Exhaust Gas effect on drag

[puppy]

Has there been a discussion on here about how exhaust gas flow affect aero drag? We have all "seen" the gases in a top fuel car(hence picture for reference). Most of the lakesters and streamliners I have seen pictures of just dump the exhaust right out to the side of the car into the passing air. I realize that optimum tube length for tuning/packaging ect, are considerations but I have to wonder what kind of drag the exhaust has to have on high speed vehicles. Thoughts?

[manta22]

There was a discussion on this some time ago. Search and you'll find it.

[Stainless1]

We dump straight back... years ago someone way smarter than us said hold your arm out the window at 100... an exhaust plume is just like that...  Now some of the big motor guys use that for down force....
When in doubt... Bonneville allows you the luxury to test... it just may take a couple of years to decide what is required and what works best.

[jacksoni]

There are a lot of posts about exhaust. This one is specific as Neil (Manta22) noted and goes for 20 pages.......
http://www.landracing.com/forum/index.php?topic=9912.0

[Dynoroom]

Not ALL lakesters have exhaust out the side... 

[Rex Schimmer]

One of the better designed exhaust systems on one of the most successful cars to ever run at the Salt.

Rex

[John Burk]

Eric Ahlstrom advised me to have the exhaust outlets lay back against the body and then cut off flush with the body . Marlo mentioned the skin drag reduction of having hot exhaust smear down the body . I guess it's been mentioned that at high speed most of drag is skin friction .

[donpearsall]

We dump straight back... years ago someone way smarter than us said hold your arm out the window at 100... an exhaust plume is just like that...

I don't know about that. It does not seem logical. Once the exhaust molecules are out of the exhaust pipe, they are no longer confined or connected to exert a force on the pipe.

Here is a a thought experiment- get in the back seat of a car with your lawn blower. Have the driver go about 60-70 mph and shoot the blower air out of the window making sure the snorkel part is not outside the window. Will you feel a sideways force on the blower? There will be a backward force from what little reaction there is of course, but I am confident there will be no sideways force creating extra drag on the car. 

Don

[kiwi belly tank]

If the pipe is flush with the body, there will need to be some sort of heat protection plus the exhaust flow is going to take the boundary with it creating more drag than having the pipe extend further out.  Modern turboprop aircraft have settled on the best option where it's not possible to put it out the back of the vehicle plus they go out both sides to equal the side thrust created.
  Sid.
 

[Stainless1]

I don't know about that. It does not seem logical. Once the exhaust molecules are out of the exhaust pipe, they are no longer confined or connected to exert a force on the pipe.

Here is a a thought experiment- get in the back seat of a car with your lawn blower. Have the driver go about 60-70 mph and shoot the blower air out of the window making sure the snorkel part is not outside the window. Will you feel a sideways force on the blower? There will be a backward force from what little reaction there is of course, but I am confident there will be no sideways force creating extra drag on the car. 

Don

Don, watch the exhaust plumes on a top fuel dragster... they visibly extend straight out of the pipes several feet even at half track....  but it is a column of air with direction and force.... and it will generate drag when you move it through the still air.... moving it through the air is trying to change it's direction and break up the plume....that takes energy...
I see exhaust plumes as added frontal area... That does act like sticking you arm out the window and holding it there. 
I didn't say it would force your car sideways, I said it is creating drag...  Kinda like sticking your arm out the window....

[jacksoni]

But Stainless, your arm, through you, is "attached to the car" so the force on your arm is transferred to the car. The plume of exhaust on a TFD is not "attached" to the car, but is moving so fast that the vector of the plume far out weighs the vector moving forward until car velocities get way up there. The prior comments about the leaf blower are similar. The force on the blower, and by it to the holder and then car, are generated by the acceleration of the air within and apply to the body of the blower. The accelerated air particles don't "push" on anything (not saying Don said that but is a common misperception). F=MA. Otherwise rockets and such in space wouldn't do anything. The leaf blower held perpendicular to the direction of the car would produce a force perpendicular to the car's motion.

[datadoc]

I can add a bit to this discussion about header thrust from my drag racing involvement. About ten years ago I was working as a consultant for one of the drag race teams. I wanted to visit this subject after some observations of the effect on the race cars after a couple of incidents. One where the car jumped in the air after the test end bends burned off the car all at the same time. Another when a car lost one bank of headers and the car changed lanes. So my thought was with all this thrust why not turn the ends of the pipes up and try and gain downforce. The team then cut the ends of the pipes and rotated the them up and rewelded them. We finally got to try the modified headers at a test session. To my surprise the car went slower and spun the tires more. From the ride height sensor data I could see that at all four corners the tires were in fact pushed down so the headers did increase downforce. After thinking about the results I came to the conclusion that with the engine mounted in the middle of the car we could only get about 1/2 the available downforce to the rear wheels. The results then showed that forward thrust had a better effect on the car that downforce thrust. My recommendation then was to turn the pipes rearward to gain forward thrust. As things sometime go nothing was done to further test this concept for a couple of years. It wasn't until the crew chief now working for a different team decided to give the idea a try. I told him that there was no doubt in my mind that it would improve performance and that he should only try it for the last 6 races of the year. This is the period where the championship is decided and the results would be so obvious and easily copied that he needed to wait until then because the other teams wouldn't have the time to respond. Well he didn't and the other teams could see what was going on and by the count down he lost his advantage. The next thing that happen was teams started to lay the headers back more and more until the front ends got so light that the cars became hard to handle. NHRA stepped in with a header angle mandate. We then started to test nozzles mounted up in the pipes at the outlet but that testing was cut short because of a rule change to outlaw anything in the pipes.
   I had a opportunity to meet with some real rocket scientist from USU here in Utah. Using the back of a napkin calculations their estimates for thrust  based on the fact that the air fuel ratio of a top fuel car is about 3 lbs of fuel to 1 lbs of air for a total mass of about 1000 lbs/min could be as high as 6,000 lbs.  Due to the increase in speed 0f the car we also thought that the Coanda effect could be at play, because the hot exhaust would attach to the body such to burn off the body wrap. The thought was that the hot less dense air could reduce drag.

[Paulin adelaide]

My thoughts were that it takes 10 000cubic foot of air to burn a gallon of gas .
If that was true it could be 5 000 cubic foot each side , and at what velocity ?

[Stainless1]

Well I'll defer to the guys obviously smarter than me, point your exhaust any direction you see fit.... but our exhaust will always point the opposite direction of the way we want to go... 
 

[tf737sb]

I like this topic!
We discussed this last night in after-hours "beer-thirty" round table. In Top Fuel racing, we have seen the advantages (and disadvantages) of aiming zoomies for thrust. There's real world experience to support it (data as well). Nitro combustion gasses expand in a big way, and continue to expand while in the header tubes, and especially when it gets to meet an oxygen rich environment again outside the header. In a Bonneville application, you can ask George Fields about how much rudder he had in the coupe with the 'E Motor' 4 Banger (Half Hemi). After a quick 'pedal at the 3, he wasn't aimed as originally intended.
To further the idea about the Coandă effect, when I built the last set of headers for the Donovan in #44, we did our best to aim for the wing. Can the faster passing gasses increase the effectiveness of an airfoil? Or does the turbulent air disturb the laminar flow over the wing? I suppose we could 'tuft' the wing elements to find out for sure, but the idea sounds pretty good to me.
As for drag, I tend to agree with the "leaf blower" theory. If there is significant aero drag from headers, it's most likely coming from the solid header tubes, not the plume of gas escaping.