Evolution of a Thoroughbread

By Al Rabe. Originally published in 'Aero Modeller', September 1978.

Al with Mustang and Sea Fury

Al Rabe with Sea Fury and Mustang, each has won the U.S.Nats.

Mustang I - Mustang II - Mustang III - Mustang IV - Mustang V - Mustang VI

World class competition stunt ships are seldom the result of casual design efforts or luck. They usually evolve over a period of years. For a design to evolve, the designer must experiment with layout changes on a succession of airplanes. He must analyse the results of these changes and apply this experience to the improvement of the design. The proper evaluation of these changes depends somewhat on his ability as a flyer. The most competent designs are usually the result of thousands of practice Ilights.

While evolution and practice are all that is usually required for competitive success with classical stunt ships, semi-scale designs require a third ingredient, revolution, to become successful. Departure from proven moments and areas creates new problems which require the practical application of new ideas. The Mustang presented here is evolutionary and revolutionary. It is the sixth of a series, the result of eleven years of experience and eight thousand practice flights. It all began in 1967 with the Mustang I.

Mustang 1

The first Mustang turned out fairly well for a novice attempt at original design. It was underpowered, tail heavy, a little overweight and suffered from the use of a super airfoil shape in the centre of the wing. This 'super' airfoil had a fairly sharp leading edge and the high point was back around 50%. It had horrible stalling characteristics for a stunt model. If the last triangle corner happened to be pulled just a little too tight that centre section would stall suddenly and the model would drop three or four feet. This result&d in a number of surprise landings in the triangle bottoms. Adding four ounces of nose weight cured the tail heaviness and improved the overall handling charactieristics to the point where those heart pounding surprises occurred less frequently.

In defence of the Mustang I, it flew fairly well and was impressive in flight. In 1968, it was the first serious attempt to bring realism to semi-scale stunters. This new concept, while dismissed by some as semi-stunt scale, helped me to qualify at my first Nats.

In addition to semi-scale realism, that first Mustang was the third stunt ship anywhere to use a removable wing, (Ha Ha - DD) it pioneered the use of significant wing dihedral and the movable rudder.

Mustang II

This is the airplane published in American Modeler June 1969. It incorporated changes to the Mustang I design to correct its obvious faults. The root rib template was changed to a more conventional section and the flaps were slightly enlarged. This improved lift somewhat and completely eliminated the Mustang I's nasty stall. The nose was lengthened 1/2in. to balance the airplane without noseweight. There were minor improvements in the structure to eliminate stress cracking and a built-up fin and rudder was shown.

While the Mustang II looked realistic for its day its outlines actually left much to be desired. when I got around to building one myself, more changes were made. The landing gear was lengthened and larger wheels were used. The belly scoop was extended aft 1in. and the tailwheel doors relocated. This produced a much nicer looking airplane than the Mustang I or the Mustang II plans.

My Mustang II's structure was also improved by usinig all built-up tail surfaces and a fibreglass pushrod. This Mustang II was also the first airplane to utilize the now familiar sliding block type adjustable leadouts. I published that feature in the Bearcat article in the American Modeler March 1970.

(So, there you have it gentlemen, ajustable leadouts are neither vintage nor classic legal - DD)

By 1968 standards, the Mustang II was a good competitive airplane. It was so easy and fun to fly, and looked good enough on the end of the lines that neither flyer nor judges seemed to notice its subtly lacking performance. Bill Netzeband published a complimentary flight evaluation of the Mustang II in his R&R column in MAN February 1970.

Mustang III

In the winter of 1969-1970 I was busy building the first successful moulded fuselage competition stunt ship, the 1970 Nats 2nd place Bearcat III. Development of an improved Mustang II was taken up by Bill Rutherford. Bill used the fuselage and tail of the Mustang II and adapted the new Bearcat III wing to build his Mustang III. This new wing had a smaller wing tip chord and a larger flap tip chord. Trading wing area for flap area substantially increased lift. With improved lift and less weight the Mustang III/Bearcat III were probably the first realistic semi-scale stunt ships capable of flying as well as most classical designs. This development occurred just in time. After a couple of years of fairly realistic Mustangs, judges were noticeably less impressed by appearance and were demanding better flight performance for winning scores. Bill won a number of contests with that airplane. Because of Bill's success with the Mustang III and mine with the Bearcat III, I published a fifteen page supplement of drawings and instructions which many builders used to upgrade their Mustang II plans to Mustang III.

Mustang IV

In the winter of 1970-1971, I ran some airfoil tests to optimise the wing design of the 1972 and 1973 Nats winning Sea Fury. Bill wanted to use those test results to build an 'improved' Mustang III. We designed a new wing using a thicker rib of new contours and reduced area. The percentage of flap was increased again as was planform taper. The idea, this time, was to use improved aerodynamics to reduce the wing size to stress appearance while just maintained performance.


Progressive development of aerofoil, flap and dihedral on successive mustangs.

Bill also enlarged the basic Mustang II fuselage by adding 1/4in. to both the height and width of the fuselage and moved the tail surfaces 1in. aft to offset the weight of the ST 60 he used. The inch longer tail moment on the Mustang IV's drastically increased elevator sensitivity and required considerable adjustment of linkages and whittling on elevators to get them trimmed out. Bill used his Mustang IV to place 3rd at the 1972 Nats.

Mustang V

In the winter of 1973-1974, after three years of more or less constant engine problems with the Sea Fury's, I decided to build a new, smaller airplane to use the better running ST 46. The ever popular Mustang seemed the most likely candidate for an all out effort. The design goals were to use Sea Fury aerodynamics and construction techniques to (1) retain the pleasant handling characteristics of Mustangs I & II, (2) substantially improve on the performance of Mustang III, (3) build a scale like airplane more realistic than Mustang IV. By combining the best characteristics of these airplanes with stunt aerodynamics developed from the experience gained from the Sea Fury's, I hoped to design an evolutionary airplane with outstanding competitive performance. I also tried to make Mustang V revolutionary by using a number of altogether new ideas and sophisticated building techniques to lessen weight while increasing the fuselage size to more realistic proportions.

The first design consideration was the power/weight ratio. From experience, I've found the ST 46 capable of hauling about 60oz. To ensure a strong pattern with outstanding vertical performance a design weight limit of 50oz. was decided upon.

I wanted to use the smallest wing possible. A small wing would enhance realism and build lighter while reducing profile and parasitic drag. Based upon airfoil tests published in the Sea Fury article I expected an approxiinate 15% improvement in lift from the new concept airfoils and flaps. A 50oz. Mustang, then should be capable of a corner radius equal to a typical 43oz. stunt ship. Since 520 sq. in. total wing/flap area would be plenty for that 'typical' airplane, it should be sufficient for the Mustang V. Distributing this on a 55in. span gave a realistic plan form and taper similar to a scale Mustang.

To keep the Mustang V on the end of the lines while manoeuvring, it would have to roll slightly away from the pilot on each control application. This roll could be caused by (1) increasing the length (area) of the inboard wing and flap (assymmety), (2) deflecting the inboard flap more than the outboard flap with unequal flap horns (Palmer's differential flaps), (3) utilizing the wing swinging inertia of tip weight. Nearly all stunt ships combine at least two of these methods. Since beginning to fly stunt, I have developed an increasing respect for the third method above, tip weight. In the early 1970s my Sea Furys and Mustunts used tip weight, without any asymmetry or differential flaps, with great success. I was perhaps more successful with equal span panels than others because I happened also to be using a movable rudder which would remove the yaw component, if any, from the tip weight induced roll. It also helps to be able to stick 3.5oz. of weight into the tip without flinching and to have a fully trimmable airplane. I am convinced my airplanes fly better in the wind and are easier to trim without asymmetry or differential flaps. This information was published in the Mustunt and Sea Fury articles in 1973. Since then many top flyers have reduced or eliminated asymmetry in their airplanes.

The Mustang V was so very weight critical, however, that a slightly less desirable configuration was used. The Mustang's wing used a small amount of asymmetry (1/2in.). Asymmetry was eliminated from the flaps by adding 1/16in. more tip chord to the outboard flap to balance the flap areas. This made the Mustang V a little less straightforward to trim but it reduced the required tip weight for a small overall weight reduction.

A fairly large stabilizer/elevator total area, located on an inch longer moment arm than the Mustang IV was used because longitudinal stability (groove) is improved when the horizontal tail area is increased and/or the tail moment is lengthened. Elevator sensitivity is determined by the percentage of that horizontal tail used for elevators. Recalling the unusual elevator sensitivity of the Mustang IV, the hinge line was placed well back to limit elevator area. The stabilizer was mounted on the Mustang V with 3/32in. incidence (tilted up) at the leading edge because my previous airplanes usually turned tighter 'inside' than 'outside'. My Mustunt article explained how this is caused by mounting the stab much higher above the wing than most other stunt ships. The unusual height is the result of using a true low wing and a scale stab location. The differential in down-wash experienced by the horizontal tail, in the high tail configuration, is greater between upright and inverted flight than the differential experienced by the horizontal tail of classical configurations. The Sea Furys and Mustunts, for example, required approximately 3/16in. down elevator trim to balance their 'inside' and 'outside' tum rates. The Mustang V's positive incidence allows the elevators to approximately 'flair' with the stabilizer when it is trimmed to turn equally. This reduces or eliminates the misalignment between the elevators and the elevator root fairings when the Mustang is trimmed. Either configuration can be easily trimmed for fully normal, balanced turning characteristics.

To approach scale realism the fuselage had to be large. Accomplishing this within weight limits required moulded balsa construction. The fuselage was drafted by laying out the stunt wing and indicating the desired moment arms and thrust line. Onto this paper was projected Mustang outlines from an accurate North American 3-view. The critical locations of the canopy, scoop, tailwheel and stab height were determined. The slimmed, but photographically derived characteristic shapes were drawn m. The result was a fuselage stretched to stunt moments but pleasing to the eye and appearing more 'right' than many supposedly scale ships. The vertical fin and rudder were enlarged so that the stabilizer and elevators would not appear too big by comparison.

To fly the Mustang in FAI events a muffler was required. Conventional muffler installations had to be avoided for several reasons. First, most mufflers were much too heavy. Second, most mufflers create too much back pressure causing the engine to run unnecessarily hot. Third, an external muffler would disturb the airflow over the inboard wing/flap root causing a loss of lift and requiring extra wing to compensate. Fourth, an external muffler would disturb the airflow over the inboard stab/elevator causing an unpredictable effect on handling characteristics and stability. Finally, an external muffler would be particularly unsightly on an otherwise 'realistic' airplane. Clearly, the Mustang's muffler had to be light, cause little or no back pressure, and go inside.

The only practical way to put the muffler inside was to 'cant' the engine and use an 'elbow' type adapter to duct exhaust gases to the muffler body located behind the engine. The adapter protrudes into the mouth of the muffler body without touching the body walls. Since the body does not touch either the engine or adapter, it could be made extremely light and attached semi-permanently to the airframe. The adapter could also be verv light as it does not support the weight and vibrating mass of the body. The result was a flow-through muffler where cooling air flows into the muffler body around the centre rather than through the centre. It weighed .9oz installed.

Engine compartment

Engine compartment shows three larger welded-on cooling fins and stand-off
baffle silencer enclosed in fuselage behind engine.

If care is taken to flare the muffler body inlet, the muffler becomes an augmented exhaust system as installed on a Cessna 310 or a Convair440. The exhaust, blasting into the muffler body, induces a flow of engine compartment air into the annular body inlet. This pumping action draws air into the cowling and past the engine thereby 'augmenting' the engine's normal ram cooling. if the muffler body is removed from the airplane, the engine will run hotter and longer.

From this, I conclude that this particular muffler design is not only augmenting the engine's cooling, but is also increasing its power, probably through a reduction of back pressure in the exhaust system.

The muffler body was completed by adding two 1/2in. stacks which protrude through the fuselage bottom. While this was obviously not a very restrictive muffler, it was very effective due, in part, to being wholly enclosed. More noise energy was dissipated by the dilution, mixing and cooling of the exhaust gases in the muffler. Canting the engine made it impractical to use conventional engine bearers extending through the tank compartment and radial mounts were too heavy. Combining the best characteristics of both bearn and radial mounting resulted in the use of short wooden beam mounts installed in the engine compartment only. These extended from the spinner ring to the firewall and were cut off flush with the back of the firewall. This mount, including the plywood spinner ring and firewall weighed 40% less than an aluminium radial mount by itself.

It was also impractical to make the fuel tank removable or adjustable. Since it had to be glued in anyway, I tried to make it part of the load carrying structure by gluing it to the firewall, then used 1/16in. balsa sheet egg-crating to transmit engine vibration to large areas of fuselage shell structure. The tank itself was a two vent, baffled stunt tank. In the top view this tank was not rectangular. With the front and back of the tank mounted flat on the forward and aft tank compartment bulkheads, the rear end of the tank was 1/4in. closer to the right side of the airplane than the front end. Canting the rear of the tank outward makes the tank more nearly tangent to the flight circle while the airplane is in flight, reducing the volume of unburned fuel in the tank when the pickup uncovers. Airplanes using this tank configuration are capable of a very clean engine cut only a lap or two after completing the cloverleaf.

This entire nose design built unusually light. It was adequately strong but a little marginal as to its effectiveness in vibration dampening. It worked OK but an out of balance propeller can damage or destroy the nose. This happened once when a wiping cloth blew into the turning prop of Mustang VI. The prop broke and the entire nose vibrated off the airplane. An all new nose was built without changing the original structural design. It was grafted onto the Mustang and has survived an additional 1525 flights, crack free.

Another obvious feature of the Mustang V was its shock absorbing landing gear. This would seem unnecessary as all of the Mustangs were excellent wheel landing aircraft. However, experience with shock gears on two Sea Furys convinced me there is no other way to make a stunt ship take-off as smoothly or land as softly. These two outstanding manoeuvres are not only scored high but seem to add a definite 'quality' to the rest of the flight.

Unfortunately, I had a great deal of trouble with the dope sealer during the finishing of Mustang V. First, dope shrinkage buckled the wing sheeting. This necessitated removal of all wing skins, repairs to the ribs, and resheetmg of the wing. To prevent a re-occurence, I plasticised the dope sealer. This time the dope failed to completely harden making it impossible to properly remove the surface sealer by sanding. The extra weight from repairs and incomplete removal of sealer drove the final weight to 56oz. Now, I use Hobbypoxy from the bare wood up.

Regardless of the finishing problems, I thought Mustang V, 'Easy Two Sugar' looked beautiful. Its small wing, long moments and scale lines gave it an undeniably 'real' look.

In the air, E2-S turned out to be a little less than I had hoped for, but much better than I had expected, considering its overweight condition. Surprisingly, the new wing handles the excess weight well. The extra weight did, however, ruin the outstanding power/weight ratio I had planned. This loss of anticipated vertical performance, more than insufficient lift, characterized the Mustang V.

The Mustang V also had poor comers. I had overcompensated for Mustang IV's elevator sensitivity by making the elevators too small. Since E2-S had a removable wing and vafiable flap/elevator ratio, an effort was made to improve the turn by increasing elevator deflection. The extra elevator travel helped the turn but had an overall undesirable effect on handling characteristics. Numerous trimming flights indicated the original one-to-one ratio worked best. Next, the CG was moved aft to improve the turn by machining weight from the engine and spinner and adding weight to the tail. The tail weight was removed when the 'groove' suffered. Finally, the central handle was modified to increase both elevator and flap deflection. These efforts improved the turn, but E2-S would always be a little 'soft' in the corners.

The appearance of the corners was also hurt by a tendency to roll or 'hinge' on hard corners. The hinging lessened as tip weight was removed. The last of the tip weight was removed at the 1974 Nats. By then E2-S was flying smoothly unless the controls were jerked. Fortunately, the weather was good, as the absence of tip weight had reduced the Mustang V's windy weather capability. After the Nats, I discovered the outboard flap was torsionally weaker (springier) than the inboard flap. It was working like Palmer's differential flaps. In hard manoeuvres the air load would reduce the outboard flap deflection allowing the inboard flap to roll the airplane. Adding extra area to the outboard flap balanced the asymmetrical roll force and allowed the use of normal tip weight for excellent line tension in the wind. It was a relief to discover that Mustang V's major trim problem was caused by a construction fault rather than a design fault. I had begun to suspect the Mustang V's semi-scale configuration was somehow spoiling the airflow over the model creating an untrimmable condition.

Excepting the slight softness of the corner, the Mustang V finally flew well. To put this in perspective, E2-S placed 3rd at the 1974 Nats, less than one point out of second place, before its problems were fully sorted out.

Mustang VI

This was a straightforward clean-up of the Mustang V design. The elevator hinge line was moved forward 3/16 and 1/2in. was added to the stab/elevator span to make the Mustang turn and groove better. To save weight, the wing was glued on as the optimum flap/ elevator ratio and stab incidence had already been determined. Mustang V's hinging tendency was eliminated -by making the outboard flap tip cord 1/8in. wider than the inboard flap tip. Both flaps were covered with five layers of epoxy resin and glass cloth for much greater stiffness and aerodynamic effectiveness. The size of the vertical fin and rudder was increased again to provide more directional stability for further protection from hinging. You don't hear much about my airplane 'wobbling' in the corners any more. Comments are heard, however, about the accurate shapes of my manoeuvres which couldn't be accomplished without the extra line tension from a fairly mature movable rudder system.

Finally, two inches of wingspan was added to the Mustang VI because Mustang V's overweight worried me. My design/construction concepts were verified when Mustang VI finished at exactly 50oz. This made the extra wing not only unnecessary but unsightly as well, particularly when painted silver. For two years now, I have threatened to cut that excess off.

The Mustang VI's flight characteristics were excellent. In addition to being easy and pleasant to fly, it was very smooth and groovy. Increased elevator sensitivity gave more corner than I could use. The extra parasitic and profile drag from the oversize wing plus the induced drag build up in an extra tight comer, would slow the Mustang VI more than it could comfortably recover in a vertical climb. It was obvious that the excellent power/weight ratio of a 50oz. airplane powered by a strong ST 46 could still be improved.

In 1976, the Mustang VI placed a very stisfactory 3rd at the Nats. While I am usually not all that pleased with 3rd, placing ahead of the World Champion, 2nd and 3rd places in the World Championships made it seem like a good day's work.

In 1977 Mustang VI was brought to maturity by an increase in power in the form of a stroked ST 46 and a prop having more pitch. Finally, I could take full advantage of the Mustang VI's corner. With a .51 motor in a 51oz. airplane the Mustang VI was then able to fly very slowly for an impressive presentation of its new sharpened comers, while retaining its vertical performance and windy weather capability.

When I increased the lap time from 5.4 sec with the ST 46 to 5.7 secs with the .51, the slower speed and the increased prop load caused cooling to become a problem again. I needed two engines, overhauled and alternated frequently to get through the 1977 contest season. At times, to combat the effects of excessive heat, ring end gaps of as much as .0I4in. were used. After the Nats, I modified a ST 46 case by welding on and machining three larger cooling fins. A cowling baffle and stand-off cylinder baffle were also added to improve cooling. With these modifications, it was possible to use a ring end gap of .006in. for a better seal and more power but the cooling problems were not yet eliminated. Since then I have increased the custom case fins to five and installed a machined head with greatly enlarged fins and a new muffler of improved design. It runs great now but I'll have to wait for summer's heat to find out if the problem has truly been licked.

On a memorable evening last August, I was awarded three trophies: 1st place Nats Open Stunt, the Walker Cup, and a PAMPA award for technical contributions to the sport of precision aerobatics. The PAMPA award mounts a figure control line flying a Mustang. I agree that if technical contributions have been made, then the Mustang best symbolizes those efforts.

Plan View

'Easy Two Sugar' Mustang V shows its exceptional scale-like layout
for a competitlon aerobatic model.

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