Interceptor 5 (oz11422)
About this Plan
Interceptor V. Radio control aerobatic model.
Quote: "Tremendous amount of research and testing behind this design which was top Easterner at Nats. Interceptor Five, by Harold deBolt.
Obviously the 'Mark V' is a further development of the Interceptor (oz4836) contest design which I have been using for the past 3 years. Like all good things the first never seems to be an all-inclusive answer. This one's great grand daddy was a mighty fine airplane with the per-formance of a thoroughbred; Mark V's Daddy won 2nd place at the Nats. I would be the first one to say that the Mark IV was not a perfect airplane, the original layout had a real glaring fault which showed up during the first flights and the fix which was used to get it to the Nats was no real cure for the problem. So, what has been done with the Mark V is to take the basic original design, use the improvements which have been proven along the way and cure the problem which occurred in the last one. The end result has to be a good one and the improve-ments which have been made along the way should be interesting to anyone who cherishes these competition designs.
To understand the Mark V completely it is necessary to go back and review the original design concepts since they are still the backbone of the airplane today. The idea was to design the ultimate by striving for the utmost in efficiency no matter what the effort and to try to obtain the best possible compromise for the engines and radios available. Fortunately for us, even though it may not be for the sport in general, the same basic engines and radios are still on top 3 years later. Today it still seems as though the .49 to .60 engines are 'it' and that no great improvement has been found to replace the quadruple-proportional radio, at least for competition flying, so it would seem that the compromise still holds.
One of the cornerstones of the original design was drag reduction; it was felt that anything which tended to hold the model back could not help but detract from its efficiency. Therefore extreme effort was put forth to streamline and clean up the airplane. This resulted in a fully cowled engine, internal control horns, Camloc wing fasteners and on the first several versions the use of Retract-Gears. It was said that the only projection from the model's skin was the needle valve!
The second most important aspect of the design was the wing. It was felt that the most valuable portion of any airplane is its wing for this is the part that makes all the rest of an airplane workable. Hence, we went to the NACA and used all the very best data available in hopes that we could have the most efficient wing from all aspects possible. First this required a wing planform that would create the least drag while remaining extremely stable and developing ample lift under all conditions. The shape which evolved is the classic double taper with round tips which is still mighty hard to beat. The 65000 series of airfoils are of the laminar flow variety, extremely stable, develop ample lift, have one of the best lift-drag ratios and are symmetrical. This is a combination which is ideal for our competition stunt flying. We carried this even further and used progressive airfoils so as to take full ad-vantage of the slight advantages found in the different versions of these. We used the 65018 in the center for maximum lift and progressed to the 65012 at the tips so that the tips would have more stability than the root of the wing. This meant that the wing would not have any 'wing drop' tendencies no matter how slow you dragged in for a landing and yet the average foil of the wing would be 15% which is considered ideal by the NACA. The effort was even carried to the construction by using the 'egg crate' variety which provides the least total weight while reducing tip weight to a mini-mum, both important for efficiency.
We carried the same sort of thinking used for the wing into the tail section where for the first time we used true airfoils with realistic lift-drag ratios, in fact we put the tail to work so to speak. To do this easily required the development of a new construction technique which worked so well that we are continuing to use it on even the simplest sort of models.
The item which contributes the most to making a model groove is the force arrangement and the concept used was to have the horizontal tail lift in pro-portion to the wing. Thus, as the model would change speeds things would stay in status-quo and no trim changes would be required. This involves the thrust line location and the lift developed by the tail through its size, airfoil, incidence setting and its moment arm. The original layout was quite good but also this is one area where definite improvement has been made as we will see later. These then are the basics of the design - efficiency through the use of a really clean airframe, an efficient wing and a force arrangement that really works for you. What has happened since the original inception is a story which can show what development can do if given a chance.
Mark I probably was the best constructed model of the series, it enjoyed a relatively long life and undoubtedly won more contests than all the others put together. Unfortunately, its one chance for real glory came at the California Nats where its pilot let it down rather badly. It did however serve well and showed us the possibilities of the design. One of the admirable qualities of the design which showed up in the course of its life was the ability to fly well in really stiff winds and make very decent scores. This was something new and very desirable when you consider that the performance did not deteriorate in the calm.
However, it was when flying in the wind that the one weakness of the model showed up, this trouble did not harm the maneuverability nor did it detract from the slow speed performance or landing qualities. In fact it was only present while flying straight or level into a strong wind. What happened was that the model would fish tail in direct proportion to its speed and the velocity of the wind. In other words the faster the model flew and the stronger the wind the more the fish tail. With the passing of time we find that this is not an uncommon problem and that other designs have been known to have the same fault. I think it is safe to say that the problem was licked with the Mark III version and that our findings could possibly help with these other designs.
The Mark II version was worked out more by Dwight Hartman than myself. The object was to adapt the basic design to a fiberglass fuselage. Dwight did an admirable job in forming the fuselage and test flights proved that the little additional weight could be com-pensated for by the use of just a bit more power. In fact the use of a .51 in place of the .49 normally used did the trick rather well.
Mark III ended up by showing the first real improvement in performance and indicated that it had the greatest potential which we have seen. I feel that it was a far better machine than the Mark IV version used in 1965. We now hope of course that through use the Mark V will be just as good as III and perhaps have just that little bit more which makes it all worth while. Unfortunately I left Mark III in Dallas before it ever had a chance to prove itself in competition. I have often been asked what happened down there but frankly I can not say anymore today than I could then, the post-mortem gave up no clues. All I know is that I suddenly lost all radio contact and no positive reason was ever found for it. However, during its short life span of about 40 to 50 flights before the Nats it could be said that Mark III literally flew right off the work bench and gave us a good look at the design changes which had been built into it.
As I have said Mark I had a tendency to fish tail in the wind and this we hoped to eliminate in Mark III. I also said that I felt that we have cured the problem but this was only after the usual blood, sweat and tears. I believe just about one whole Winter was spent by Ken Person, myself, and Cornell Aeronautical Laboratory people re-searching the problem before the final highly problematical solution was found. A great deal of credit must be given to Ken and Cornell for their interest and their knowledge of the problem.
To get the facts straight it should first be said that a great deal of ex-perimentation was done on Mark I to try to find a solution. It should also be said that this same problem occurred with Mark I versions flown on reeds by Person and on another using proportional flown by Jack Roth. At first it was felt that the problem was caused by lack of vertical fin area or by the relation of the amount of fin area to the dihedral of the wing. In fact, initial research indicated that this was probably the cause of the fish tailing, at least as far as the full scale people were concerned. As a result of this thinking both Ken and I did a lot of changing of the vertical area, we added it, subtracted it, placed it high and then tried low, but the fact remained that anything we did had absolutely no effect on the fish tail!
So, obviously that was not the cause of the problem, there had to be a culprit somewhere else. However, all this work did not go for naught as we found a distinct advantage from the use of a sub-fin. We found that the model would hold a straight course in some maneuvers much better when about 1/3 of the total fin area was below the thrust line. What it amounted to was that maneuvers such as the tail slide, wing over and the start of inside loops would track more easily when the sub-fin was on than when it was not. It would appear that this is a place for more research and possibly it could be an easy improvement to any of our models.
When the solution as we found it to the fish tail was first proposed it came from Cornell to Ken Person. Those people had decided that if this was not the solution the only other way of determining one would be to use a wind tunnel where very close observations could be made and as a result we were giving it serious consideration as we did not put much faith in the proposed solution. The idea as put forth was to change our thinking about the design rather drastically. It was suggested that what we had was no longer a model airplane in the general sense of the word but instead a miniature aircraft in the true sense of the word and one which possessed many of the problems inherent in the man-carrying machines. The main reason for the switch in thinking was the flying speed, it is one thing to have a model air-plane floundering around in the air at speeds up to 50 mph and quite another to have one of same size zipping along over 100 mph! You can get away with a lot of things at 50 mph which just can not be tolerated at twice the speed. It's as simple as that, especially if you realize that we do have a scale effect. The shocker was when it was suggested that scale-effect-wise a model of 5-1/2 foot span flying at over 100 mph could be compared easily to one seven times its size flying at 700 mph or close to the speed of sound and in the area of compressibility!
In any event and whether you go along with the suggestion or not it is a fact that full size machines have run into a fish tailing problem when they have approached the compressibility area which lies just below the speed of sound. Remember that our model only encountered this problem at maximum speeds and even then the air speed had to be added to by wind..."
Interceptor 5, American Modeler, January/February 1966.
Direct submission to Outerzone.
Supplementary file notes
Article pages, thanks to RFJ.
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SteveWMD - 09/08/2019
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