Hugh Fairhead has e-mailed us with an interesting piece about a general misconception that model engine performance improves with the weather! What do you think about Hugh's ideas? Let us know your views by e-mailing us here at Flying Sites.

Peter
Browsing through one of the model aeroplane comics recently whilst in a well known bookshop, I was appalled to read an article on engines which categorically stated that engine performance improved in weather and that model's flying performances were also enhanced.

This may be a popular conception but is factually quite the reverse of the truth due to the incontrovertible that moist air is quite a bit less dense than dry air - have a look at where the clouds are.

The power of an engine is dependent on the WEIGHT of mixture burnt in the cylinder per second, and since the weight per volume or moist air is less than dry air, it follows that the power output must be lower. There will not be sufficient weight of air to burn all of the fuel induced into the cylinder ie; the engine will be running too rich. Adjusting the needle valve to correct this will improve matters, but only by reducing the amount of fuel coming out of the exhaust unburned. A certain weight of air will only burn a certain weight of fuel completely, and whilst an engine will still run with this ratio a little bit out from its ideal range, performance will suffer and engine life may be reduced.

Ideally engines should be readjusted to suit the prevailing atmospheric conditions. Alternatively, it should at least be borne in mind that there will be a reduction in power output in damp air.

In very hot conditions where air density is low, it often happens with full-size aircraft that the take off run is extended by a third and the climb out is dangerously shallow due to reduced engine power.

Weather conditions also affect the aerodynamics of flight due to the fact that lift is of course dependent on air density - no air = no flight.

As far as model flying is concerned, the pilot will find that at the same apparent speed, the model seems sluggish on the controls, takes longer to get into the air on take off, and has a relatively poor rate of climb. Remember lift is greater in cold dry air than it is in hot air or damp air.

Propeller performance is also dependent on air density because the propeller can be equated to a revolving wing and is subject to the same aerodynamic laws.

Our type of flying is done by observing the models' behaviour, and it's response to control input. We have no absolute knowledge of airspeed and therefore we learn to fly by judgements, which can only come from experience. Knowing how different weather conditions can affect our flying performance must purely be advantageous.

How about looking at wind shear and "ground effect".

Regards Hugh Fairhead.
Hugh can be e-mailed by clicking here.

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Jonthan Rabbit takes up the discussion, with a scientific slant to the argument....

03/02/03
With reference to Hugh Fairhead's email about model engine performance and weather. Hugh may be stating some correct principles, however his methods of describing and proving them are somewhat dubious.

I was amused to read that he was "appalled to read an article on engines which categorically stated that engine performance improved in weather". The entire letter following contradicts this by asserting that aerodynamics are indeed weather dependent . I should ask him to clarify what he means by 'weather'?

I was also amused that something could be "factually quite the reverse of the truth", but this is by the bye.

He is correct about the power output of an engine increasing with air density, as the mass of air drawn into the engine is the important variable. This is, after all, how the throttle works, since it creates a pressure differential across the butterfly/orifice, lowering the pressure of the air/fuel mixture downstream. This reduces the mass of air/fuel mixture entering the cylinder. The further the throttle is closed, the less dense the air/fuel mixture is entering the cylinder, and the less power produced. Simple and obvious!

The effect of humidity, however is less obvious. Its effect on air density, however, is fairly minor, when compared to the impact of normal barometric pressure changes we see with the 'weather'.. It is true that humid air is less dense (for a given pressure and temperature), this being due to the molecular weight of water (well the hydrogen in water) being less than that of the components of air that it replaces (oxygen and nitrogen predominately). I'm not sure if the location of the clouds is a good proof of this principle, since their formation is probably it is more to do with the where it 'falls' on the saturation curve for air/water solubility. I'll have to do some more research to better describe this phenomena.

Cheers,
Jonathan

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18/05/03

S.P Taylor takes an opposing view!

Sir,
How folks love to complicate things. Anyone who has been around modellers and model flying fields for any length of time will be aware that there is much 'lore'. Half truths, mis-interpretations and pseudo-science are the order of the day. Much of it is simply wrong, or at best irrelevant.

In the real world, unless you are going to fly your model in the High Andes, Central Africa or some other 'Hot and High' region of the world, the simple truth is that in the UK you are highly unlikely to notice any real difference in model performance. Yes it will, strictly-speaking, operate better on a cold winters day with high barometric pressure than it will on a hot summers day with low barometric pressure when the air density is lower. And...?

Yes, sure your mixture will be slightly different, - but then again, you re-set it for every flight anyway. So what.

As for ground-effect, yes, it is a real phenomenon, applicable to aircraft of any size. The same applies to wind-shear, although to a lesser extent. One flies accordingly. It's all there in proper authoritative factually-written textbooks for those sufficiently inclined to find out the real facts, rather than absorb lore, an incoherrant and distant relative.

As a pilot of both models and full-sized for many years I try to make life simpler, not more complicated!

S.P.Taylor.

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