DANGEROUS LI-POS
John A Thompson
Recently I bought a helicopter directly from Hong Kong and was offered a 3s2p Li-Po update at a special price. The heli is fine, it is from a well known maker, and the battery seemed to be OK as well, although it had no maker's name on it, just a CE mark. However I decided to fit a balancing lead to it and upon removing the covering was amazed to find that two of the six cells were smaller than the others (see below)! This means that the the two parallel cells are of a lower capacity and so could easily become overloaded and so are potentially dangerous. So much for the CE mark. An e-mail to the supplier brought a prompt reply and profound apology. He seems to be the innocent party and has replaced the item with a branded one complete with balancing lead. As he says, he has never had cause to look inside a LiPo before. He is now looking for an alternative supplier.
AS YOU WERE
TORE LOODIN
The
accompanying pictures
depict
my first proportional RC system a “Quadruplex" imported from
the U.S.A. It was one of the first three systems imported to Sweden. The
transmitter was valve operated but the receiver was transistorised, a
big novelty at that time in the early years of the sixties. The price
I paid was about the same as that for a good used car! The system was
analogue, pulsing at three functions - elevator, ailerons and rudder.
By a “pulse omission detector” you could achieve a non-proportional
throttle function. I think this was the best working system among the
then analogue ones, and I flew it for many years.
The
joystick was very nice to handle. Through twisting the knob you achieved
the rudder function. When we saw the receiver (right), we exclaimed
how small! Now we were finally able to fly very small models! In one of
the pictures I have placed a modern receiver at the left side of the transmitter.
But I have some smaller - not larger than a stamp - for my park flyer.
Below are further shots of the innards of the Multiplex and, below
right, the pulsing servo board with reinforcing transistors.
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Counting the Molecules
TIM COSTELLO
Last month (full scale February), after talking about Dutch Rolling etc., I made a closing reference to Reynolds Number and the relevance it has to our model flying. Martin Simons explains it beautifully in his book Model Aircraft Aerodynamics. In a nutshell, the Reynolds Number is a measure of the quantities of molecules of air that pass over a model wing in a given time. Generally, the more the merrier. In practical terms that simply means that a big model will fly better than a smaller version of the same design. From that you would therefore assume that a very low aspect ratio wing model should fly more efficiently than, say, a high aspect ratio glider. More molecules per unit area and all that.
Unfortunately life can be cruel to us modellers and it seems that the induced drag of the airflow curling in a vortex around the wing-tips of the stubbier wing assumes a greater importance than the amount of molecules rushing over and under it. It should follow then that a cunning way to increase the R.N. is simply to fly faster and indeed this is the case. Bigger models and faster, smaller ones actually do fly steadier around the club field.
There is nothing worse than to see a model being flown too slowly, wallowing around the sky in the downwind leg looking like its centre of gravity is too far back. But when it turns into the wind it quixotically exhibits a classic case of the “ballooning-into-wind” syndrome, damning the aeronautical pundits in the process. After all, we all know that a model does not suddenly gain height when it turns into wind and it certainly does not lose height suddenly when turning downwind – don’t we?
The practical importance of the Reynolds Number is easy to see in those very small and light indoor/park fly models. I have one myself, a little 14-inch span all-electric 3-channel job. You can almost count the number of molecules on the fingers of one hand as it flits around. Sometimes, if I don’t launch it straight and level at the right speed it just tumbles and drops to the grass. A case of not having a high enough Reynolds Number just at that point.
Pity
the poor pilots of the fabulous Lockheed U-2 high altitude reconnaissance
aircraft (right). It was easy enough to take off but landings were
to be approached with caution. Military pilots are not used to landing
powered gliders. When landing, some pilots were taken unawares by the
sheer gliding efficiency of the U-2. When they could see it was going
to over-shoot the runway they used to give it some down elevator to get
it down. To their surprise that just caused the aircraft to speed up.
That was quickly followed by pulling the nose up too suddenly, a wobbly
stall and crash followed, partly thanks to our previous friend, the ‘Pilot
Induced Pitch Oscillation’.
But it was at high altitude that the Reynold’s Number came to be both feared and revered. In the rarefied very thin air there were fewer molecules flowing over the wing, that is, a lower, less efficient R.N.
Due to being powered by only one jet engine the Lockheed U-2 was already underpowered by any aeronautical standard. The pilots quickly discovered that if not flown using the barest minimum of control inputs the aircraft started to buffet. This signalled the onset of a high altitude stall. In fact the cruising speed and the stalling speed of the U-2 began to converge, such that the aircraft would suddenly stall at cruising speed and begin to literally tumble out of the sky.
The extremities of the aircraft carried too much inertia to allow the pilot to pull out in time, so he would simply bail out, hoping he was over friendly territory. Gary Powers was not and joined Rudolf Hess in the annals of Famous Bailouts of the 20th Century.
Tim Costello, Dublin February 2006