Landracing Forum
Bonneville Salt Flats Discussion => Bonneville General Chat => Topic started by: WhizzbangK.C. on June 06, 2010, 01:54:59 AM
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Watching NHRA qualifying on ESPN2 tonight. One of the funny cars had the left side header break off during the run. The downward thrust from the open exhaust ports was enough to lift the car off the ground! They showed the high speed-high definition footage and you could see that the frame of the car was bowing upward as the engine pushed it up.
It's been discussed on more than one occasion how much benefit one can get from proper exhaust placement, and I thought that this was pretty interesting. I know that no one is going to be running a top fuel spec engine in LSR to make the kind of thrust that they make, but a lot of folks run pretty healthy stuff and to me this seems like proof that there are gains to be had from exhaust flow.
I thought it was interesting anyway. Not that my little 250 Triumph could ever show anything from it, LOL.
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This is an old NASA paper on using a piston engine exhaust ejector to improve air mass flow through a radiator cooling duct. Tony
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091895_1993091895.pdf
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Anyone who remembers Convair aircraft with P&W 2800 and "Exhaust Augmentation" is a believer.
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Not pointing your exhaust through the air that you are trying to keep stuck to the sides of your vehicle is a good start so zoomies and cowl flush exhaust ports although cool looking are slowing you down.. Exhaust gas is hot , and cooling rapidly...that means it is shrinking ...........pointing that into your wake is a good move as it will fill AND tame the low pressure area that many cars and bikes have following them that is responsible for a large part of the total drag.
Any thrust you get is a bonus on top of the above...
EDIT:of course if there is any thrust you want it pointing STRAIGHT backwards, that's a no-brainer.
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Ask Joe Amo about using exhaust to fill the vacuum behind the bike.
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Ask Joe Amo about using exhaust to fill the vacuum behind the bike.
Amen, Amo. .....and de Stoop our daily bread........ :wink:
(http://lh4.ggpht.com/_u0_IOr1cg0w/S6icK71YwRI/AAAAAAAAALs/1dxszc3yrW4/s800/FeverRearView.jpg)
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Raises a thought -
While the thrust factor would be minimal on a low output engine, might the flow be utilized to evacuate high pressure underneath a car to minimize lift? :roll:
Thinking out loud again.
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Amen, Amo. .....and de Stoop our daily bread........ now thats funny
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jon amo did, i did not....:)
Joe :)
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Sorry for the cornfusion, Joe -- I thought it was your bike. Ah, isn't old age great? I can remember things that never happened.
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You might note this report done in "1935" regarding exhaust augmented thrust for aircraft.
http://contrails.free.fr/refroid_meredith_en.php
It would also be useful for landspeed builders to read up on the radiator extractor system on the P-51 fighter, that derived significant thrust in flight from properly expanded radiator heated airflow.
Larry
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Great discussion. Alex Tremulis also looked into this during the development of the 1948 Tucker. Here's his description from a 1987 paper he wrote for SAE:
(http://i652.photobucket.com/albums/uu248/gyronaut/Poormansjetpropulsion.jpg)
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That funny car also lost the downforce from the upturned zoomies which helps hold the the front end down during acceleration. The car also made a left turn from the right side zoomies still working.
I have read that some aircraft fighters got up to 160 thrust hp from their headers and some engine designers didn't use turbos because of losing that thrust.
We believe that we get downforce and thrust from the zoomies on our 222 camaro.
JL222
It always amazes me when those funny cars get so far out of shape and still go down the track right side up.
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jon amo did, i did not....:)
Joe :)
er, apologies Joe , I too thought it came out of your rear end...
so to speak.
no doubt you've been blamed for lots of things he did over the years,
probably balanced out in the long run.... :wink:
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Hi ya'll,
I've seen flops go up even when the headers stay on. I saw a the picture yesterday of the explosion but I couldn't tell if the headers were still attached to the head. The height is usually due to the explosion under the body, just like a can with a cherry bomb under it.
POPS
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On some of the turbocharger pro-mod cars they have "bull horns" ( exhaust out of the turbo turned up out the side of the car) for downforce.
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Hi ya'll,
I've seen flops go up even when the headers stay on. I saw a the picture yesterday of the explosion but I couldn't tell if the headers were still attached to the head. The height is usually due to the explosion under the body, just like a can with a cherry bomb under it.
POPS
When I was watching they showed the run over and over + I can back up the video and stop it myself. there was no explosion and the header did come way off. All the fire under the car was from the exhaust ports minus the headers before he shut it down.
JL222
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Rob had his Tempest up on the dyno a few weeks ago. When he was making a full pull, the exhaust had the trees waving wildly out to about 75 or 80 feet! The loose ends of the tie-downs were standing straight out (about 10 feet) like a flag in a hurricane. There was a piece of 1.24 tubing about 5 feet long that was caught in the wind stream and it flew about 30 feet.
Next time we have it up on the dyno, at a minimum I want to put an anemometer in the flow to see what kind of speed it has. Might even put a cart with some weight just to see if we can measure the force.
btw, that was through mufflers too! :evil:
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Brandon,
Good morning, how are you? Talk with Rob regularly. Sounds like Vinny is getting ready for flashlight drags or is it Maxton this time.
Detective Dave
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Brandon,
Good morning, how are you? Talk with Rob regularly. Sounds like Vinny is getting ready for flashlight drags or is it Maxton this time.
Detective Dave
Doing well, thanks for asking Dave!
Vinnie made the flashlight drags two weekends ago. Time for new set of hides though, it is hard keeping the skins on 1,400HP and an (unprepared) runway! btw, he is now making big power on pump gas (94), that is what he was working on with the dyno. I think it was with only 17lbs of boost and 94 octane it was 1,050HP. Sure is fun to watch people's reaction to seeing an almost stock '63 Pontiac run 8.92/163 in the 1/4!
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I stumbled across this while looking for information on the Republic XF-12:
"All of the exhaust from the P&W R-4360 was ducted straight out of the back of the nacelles. This provided additional thrust. Research showed that roughly 250 equivalent horsepower was generated by each engine exhaust during high speed cruise at 40,000 ft."
The XF-12 was a very efficient aerodynamic design, well worth looking at closely.
Regards, Neil Tucson, AZ
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And then of course there is this. Italian designs are always so elegant aren't they? :-D
http://fly.historicwings.com/2012/10/the-flying-barrel/
Robin
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That one looks like a cross between a ducted fan and a Custer channel wing.
When I think of beautiful Italian designs, I think of Pinin Farina, Bertone, or Sophia Loren. :-D
Regards, Neil Tucson, AZ
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That has to be the "Edsel" of airplane design. :evil:
Doug :cheers: :cheers: :cheers:
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That plane proves that with enough power and big enough wings even a pig can fly!!
Rex
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I do know that on the 411 streamliner, Tom & Gene went to a considerable amount of design and construction to make the exhaust exit at exactly the right place Tom wanted it to create a vortex and assist pulling air from under the car and around the back. Tom told me once that at speeds over 400mph exiting your exhaust straight out the side can cause enough aero drag that it would be like giving up 800hp or so.
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Thrust
Exhaust exiting a typical exhaust pipe does not create significant thrust. When a mass is accelerated in one direction thrust is the force in the opposite direction. This happens at the point where the acceleration occurs which is typically not the exhaust tip. Most of the thrust occurs when the exhaust goes from stationary in the cylinder to vMax as it exits the exhaust port. There are only a few PSI in a properly sized exhaust pipe, releasing that pressure at the tip creates minimal thrust. There is typically very little fuel still burning/expanding in the pipe as it approaches the tip. Air already moving at speed is not thrust.
Inertia
“So when I put my hand at the end of a tailpipe I can feel the thrust.” No, that is the inertia in the moving air. Just because the air is moving fast does not mean that there is thrust present. Analogy: Toss a medicine ball. As you throw/accelerate the ball your body falls in the opposite direction so you have to brace myself to counter that. That is thrust. Once the medicine ball is done accelerating it no longer has any effect on you but it still has inertia. When the ball encounters the hands of someone who is catching it, that transfer of energy has no effect on you, the person who threw it. Energized exhaust still may be useful for things like filling the negative pressure behind the car (reducing drag), or drawing air thru a venturi to create negative pressure in ductwork or even to help seal a diffuser. It may also have negative effects like separating the air already attached to the body of the vehicle which creates drag.
Zoomies create downforce therefore thrust?
The above statement demonstrates a misunderstanding as to why zoomies create downforce. It is the bend on the tube creates a centripetal force changing the direction of the exhaust. It’s the same force that presses your butt into the seat and tires into the pavement when you drive fast thru a dip in the road. If thrust was the principle by which zoomies worked TF exhausts would exit straight up in order to maximize downforce, but they don’t. They are angled upward at approximately the same angle as they went downward. This is because the direction of force they create is the difference in those two angles and not in-line with the exhaust exit.
Exhaust Thrust, P&W R4360-VDT
Most articles refer to the VDT engine make it sound like they just pointed an exhaust pipe out the back of the plane and it provided significant amounts of free thrust. A misleading oversimplification of the design. At cruise speeds it ran at wide open throttle, had no wastegate and controlled boost with a variable nozzle at the exhaust tip. The design intentionally created massive exhaust back pressure so they could release it at the nozzle and create thrust. The purpose was to gain high speed cruising efficiency not max power. When they wanted to create max power they would open the exhaust nozzle, cutting the ‘thrust’ and allow the turbos to create boost and put peak power.