Cobra Kit built helicopters page 2
|The Helicom Commuter Jr kit helicopter Model H-1A of the 1960's powered by a Continental C90-12F four stroke four cylinder horizontally opposed air cooled engine providing a top speed of 121 kmh and range of 273 km. There are many still flying today.|
|Helicom Commuter Jr single seater|
The freewheeling unit, which is a must in a helicopter if one is to survive an engine failure, was designed for him by Borg Warner - the worlds largest manufacturrers of freewheeling units in the world. This is very similar to the freewheeling units on commercially made helicopters on the market today.
The transmission design saves a great deal of money. In most American helicopters, a planetary gear system permits the blades to turn in the same direction as the engine, but these systems are very costly. Emigh's design drives the rotor blades in a direction opposite to that of the engine. This design is simple and inexpensive and is a direct gear drive. This is not new, as most French helicopters are like this now.
After successful completion and extensive testing of their single - seat Helicom, they decided it was time to offer it to the public. Many single - seat Helicom Commuters used 20 - foot blades. Then in 1969, a two - seater emerged and the forerunner of the Commuter I I - A. A switch was made to the Lycoming engine and to 23 - foot blades at this time. The two - seater started out using a 125 - hp Lycoming but, as designs usually go, weight was added, so the move to a 150 - hp engine and 25 - foot blades made a real performer out of the Helicom Commuter. This combination seems perfect - providing the ability to carry a great load while still being an inexpensive, well - refined kit helicopter.
|Helicom Commuter II A two seater|
During the next 20 years, the company passed through several hands, but the product never changed. Some cosmetic changes were made to the cockpit area in an attempt to streamline or modernise it a bit, but the reliable components designed by Emigh were left alone.
Then, recently, the rights to the aircraft were purchased by Gary Helton, of Danville, Indiana who has invested much money and time in upgrading the Helicom Commuter I I into a modern - looking machine using the latest in space age techniques. Gary named his new helicopter the Cobra.
The first Cobras were powered with Mazda Rotary engines and utilized a welded steel framework. They some - what resembled the Rotorway Scorpion I I helicopters but with a right angle main rotor gearbox, shaft driven tail rotor and triangulated steel tube tail boom. While they were fine machines, Gary felt they looked angular, and worse, somewhat dated. A complete facelift and upgrading were in order.
He came to the conclusion that for a very reliable helicopter which would be capable of going away from the airport on long cross country flights, the system of the geared main transmission, tail rotor drive shaft and tail rotor gear box which Mr. Emigh had designed for the Commuter series helicopter was still the best overall system for the homebuilt helicopter. This system has been successfully used for over 34 years by homebuilders in their Helicom Commuters all over the world. The original Commuter I I A for which Gary and his people were offering as plans and kits had a rather boxy looking enclosure, since when this machine was being designed the main helicopter of that era was the Bell 47 to which his helicopter has a slight resemblance in looks as well as in its transmission and rotor systems.
|Helicom Commuter II B late model two seater|
What they did in the Cobra is to use the same transmission system, rotor systems, tail rotor drive shaft systems, tail rotor gearbox system, the control system which was used in the Commuter I I A helicopter, along with the aircraft engine, the Lycoming 150 hp. They installed this complete system in a Kevlar sandwich structure which is very strong, lightweight, easy to assemble, as well as being very stylish. Although there is nothing wrong with the original Commuter I I A airframe, the new Cobra enjoys various advantages which were not available when the Commuter I I A was designed:
- It's a more streamlined and stylish helicopter.
- It's easier to build, since less welding and fitting is required because of the pylons and joggles are pre - moulded into the composite parts.
- Faster to build because of the pre - moulded parts used.
- More comfort to the pilot and passenger as the enclosure can be sealed and heated for winter flying.
- Faster cruising speed due to the aerodynamic design.
- Wider cabin along with more leg room and more head room.
- More soundproof due to the engine installation.
|Gary Helton's composite bodied COBRA helicopter|
Although the Cobra cost a little more than the Commuter I I A (due to the expense of the Kevlar composite structure) the people at Cobra think they are the best kit built helicopters on the market today. The drive train and engine systems are very comparable to commercial helicopters which cost many more times the price of our kit helicopter.
When Helton decided to upgrade the Cobra he utilised modern construction techniques including Kevlar and other composite components.
The main shell is approximately 22 feet long and is made of a composite sandwhich - type of construction and uses Kevlar as its main structural material. Kevlar is the same material used in bulletproof vests and is very strong and lightweight, but expensive. By using the composite sandwich type of construction in this structural shell, they could vary the thickness, weight and direction of weave of the material in different areas which require different strengths for the load applied to the airframe. Also, with this type of construction, they could model a very strong lightweight part into a functional streamlined load - bearing airframe structure.
The engine and transmission are moulded into the main lower airframe structure. The engine mount and transmission mount are bolted to the pylon. This eliminates a great deal of the labour of welding up this type of pylon, previously required by other kit manufacturers.
|MUSTANG 503 single seater|
Also, the tail rotor pylon is moulded into the tail section. This pylon supports the tail rotor drive shaft which is simply bolted to it again, saving the builder construction time by not requiring special jigs to be built. Also, the main landing gear bolts to joggles which are moulded in the main airframe structure. The main airframe can be installed on its landing gear with just a few hours work.
The remainder of the composite helicopter components which are made of a composite material are non - structural tail boom coverings, transmission coverings, main cabin covering, and front rails, along with the two place seats. These coverings are non - structural and made of fiberglass with the necessary joggles moulded in for proper alignment and attachment to the airframe. These parts are bolted or riveted to the main airframe, with very little fiberglass work required to be done by the builder.
|MUSTANG 503 HELICOPTER|
|Engine type||Rotax 503 (50-hp)|
|Maximum speed||75 mph|
|Rate of climb||500 fpm|
|Hover in ground effect||6000 ft|
|Service ceiling||10,500 ft|
|Empty weight||366 lb|
|Design gross weight||600 lb|
|Useful load||232 lb|
|Length overall||22 ft|
|Skid width||4.0 ft|
|Disc loading||1.91 p/sf|
|Power loading||12 lbs/hp|
|Main rotor diameter||20 ft|
|Tail rotor diameter||42.0 in|
|Fuel capacity||5 gal|
These parts, along with the lower airframe structure, are laid up in female moulds and are supplied to the customer with only one bulkhead to be fiberglassed in place by the builder. By using female moulds, Cobra can supply the builder with a good quality part which requires very little, if any, work to the outside of the part before painting.
Normally, all that's required is light sanding in order that the paint with which you will be covering your helicopter will bond to the fiberglass. Also, by using the female mould process, we can lower the weight of the part while still making it structurally stronger than one fabricated by the builder. Although making the original female moulds is extremely expensive, we feel that the quality of the parts which can be produced from them are well worth the initial expense. Thus the homebuilder gets a strong, lightweight part of good quality which is easy to assemble.
Many builders beleive that helicopters sre hard to fly, but that's only because they're not at all familiar with the controls and some of the terms and this intimidates them somewhat. Their first - asked question has always been: "What happens if the engine quits?"
As far as controls are concerned, a helicopter has a stick and a pair of pedals, just like an airplane, and in forward flight, these controls react just as they would in and fixed - wing craft. The difference comes in the hovering mode: movement of the main control stick, called the "cyclic" since it changes pitch of each rotor blade once (cyclically) per revolution, will cause the helicopter to move in the same direction, whether forward, backward, or sideways. The pedals are analogous to rudders, and control direction in which the nose points (but not necessarily the way the helicopter is going) by changing the pitch of the tail rotor.
There's an additional control stick, though, called the collective, since it controls the pitch of all rotor blades at once. This stick is mounted horizontally, like a hand brake, next to the pilot's left hip. Pull it up, pitch increases and the helicopter climbs; lower it and the opposite occurs. On recipical engine powered helicopters, there's a motorcycle - stlye twist grip on the end of the collective, and one of the most important things the pilot learns is proper co - ordination of collective and throttle - obviously, more power must be applied as collective pitch increases. At the same time, a pedal adjustment has to be made for and change in power, since more power being delivered to the rotor also means more torque trying to turn the entire helicopter in the opposite direction.
Now then, about engine failures. Contrary to popular beleif, the helicopter doesn't plummet earthward if the engine fails; at least not if the pilot has his techniques down. Instead it enters a flight regime known as "autorotation". The derivation of this term is obscure; some feel that it's used becasue the blades continue to turn by themselves; others joke that an early helicopter pilot once decided the the blades "oughta rotate" in an emergancy.
|MUSTANG 503 single seater|
For a successful autorotation from anything higher than about three feet above skid height, a helicopter has to have a certain amount of energy available (below that height, the auto rotating blades themselves store enough kinetic energy to cushion a touch down). This energy can either be kinetic, i.e., speed, or potential, i.e., height. The helicopter operation manual expresses the required energy in terms of a curve, the so - called "height - velocity diagram". (Or more familiary, the "dead - man's curve"). Typically, a successful autorotation can be executed from the hover up to a few feet in height; any higher and you need some forward speed. As you go higher yet, you need less and less speed, since you can dive to get it; from about 300 feet you can once again shoot a good auto from a hover.
The easiest way to understand autorotation is to think of the helicopter not as a rotorcraft, but as a very small airplane (the rotor blade) flying in a very tight circle. In normal flight (with the collective raised) the "wings" fly at a positive angle of attack, producing lift and drag, just like an airplane. The drag is overcome by thrust, in this case provided by the engine turning the rotor.
Now, the engine stops. What would we do in an airplane? We'd reduce the angle of attack, i.e., lower the nose to maintain some airspeed and keep the wing from stalling. Now it's flying at a lower angle of attack, producing less lift, but some thrust. The airplane continues to glide forward. Exactly the same thing happens in a collective: the blade or wings nose down, and their thrust keeps the rotor turning. The tail rotor also continues to turn, since it's geared to the engine, of course. A large pedal adjustment has to be made after the engine failure since there's no more torque.
NOTE: Cobra Helicopters are no longer in existence and their Cobra, Mustang and Predator helicopters are longer in production. Article date 1990 courtesy Norm Goyer.