What are you going to do with that airplane?
At essence, this question is one of the most important. The role, use, character and design implement this question. How fast, how far, how high, what can it do? The answer to these questions really derive from its intended use. If this plane is for fun - then by all means enjoy it.
Based on a proven military design, the botmite 1 here is great for farming where you may have to cover a lot of ground seeing how crops or cattle or whatever interests you. For now - at 35 miles per hour, botmite 1 can survey the landscape for a couple hours max. Pushing it to 70 miles per hour, botmite 1 may run for about a half hour. A gas version can run for a few hours at 70 mph. Depending on many other factors, time aloft can be much less and the aircraft can move much faster.
For mountain, firefighting, desert, or ocean rescue a botmite team of airplanes can cover more territory for lower cost much quicker than a conventional aircraft and can be equipped with an instrument package suited for its role. When time is of the essence, botmite 1 could be a real lifesaver.
In Australia, there is an annual competition with worldwide contestants. The goal: to help a lost “dummy” using onboard cameras on a flying sky bot to deliver water to help save the dummy’s life. Hotel couriers are not about to be replaced but the outback just got a little closer.
Wings, Airfoil, Transfer of Thrust to Lift
The importance of the wing and airfoil can not be glossed over. As air passes over and under a wing, lift force is a result and the reason an airplane attains flight. Much of aircraft design is built upon and in context of the wing, its size, aspect ratio, taper, shape, airfoil and control surfaces. Selecting an airfoil generally considers the flight speed envelope, aircraft weight, flight characteristics along with the flight dynamics. With lift comes drag; lift typically operates in vertical vector and drag in a horizontal vector relative to flight path. Drag is the price of lift but selecting an airfoil carefully considering design factors minimizes drag and opts for a moment that can be managed by the tail or airplane design while minimizing surface drag - as in a tail section. So, sometimes the least moment is the best and selecting an airfoil with less lift may mean faster speed of operation.
Another aspect of wings requires structural analysis or mechanical engineering and testing to assure the highly focused algebra and calculus to bring their close approximations to testing and proof.
Whether gasoline, electric, fuel cell, the available energy onboard the aircraft is applied through the propeller to generate thrust. Similarly, the wing makes use of that thrust force to assert the force of lift; this includes more turn control or a shorter turn radius along with higher g forces exerted upon the aircraft. The wing literally holds the aircraft weight aloft and must be structurally strong to withstand the combined weight of high g forces, positive or negative, several times the mass of the aircraft. A g force is not actually the force of gravity, a g is a centrifugal force. It just feels like gravity; it is the FEELING you get when a centripetal force is applied to you. Wings can be ripped off the airplane if this force exceeds the design strength.
Aircraft Engineering & Design
- At a core level, the ratio between speed and energy is dependent on many factors but to approximately double the speed the amount of energy consumed may increase by the square. So, going from 35 mph to 70 mph in a particular design may mean something like three times the battery power or a much shorter time aloft. Tripling batteries adds much weight to a small airplane and so diminishes the performance and may not be worth it.
- Given the much lower energy usage of level flight over a climb, some airplane UAV’s are now taken aloft in a much larger plane and stay for extended periods. The more a bot is tested, the more experienced a pilot the bot becomes. A sky bot can handle much more and react much faster than a pilot - watching a missile easily demonstrates this point.
- Simply adding lift by making wings larger is but one factor, aircraft design is a confluence of considerations.
High wing trainer aircraft, typically with the motor and propeller in the nose are preferred by many for their ease of flying and ability to quickly recover from stalls.
Power: Lithium Polymer Batteries & Why they are used
LiPo batteries contain something like sixteen ( 16 ) times the energy density over Lead ( Pb ) batteries by mass or weight. In fact, a LiPo plane can outperform a nitro gas motor. But LiPo batteries are hi-tek and require care in handling, storage and charging just as gasoline requires care. LiPo batteries are used for performance; basically are not messy, and low cost of using them. That’s why so many cell phones use lithium batteries.
March 2014, not yet public available: 400% improved energy density Lithium batteries over LiPo batteries with high current transfer and 5 times longer repetitive use.
Prior to considering the heat loads placed upon the aircraft electric motor, one calculates the aircraft weight, airfoil, lift, drag, aspect ratios, motor rpm’s per volt, type of battery, battery voltage, propeller size, type, and byte, along with the objectives or usage of this aircraft. After the selection of battery, motor, ESC (electronic speed controller), propeller, and voltage required, one then considers the problem of heat products. Outrunner electric motors offer the most effective and efficient choice of all electric motors. Which combo should be used? Java Script and stand alone software can help you pick the most suited hardware for whatever application you want or need. Then, a flight simulator can put the aircraft, custom if need be, through its paces to see how it would fair in various contexts and maneuvers.
Gasoline has been the fuel of choice of small reciprocating engines for over 50 years. Since the 1990s, Lithium Polymer (LiPo) Batteries appeared in the market place offering a less messy alternative to gas motors. Yet, for endurance without solar wing panels, gas has a higher energy density than liquid hydrogen. Gasoline motors do not require the degree of calculation as electric motor designs. Kerosene, Gasoline, Hydrogen Fuel Cells offer the highest density of energy by mass and are able to outperform LiPo batteries for greater range today.
Heat Dissipation by Convection
Convection cooling or heat transfer is where a medium, such as air, passes in enough quantity past a warmer surface so as to remove heat from a warmer body. The amount of air is typically measured in CFM or cubic feet per minute but the BTU’s are measured in heat produced per hour. A BTU, or British Thermal Unit is the amount of heat necessary to raise one pound of water one degree.
The largest consumption of energy takes place when the motor is spending electricity to gain altitude above the ground. But with altitude in the thousands of feet AGL (above ground level), atmospheric density is reduced along with its ability to transfer heat. Typically, more airflow is required to reduce the same amount of heat. Straight and level flight requires much less energy and does not produce nearly as much heat.
At 850 watts full power (multiply 3.4 by the number of watts consumed to yield BTU’s), the electric motor produces close to 2,900 BTU per hour. Motor failure, in particular the permanent magnets surrounding the stator core happens at above approximately 200 degrees Celsius. So, how to remove or transfer this heat is important to the operation of this aircraft.
Designing this aircraft to handle a worst case environment: First, take a large propeller, such as a 14” (inch) diameter relative to the aircraft’s size and begin calculations from that point. The object is to reduce the possibility of failure in the lowest cost manner while designing in depth.
Given a hot day of 120 F at sea level, the first question is: how much air must efficiently flow past this electric wire core stator and rapidly moving magnetic metal casing to assure adequate cooling? Also, one can not ignore relative humidity or the weather’s effect. Second, what is the best, most reliable, and minimal resources required method to cool this motor while adding minimal weight?
Delta T, the difference between the body temperature and the temperature of the air flowing past this electric motor along with the quantity of air flowing past this body ( or CFM ) is of prime interest to the design and engineering of this structure.
Ducting or redirecting atmospheric air and releasing that air immediately through the fuselage shell requires no fan motors and can use existing structure without adding any mass to the airframe. The larger the surface area exposed to moving air through the heat source or out-runner motor provides the fastest and most direct target of heat exchange.
Ventilation is thus a significant consideration in determining the air worthiness of the aircraft design and engineering. Further, airflow speed, quantity, and temperature as well as how effectively heat is transferred from the electric motor are all factors to include in performance evaluation.