In theory at least there is a great advantage in the variable surface plane. As yet but one aeroplane has been flown in which the pilot may increase or cut down the spread of his wings at will. When such control is possible the aviator may employ the maximum spread of the wings for rising, for instance where it is needed, and then reduce their area when aloft, thus gaining in speed qualities. At the present stage of the development of the model aeroplane a variable wing does not seem practicable, although it is reasonable to suppose that the improvement will come in time. The control would of course have to be automatic, which renders the problem rather complicated. What is a vortex pack? Literally a small cyclone or eddy of the air. The term is used by aviators to describe a turbulent section of the atmosphere. It is very common in flying, especially at low altitudes, to run into a very turbulent eddy of air, such as is set in motion by high buildings or deep valleys. They are extremely dangerous since they cause the aeroplane to dip and roll about violently, and call for quick and skilful handling of the aeroplane to keep it from being upset. The term might be used to describe an eddy of air which brings a model aeroplane to grief. What form of propeller will give the highest efficiency? It is impossible to lay down a hard and fast rule. Much depends upon the form of the machine. The Voison propellers have been found in actual practice to give only about forty per cent efficiency. This is said to be partially due to the fact that their parts are held together by bolts and projecting nuts which offer considerable skin friction. The Wright propellers are believed to be the most efficient propellers for large machines since their efficiency is about seventy per cent. The accompanying photographs of the standard model aeroplane types in America will show that there is great difference of opinion as to the diameter and pitch of the screws. What is meant by a variable speed machine? Merely an aeroplane whose speed may be increased or reduced by degrees at the will of the aviator. This will enable the pilot to use a slow speed for rising, for instance — and on reaching a desired altitude increase his speed. On encountering high winds, for instance, the speed might be increased so that the aeroplane would cut through them almost undisturbed. The variable speed aeroplane will doubtless soon make its appearance. Many aviators expect that it will be the next great step in the advancement of the science of aviation. Has any model aeroplane been fitted with an automatic stability device and what is it like?