Mid-engine, Rear- engines superior for aero?

[Unheard]

this is something i have thought about quite a bit, the engine placement is not that important really its more to do with engine design and placement of the radiator.

A flat 4 such as in a subaru or porsche allow for a lower profile to the car. subarus
with flat 4s upfront can achieve a very low bonnet line.

There is nothing at all to say that a front mounted engine has to have a radiator mounted at the front and have the hot aor swilling around the engine bay it is just just the way it has always been done. there are several other ways to look at it.

a standard setup air comes in at the front through the radiator, once through the radiator the air is expanded so it just has to exit where it can out of the engine bay. so to refine this you could have ducting incorporated into a bellypan that ducts the hot air under the car and exits at the rear of the car. At the same time the air could pass over the exhaust and cat to help cool it killing 2 birds with one stone

The other is to actualy have the radiator mounted at the back of the car. you have to remember that a radiator dose not have to take the form of what we have always known with little fins. It just has to have the correct surface area to cool enough so the radiator could actually be incorporated in to the whole rear end of the cars body pannels, or a rear wing fin anything the imagination can come up with obviously there
are issues to work our with the car at idle with no air flow but if you set yourself a harder challenge in the first place you come up with better answers to problems
a smaller pipe bore would be more logical if running pipe down the length of a car, .

Then there is another idea i had and one i have spent some time looking for infomation on thermal jets to work out the feasibility of my theoretical concept

Instead of having a radiator made up of fins it could be made from micro bore metal piping, so the air flows in at the front gets expanded in the pipes and the micro pipes then get channeled down the car to the rear end because you will have fine thermal jets theoretical the jets could be used to change the dynamic of the wake at the back. Eather acting as a thermal cushion for the colder air to ride on or by spiraling the jets to create a worm air vortex

The 911 constantly out preforms cars with equal power and its always put down to handling but its would seem obvious to me that the hot engine at the back actually has a big factor in this. After all an engine is expanding air to shift the pistons its just thermal expansion of air and water vapor. as an engine wastes most of the potential energy of the gas put into the car in heat-loss and noise it seems logical to put the waste heat to good use, i have more ideas on that but i will keep that for another topic

good weight distribution can be achieved with rear mid of front engines its just down to good design.

edit..

Just noticed 'Body integrated radiator method, requires no grill or openings...' looks to have some of the same ideas so i am going to go read it now

[MazdaMatt]

Porches win because they are extremely well engineered, highly refined cars with excellent weight distro (in all planes). If the heat produced thrust, you'd just start rolling when you let off the break, no?

I think this body-panel cooling concept is a little silly. If you took all the fins on my rad and spread them out into a sheet, you'd have a HUUUUGE area.

Oh yeah, and you don't have to have some non-conventional rad to have it in the back, that's what fans and ducts are for.

[aerohead]

[QUOTE=Bicycle Bob;52205]

Originally Posted by aerohead

RiseAbove,in the phil knox aerodynamic photo albums,under Book Illustrations,second row from bottom of 1st page,is an illustration of Walter Korff's zero-drag,cooling system design.I---------------------------------------------------------- (reply aerohead)
Within the 1963 SAE Paper,in which Korff published the cooling system diagram,he suggests,that for passenger cars,a cooling system,as depicted,could impart no drag,elliminating 12% of overall drag.I have no means to verify his claims.And as he is the Phd.,with access to the best wind tunnel facilities in the U.S.,I am in no position to argue his claims.----------------------------------------------------------(quote=Bicycle Bob)
The label actually reads "ideal low drag cooling system." I used to be inspired by the North American Mustang aircraft, on which it was claimed that the radiator enclosure had negative drag - sort of like a hot-air jet engine. However, the Voyager team did their own testing, and could not get the cooling drag to be less than 20% of the total on their whole aircraft. One has to assume that they were competent researchers.------------------------------------------------------(reply aerohead)
It is claimed that the outlet of the P-51 heat-exchanger did produce thrust,as a function of air expansion.Again,I can only report what is published,and make a leap of faith,that the authors know what they're talking about.With respect to Voyager,I hold the Rutan Brothers,and their team at Scaled Composites with the highest esteem.If they say they could not go below 20% drag with the cooling system,I'd bet on those numbers any day.However,the topic was motor vehicles,and I would still consider Korff's claims with high confidence.---------------------------------------------------(quote= Bicycle Bob)
The front end provides the most direct flow to the rad, and does not have to disturb the rest of the shape. There may be some existing cars that could be improved by relocating the rad and ducting to the rear in a combined effort to solve other problems - it is not that difficult - VW did it on the wasserboxer Vanagon, in the other direction. ----------------------------(reply= aerohead)
The Ford Probe and GM PNGV concepts use rear cooling,and achieve lowest Cds for "producible" concepts.Ford GT-style cooling,up front might have offered as low,or lower drag.Different themes.Different approaches.
However, I agree with Mr. Lee - a front engine is good for stability, which becomes much more important on a light, streamlined car.

------------------------------------- (reply=aerohead)

The issue of CG vs CP was important enough that Hucho devoted a good portion of an entire chapter on it.Evidently,crashes,if not fatalities are associated with high speed aerodynamics,as played out in events since the 1920s,when streamlining took off.Perhaps it would be prudent to take each vehicle on a case specific basis.When L.J.K.Setright road tested the Citroen CX,in the 1980s,he claimed that the front-engine/front-drive layout was the defining criteria for the cars rock solid stability,even at top speed,with crosswind.

[Unheard]

I didnt at any point say the heat would produce thrust and in any case even if it did it would only be proportional to the speed of the air so when braking the air flow slows.
it would only cause the car to have problems if there was a forced propulsion like a jet,

The aston martin v8 vantage was developed with the goal of being faster than the 911 it was endlessly tested on the nurburgring with that goal it has more power than the 911 and its a very very highly developed yet its still not as fast as the 911. Now i am not saying that the sole reason for that is having a rear radiator but i am sure it is a contributing factor, hot air is less dence so it changes the dynamics of the wake behind the car, drag coefficient is one of the most important aspects of a fast car so it stands to reason that if there is something causing less drag its going effect the cars speed.

i know you dont have to have a non conventunal rad but you need space to mount one and somewhere to suck the air in so from a design point of view its would be wiser to stray away from the slab stile radiator.

i dont think its a stupid idea having a radiator built into body pannels, you are thinking in the conventual sence of how a car is designed but if the car is actualy designed with that aspect in mind its not too hard to implament. it may not be the sole radiator on the car there could be small heater matrix size rads with vents from the rear wheels, vents reduce the air friction on the wheels anyway so again it would not be a bad idea. if engine pistons cylinder and heads where ceramic coated along with the exhaust manifold less heat would need to be removed from the block in the first place
as heat would not transmit as well through the metal components. That then cuts the thermal differential the components are subject to resulting in a much longer wearing engine and its is a very good way of getting the most energy you can from the detonation.

[equation112]

I feel I have to disagree with much in this post Matt... I'll break it down into smaller chunks.

Originally Posted by MazdaMatt

The oversteer/understeer characteristics of a street vehicle are determined by the weight distribution and wheel alignment. 99% of new cars are front engine because all that weight at the front causes the front to break away first so the car ploughs when the driver panics and slams the breaks and cranks the wheel. It is easier to survive a front-on collision, that is why they do this.

first off, while you are partially correct that in a street vehicle (indeed even race vehicles to a large extent) over and understeer are determined by weight distro and wheel alignment, you are overlooking an equally important (perhaps even more important) aspect of the equation: suspension setup - in particular the relative stiffness of the sway bars front to back.

Second, 99% of new cars are front engine/FWD (just call them FF and be done... FR would be front engine/rwd, MR is mid/rwd etc.) not because of steering characteristics, but because it is easier to design (and cheaper to build) them that way, and also allows the manufacturer to maximize a given vehicle's utility so it can appeal to as many potential buyers as possible. let's face it: MR configurations have very low useable interior volume when compared to FR's (hence why that layout is generally reserved for high-performance vehicles), and FR's are inferior to FF's although not by as large a margin.

I also have to take issue to your characterization of understeer as being preferable to oversteer. there is a saying you might be familiar with:

oversteer makes passengers go 'holy $h!T!!!' while understeer makes drivers go 'holy $h!T!!!'.

A small caveat here... understeer *is* preferable to snap oversteer, but a smooth and gentle oversteer is *far* preferable to any understeer, regardless of the drivetrain layout of the vehicle. a properly setup suspension will always tend towards oversteer. as an example, all of my scariest moments in cars have been during understeer events, where my car threatened to slam into a wall or launch itself into the air towards the outside of a curve (many ramps around here have large curves with no railing on the outside radius). every time I have experienced oversteer it was such a non-event that my heart didn't even bother to beat faster.

I went for a few ride-alongs in a friends MR2 at autocross and he was always battling oversteer becaues the rear end has more sideways momentum in a turn making it more likely to break traction.

there are a number of variables there to consider... what year is the MR2? (the 1991 and 1992 SW20 models were prone to snap oversteer because of their suspension setup, which was nerfed by toyota for the 1993 and later models). also, oversteer should be seen by an autocrosser as something to be used and not fought - when the back breaks loose, drift it. that or your friend had something setup in a non-optimal way - it could have been as simple as using the wrong tires for that surface that day. certainly there are other avenues of investigation that I am not mentioning. if the MR2 in question suffered from understeer, your friend would have had a hard time being competitive - and might not have been able to stay on the course.

As for the original topic, you could make one hell of a sleek frontal shape, top, bottom and sides if there were no ducts there for cooling or intake. A RR car definately has more aero friendly potential. I can't think of a high-speed supercar with the engine up front.

RR cars with engines any larger than the original VW beetle and of course the 'thing' have severe handling problems - a serious tendency towards snap understeer in particular.

[Kohote]

^^^ excellent post^^^
You are definitely correct about the oversteer vs. understeer. A tendency to oversteer allows the driver to rotate the car around the corner better and is preferable to understeer. Although it takes a lot more skill to drive a car like this at the limit and be as fast as possible as compared to a tight setup where anybody can just lift off. Just watch the videos of senna driving the nsx around suzuka-beautiful trail braking oversteer.
Porsches are not known for perfect handling, they tend to suffer from turn-in understeer and then snap oversteer if completely lifted off the gas. The heavy rear end is great for braking and is probably the only real advantage of a rr setup.

[equation112]

ah yes... understeer is scary. thing is I just dont see why many people buy into the idea that understeer is more forgiving, when the very definition of understeer is the complete and total loss of steering effectiveness. this is doubly bad in a FF type car, where understeer results in both a loss of steering AND propulsion AND over half the braking power simultaneously. suprisingly enough however the solution to both over and understeer is pretty simple: MORE power (ie NOT more brakes, because braking in a curve is likely what caused the problem in the first place. drifting a car during oversteer really isnt hard at all, because you still have the ability to control steering, the majority of the braking and power... matter of fact a car in oversteer is actually nowhere near it's true handling limits, while a car in understeer is far beyond its' limits. what bites people about it is the fact that it this is all somewhat counter-intuitive.

I forgot all about the 911's... yeah their handling is not all that impressive, though the new AWD models have rectified the deficiencies to a great degree.

[aerohead]

[QUOTE=Bicycle Bob;52205]

Originally Posted by aerohead

RiseAbove,in the phil knox aerodynamic photo albums,under Book Illustrations,second row from bottom of 1st page,is an illustration of Walter Korff's zero-drag,cooling system design.I

The label actually reads "ideal low drag cooling system." I used to be inspired by the North American Mustang aircraft, on which it was claimed that the radiator enclosure had negative drag - sort of like a hot-air jet engine. However, the Voyager team did their own testing, and could not get the cooling drag to be less than 20% of the total on their whole aircraft. One has to assume that they were competent researchers.

The front end provides the most direct flow to the rad, and does not have to disturb the rest of the shape. There may be some existing cars that could be improved by relocating the rad and ducting to the rear in a combined effort to solve other problems - it is not that difficult - VW did it on the wasserboxer Vanagon, in the other direction.

However, I agree with Mr. Lee - a front engine is good for stability, which becomes much more important on a light, streamlined car.

I wanted to mention that in my photo album, the label under Korff's diagram is mine,not Korff's.I used the word "ideal" because I thought any cooling system design that could totally eliminate this component of drag was "ideal".

[aerohead]

Originally Posted by Frank Lee

yer over- and under-steers are ternt around.

Having had several rear-engine vehicles, I can say I like the handling dynamics of them- nice, feather-light and responsive steering, not to mention superior traction. From a strictly utilitarian standpoint, though, it is better to have the trunk in the back, both for super heavy and oversize loads. And they are more sensitive to crosswind gusts.

I went back into my library to make sure I was using proper terminology when describing terminal understeer for rear engined,rear drive cars.I believe I chose the correct word.Forgive me while I digress.---------------------------- I the 1930s,40s,and 50s,Mercedes,Jaguar,and Bristol were plagued with crosswind stability and gust issues within their racing programs and higher speed passenger cars,as enabled by better,faster highways.------------------------------------ Kamm's K-cars were notoriously unstable in side gust conditions.It is believed that race driver Bernd Rosemeyer's death was caused by the aerodynamic instability of his Auto Union competition car,whose bodywork deformed and buckled under aerodynamic stresses,creating assymetrical induced lift which pushed it out of control----------------------------------------- In the report on the 275 mph crash of the HONDA Hawk land speed record motorcycle streamliner,it is mentioned that Bonneville racers have died in aerodynamic instability events,at velocities as low as 60-mph.------------------------------ Korff's comments concerning disregard for C.P./C.G. issues,would be analogous to shooting arrows feathers-first.--------------------------------------- My comment about casual remarks about aero-modded rear-engined cars comes from Korff's insistence that C.P. equal, follow the C.G..In a side gust condition,the apparent wind force vector will act on what Hucho refers to as the "lever arm" between CP and CG,and the front of the car can be pushed into a high speed understeer,turning the front of the car off centerline into an even larger cross-sectional area against the the air,in turn,amplifying the yawing moment with potentially fatal consequences.------------------------------- Korff considers the cars to be directionally unstable at high speed,difficult to manage,and a menace to other cars and its driver."C.P. should be at or behind C.G. for high speed stability."------------------------------------- Since,as mentioned,oversteer "freaks out" most motorists,low speed understeer is designed into basically all passenger cars.If the front breaks away,just steer in the direction you want to go,and apply more throttle.------------------------------------------- If you loose the front end at high speed your basically screwed.If you loose the rear,you can apply opposite-lock steering and judicious use of throttle to coax the front end through a curve or back into a lane.-------------------------------------- Parting remark,CP is so difficult to ascertain it requires a windtunnel to determine its location.Our ecomodder friends in Europe,with access to the Autobahn,could be tempted to test the top speed potential of their cars.Since we have no control over the weatheri it's extremely important to understand the forces we're playing with.Lives depend on it.I've done some pretty stupid things and circumstances have played out such that I'm still around.Don't flirt with disaster.If you do,do it where there are EMTs and ambulances.Saving fuel is a noble thing but its not worth your life.

[aerohead]

Originally Posted by MazdaMatt

I've never seen a cross wind that will actually screech my tires sideways causing me to correct. The oversteer/understeer characteristics of a street vehicle are determined by the weight distribution and wheel alignment. 99% of new cars are front engine because all that weight at the front causes the front to break away first so the car ploughs when the driver panics and slams the breaks and cranks the wheel. It is easier to survive a front-on collision, that is why they do this.

I went for a few ride-alongs in a friends MR2 at autocross and he was always battling oversteer becaues the rear end has more sideways momentum in a turn making it more likely to break traction.

As for the original topic, you could make one hell of a sleek frontal shape, top, bottom and sides if there were no ducts there for cooling or intake. A RR car definately has more aero friendly potential. I can't think of a high-speed supercar with the engine up front.

I don't think that velocities during autocrossing can approach the velocities I'm concerned with.