Spitfire - Efficiency of Cooling and Thrust

I think you know the answer, but, just like me are too lazy/dissinclined to do the arithmetic!
Or to put it another way, I don't s'pose the trust is any more than the drag.
I would imagine the the thing was designed empirically anyway, start with a reasoanble sized rad and make it smaller and smaller until you get overheating and then go back a bit. I don't s'pose that thrust thing was ever even considered.
I agree there is some wince making 'science' on TV, but maybe some is better than none? Please no more Richard 'look at me being a cute schoolboy with flopsy hair with one fixed mode of presenting' Hammond...it was only amusing the first 'n' times.
Del

The added heat will provide a minuscule amount of additional thrust. How much?

I don't know.

This is the ramjet effect. The air is heated and expands as a result, giving a higher pressure and/or velocity of air at the exit. Direct conversion of heat energy into thrust. It was apparently quite effective on the P51, - a later design - although I do not know the numbers.

Ramjet efficiency goes up rapidly with speed. It starts becoming usable at about 400 mph and gets much more efficient at Mach 1 and faster. The SR-71 is basically a ramjet driven airplane above Mach 1. The new SCRAMJETs take this the next step above Mach 3.

In a ramjet or scramjet, fuel is added and combusted, providing LARGE amounts of heat. Here the only heat added is from the radiator. No comparison!

This is commonly quoted as a net thrust (for these water cooled aircraft) and I don't see any reason why it would not be so with sensible design.

I don't see any reason why it would not be so with sensible design.

Hmmm, maybe because most of the energy from the expansion of the combustion has already been used to move pistons up and down and all that remains is some low grade waste heat which isn't at pressure (remember this isn't even waste exhaust gas, it's the cooling air).
OK you car isn't quite the same as an Spitfire, but how much spare energy is there in the hot water in the radiator...maybe enough for a fry up or a cup of tea?
Agreed ow't is better than now't and with a Messershmitt chasing you, then anything helps, but I would venture to suggest that the aerodynamics of the plane and the air intake were more significant than the resultant thrust of warming air up by 100degrees or so, I mean, it's not being turned to steam!
Del

The thermodynamic efficiency of a well designed gasoline engine is about 20-25%. So you are dumping a lot of heat into this oil cooler at about 180 F, at least three times what you are getting out to the prop. The thermodynamic efficiency of heat to thrust may be low, but it was enough to specifically design the P-51 cooling system to use it. A moderate amount of extra thrust at 400 mph amounts to a lot of power.

A moderate amount of extra thrust at 400 mph amounts to a lot of power.
What??? That statement doesn't stand up to careful examination.

A 'moderate amount' of extra power results in little extra velocity when drag is proportional to the cube of velocity.

Del
<slaps furry head and exits monitor left>

"A moderate amount of extra thrust at 400 mph amounts to a lot of power. What??? That statement doesn't stand up to careful examination.

A 'moderate amount' of extra power results in little extra velocity when drag is proportional to the cube of velocity."

You're looking at it backwards. Power equals thrust times velocity, so one extra pound of thrust at 400 mph is 1 lb * 400*1.5 ft/s = 600 lb*ft/s, a little over one horsepower for each pound of additional thrust. Bear in mind this is net, delivered horsepower - equivalent to at least five thermal horsepower of fuel consumption if it had been generated by putting in a bigger engine.

This is why engineers of the Thirties and Forties were so eager to derive extra thrust from engine exhaust - with or without augmentors - and from coolant heat. It also motivated efforts to use exhaust flow to pump cooling air.

ArrrrHHHGGGG.
Jeez... NO... I'm not looking at it backwards!
That's exactly the point I was making (obviously, far too subtely)!
He'd missused his terms!

The point I was making is that it WON'T produce a 'moderate amount of THRUST.
It WILL produce a moderate amount of POWER.

You are in fact agreeing with me! (I'd alread marked your answer as 'Good')
(Not to self type slowly and in block caps so people will understand cat language)
Del
(And the doc wonders why I have high blood pressure... clam calm)

"ArrrrHHHGGGG.

Jeez... NO... I'm not looking at it backwards!
That's exactly the point I was making (obviously, far too subtely)!

He'd missused his terms!"

Ooops.

I think if we are to reach a sensible conclusion it needs someone to put some figures to it. If they show a useful amount of power I'll happily grovel and distribute cases of KrisDel^TM virtual beer to all concerned.
Del