Morris Minor Owners Club

Had brilliant idea! I’ve a cheap electric power meter that I bought from a Lidl supermarket and an old Black & Decker variable speed drill. By recording the power consumption of the drill with and without the fan mounted in it, I can subtract one from the other and get the power consumption of just the fan on its own. The drill is an elderly B & D163V of 550 watt power with a nominal speed range of 0-3000 rpm, but actually its top unloaded speed is only 2800 rpm. (Our local maximum voltage is rarely above 230 volts – perhaps that’s something to do with it. The drill’s speed was measured with a laser reader bought from Maplin.)
With the fan installed the drill’s max speed was 2520 rpm at which the motor took total of 141 watts;
Calculating for the fan alone, 1500 rpm needed 11 watts, 2000 rpm took 22 watts and 2500 rpm took 40 watts.
In the engine the fan is mounted on a 3 inch pulley and driven via the belt from the engine mounted 4 inch pulley, i.e. the fan runs at 4/3rds engine speed. So if the engine is at max speed of 5200 rpm (?) the fan is running at 6933 rpm, say a round 6900 rpm. So can how do we extrapolate the power requirements from 1500, 2000 and 2500 rpm to 6900 rpm? Well, I’ve always understood that propeller type fans follow a cubic law, i.e. if you double their speed you cube the power required to do it. The only link that I can find that supports this is http://www.imeko2009.it.pt/Papers/FP_664.pdf and in lieu of anything else that’ll do for me.
So scaling the above power consumptions to 6900 rpm
From the 1500 figure we get at 6900 a need for 1070w i.e. 1.44 hp
From the 2000 figure we get at 6900 a need for 903w i.e. 1.21 hp
From the 2500 figure we get at 6900 a need for 841w i.e. 1.13hp

Now being conscious of the errors everywhere and then the effect of their scaling upwards, I’d say that all this indicates that the fan required about I hp at max speed. Anyone got any comments or prepared to repeat it? Especially with a faster drill?
Regards, MikeN.

i believe david vizard has a section in his bible about the power loss from engine driven fans. measured with engine dynos. i can't put my hands on my copy at the moment, but it would be interesting to see how close his readings are to your calculations!

any way you can calculate how much noiser it is too?

Well - the fan will probably 'stall' at higher revs and so won't use all that power. Could also argue there is a ram effect of air coming in the front of the car - so the fan doesn't have so much air to push.......... But Rover did realise the fan uses power - and later Minis etc fans were fitted on a larger diameter water pump pulley - which of course can be fitted to a Minor.. I always used to run my Minor without a fan - but one time when I put the car in for it's MOT they started it and then put it up on the ramp and left it running there for over 30 mins until it boiled furiously....... They told me this when I went back to collect it....... Topped up the water - but next day coming home from work in it - it blew the head gasket in dramatic fashion.
The small fan belt can only transmit limited power (I think ~ 1 bhp) - it has to drive the water pump and the alternator too....... Alternator can supply ~ 200 watts of power and it's not 100% efficient - so say 250 watts input ? I've no idea how much the water pump will take but it will be significant. The noise reduction without fan is worth having!

This is presumably with a 2 blade fan, my 1275 and Series MM both have 4 blade fans, how would that effect the calculations? Presumably the length of fan blade would to be taken into consideration. 1BHP is probably noticeable on a Series MM

How do the 2-blade fans compare for noise v. the 4-blade fans?

Thanks for all your comments.
Chrisryder; I can’t find anything useful in Visard about fans and such as there is, is mini orientated (see chap 20). Regarding noise, that’s not easy to measure, especially teasing contribution of one among many – and on my car the current engine blotted out everything else! However, when I’ve got the new engine in, I’ll give it a try.
Bmcecosse; if the blades “stall” in the aerodynamic sense then their drag would rise steeply, as would the corresponding power they’d absorb. Regarding air coming thro’ the rad, yes, I’d expect that to offload the fan, but at only fast forward speeds. You’ll notice that the fan’s cross-sectional aerofoil is basically two flat plates with the leading one being in the plane of rotation – not what you want with any forward speed! But I suspect this is deliberate with the aim of providing max airflow when pulling hard at low forward speeds, e.g. fully loaded or towing and in low gear. (One should remember that most British vehicles immediately post-war were designed with “Empire use” in mind, in this case the worry being “hot and high”. A neighbour of mine used a frequently overloaded Traveller in the 60s in mountainous areas of Nigeria and he says that boiling was never a problem.) Regarding the power the belt can transmit, like you I’d expect it only to be able to do about 1hp, but remember as the requirements of the fan follow a cubic law, so at a road speed of 60 mph with a fan speed of about 5200 rpm I reckon its taking only about 0.6 hp.
Mike Perry; yes this is the two blade fan of 12 inch dia. Very roughly, doubling the blades doubles the power required – but give it a try yourself!
What I’m hoping is that the methodology of using a power meter and a variable speed drill is OK. Am I right? If so, then it might be extended to say, running a distributor or a wheel hub, and here the speed of the drill will be sufficient to not require assumptions on ”fiddle factors” to scale up to the right revs.
Regards, Mike Nash.

I was wondering if there might be a cavitation effect at higher speeds?

I cant imagine that would be a huge issue, and i've never heard of anyones fan being damaged as you may expect from this?

I suggested it would 'stall' - by which I intended the same idea as your cavitation -which i think only happens in a liquid - the 'damage' occurring as bubbles of gas are formed and then collapse in on them selves.........

It wasn't damage I had in mind but energy not being transferred to moving air.

I've used a dinghy with an outboard motor where if it was revved too much the prop cavitated and it would stop moving.

I assume that air car be treated as a medium similar to a liquid in fluid dynamics.

Even at its max speed of about 6900 rpm (with the engine at 5200 rpm) the fan's tips are only going at about 360 ft/sec; they'd have to go almost three times as fast to approach transonic i.e. towards 1000 ft/sec, to run into trouble with the airflow. Even then the damage typcally seen with cavitation in water wouldn't occur - but they'd have flown apart long before then!

But the noise would be something! You can hear the baying that occurs when a Hercules climbs away overhead or (if you're lucky) you have a ancient Harvard circling over you. Common enough, I'm pleased to say, in north Hampshire. MikeN.

Just thought; there must quite a lot of cavitation in the pump.

There is ! One reason why they fitted a larger pulley - to slow it down.