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
us here at Flying Sites.
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.
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
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.
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.
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
conditions also affect the aerodynamics of flight due to the fact
that lift is of course dependent on air density - no air = no
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.
performance is also dependent on air density because the propeller
can be equated to a revolving wing and is subject to the same
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.
about looking at wind shear and "ground effect".
Hugh can be e-mailed by clicking
Rabbit takes up the discussion, with a scientific slant to the argument....
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'?
was also amused that something could be "factually quite
the reverse of the truth", but this is by the bye.
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
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
Taylor takes an opposing view!
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.
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.
a pilot of both models and full-sized for many years I try to
make life simpler, not more complicated!
anyone any information about wind shear and ground affect in relation
to model aircraft? If you have point of view on this subject please
write to us here at 'Flying Sites' and we will give it an airing
on our 'Feedback ' page.
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