissed and not in the mood.  LOL

Actually Stainless pretends that he doesn't understand computers when in reality he has installed a virus on yours that sends your posts to his where they are posted to landracing.com if considered worthy under his name  and then guess what you are left with  ,

Sum

P.S. Don't accept e-mails from Stainless with attachment and you will be safe 

[Carl Johansson]

Carl "lay speak" Johansson,
The down force on the base of the windshield part of the hood was not as great as the lift force on the forward part. 
Hope this helps clear it up.  Not really sure the high pressure extends as far forward as mentioned and the farther forward you go, the less pressure there will be.  Design, slope and shapes all affect the pressure area.
Add Weight and hood pins, have fun and make sure the back window is secured now that you know the GM latch is inadequate you may find other weak spots. 

 back window is already secured with straps over the window.
I'm not sure the lift force of the wing effect on the front of the hood is that strong -  remember it is close to the fulcrum so it would be acting as a class 3 lever -    and it would be opposing the high pressure -  which is functioning as a class 2 lever -  so the forces would have to be enormous to lift the hood.  if that were the case -  this would account for the legendary myth of the front end of c-4 corvettes getting light -  but since engineering data from GM indicate only 200 lbs of lift from the body -  i don't see how that would work! I can't wait to see jack take me to task over this now he is all pissed off and it's getting close to 10 am -  and we all know what that means.

we are talking about a water tank for weight -  that would be easy -  but it would be over the rear axle -  so it should have no effect on the hood - (unless the weight lifts the front end -  if thats the case - I need new suspension!)

carl 'I'm still stuck on airflow into the engine compartment" Johansson

[1212FBGS]

My guess is the high pressure at the windshield base is only about 5" forward due to the steep rearward angle of the shield. that hood is huge! i will also bet the the GM propaganda omitted a "1" and the real lift numbers are around 1200lbs Carl those side vents look cool but can you actuall see into the motor compartment? open the inner panels up so you get real flow through them, but remember now your disturbing the airflow around the sides of the car. You need 2 times the area of the radiator to exhaust the pressurization. Duct tape or zip tie cardboard to the front of the radiator to reduce the amount of air into the engine compartment and ultimately under the car.
kent

[JackD]

WWJD
Put water in the gas tank you have and gas in a tank that is of the capacity you need.
On one of my RWD door slammers, the old gas tank was under the rear deck and I made a removable hatch to provide access to it from inside the car.
I put the desired weight of lead shot inside.
The only visible modification was the huge straps to the outside of the tank that lookers liked but didn't really know why.
Others with the same car body never understood why it seemed I had no added ballast.
 
OBTW: I have been suspect of that person for awhile and will be watching him closer now. LOL

Oh, and perhaps the hood was staying in place and the car was forced down
I dont think so.

[1212FBGS]

cardboard gave me 5mph last weekend...
kent

[JackD]

Kent is in the business of aero stuff and can sell you the trick cardboard and 200mph tape that you can fit to your individual needs.
He will expect you to display his sticker also.
His web for the business does not mention LSR stuff and you have to know the secret handshake.

[Stainless1]

It is very disturbing that Jack and I had some of the same thoughts...    Sums analysis seem the most plausable, especially considering that "special relationship" I have with the webmaster...  Sorry to upset you Jack, but relax, take a nap, and enjoy the other posts on the site... 

[Speed Limit 1000]

Stainless, Remember!

Don´t ask, don´t tell.