"Jaunty Alouette" is a play on the French folk song "Gentille Alouette" (which means "nice lark"). This ducted fan control line model by Roy Clough, Jr., is the result of many evolutionary stages of ducted fan propulsion systems. Early ducted fans were basically a wide-blade, small diameter propeller placed inside a tube (duct). Not a lot of engineering went into the design to maximize efficiency. Over time, cones were placed before and after the engine and propeller (impeller, or fan) in order to direct the air mass in ways that created the greatest velocity, and hence the greatest thrust. Modern ducted fan units are quite different than any shown here, most notable being the shortness in the length of the ducted region. Total blade frontal area is also about half that used in early prototypes.
By Roy L. Clough, Jr.
Most practical and most easily duplicated turbine jet aircraft model developed here or abroad is this control line cutie...
This model was developed with the idea of proposing a new competition class: control-line speed, turbine jets. We further propose this class be limited to engines of .049 displacement; that nozzle cross-section area be held to 6 1/2 sq. in.; that no afterburners be permitted; that no dollies or drop-off gear be used and the minimum line length will be 25 feet.
Now, what the heck? Are there not enough different classes already? Why try to introduce another one, and why the limitations?
Well, look around at present speed, free flight and stunt jobs. For several years now the basic design philosophy has been static. It is pretty well settled how to go about designing a good model in any category. So many people have been refining the things for so long that today it is no longer a question of how to design, but rather which engine, with which prop and which fuel seems most suited to the weather conditions on the day of the official flights.
The pioneering is well over; today the emphasis is on handling technique and know-how. Therefore it seems timely to introduce something new, which is not settled, a new area of design experimentation with plenty of opportunity for the fellow with ideas, imagination and a desire to use them. We think the ducted fan speed model is a natural for this purpose.
The proposed limitations we consider wise. By keeping the displacement down the confusion of several classes within a category is avoided. Outlawing afterburners makes sense because of the inherent fire hazard and because it will place emphasis on efficient duct and fan construction; the elimination of dollies and drop-off gear should produce some interesting functionally integrated designs; and the specification of a minimum line length of 25 feet will prevent minimal-thrust jobs from cheating take-offs via centrifugal force.
Purcell's Ducted Fan (original turbine jet) duplicated in this model set-up produced 3 oz. of thrust with K&B .049.
Sound interesting? Then a good time to start is right now, and a good model to start with is Jaunty Alouette, a swallow-winged blowhard that will probably revise heavily upward any estimate you may hold at present on ducted fan performance.
What a scaled-up, man-carrying fighter version of the model might look like illustrated on this issue's front cover.
Newbold's Duct (British) was next step. It gave a thrust of 3 1/4 oz. when driven by a Cub .074. Used rim-bucket type blower.
The original model, several times rebuilt, weighs over 9 1/2 oz., without fuel in the tank. This is about 3 1/2 oz. over what is generally considered top weight for a free flight .049 fan. Yet it becomes airborne in 10-20 feet, depending upon the surface, and tops 40 mph in flight - which is the scale equivalent of a big jet edging Mach 1. It does this, incidentally, on a moderate-performing engine. We purposely did not use a really hot mill in the job because we wished to emphasize design efficiency. This means that with a hot .049, such as the Thermal Hopper, and with a reasonable weight of 7 1/2 oz. (which will be easy to hit because we took no regard whatever of excess glue, dope and wood weight) you will have no trouble bettering the already impressive performance of the original by a good margin.
Clough's Pressure Jet was extremely simple in its method of operation. Gave 1.8 oz. thrust with Wen-Mac .049 (on hot fuel).
The theory behind the design of this duct evolved out of quite a bit of experimentation with several arrangements including the original Purcell ducted fan basics; the British Newbold duct and the author's centrifugal blower pressure-jets and experiments with improved induction methods, and turbine design.
Four small sketches accompanying this article show how the present duct evolved:
The man who started it all, Thomas Purcell, Jr., used a flower petal fan running in a tight tube, that is, with very little clearance between the fan tips and the walls. The hatch must fit tightly for best results with this arrangement, and, properly put together and carefully tuned it can produce about 3 oz. thrust with an .049. It is quite well suited to scale-type-models because the diameter is not excessive. The drawbacks are chiefly structural difficulties, tying the various components together without excessive weight or weaknesses developing.
"Meld" Duct borrowed from everybody. It offered over 2 oz. thrust when utilizing an Infant .020. Ram effect was apparent.
After Purcell the most significant departure is the Newbold duct by an English builder of that name. This is radically different, using a rim-bucket type blower (similar to full scale), a pusher engine installation, cone choke in the tail pipe and flow straightening stator buckets behind the blower. This type produces thrust quite efficiently-provided the blower is properly located, and from a number of examples we built of this one, we found the blower location to be very critical. Moving it ahead or back 1/2" might cause thrust fluctuations of as much as 50% and further, unlike Purcell's duct, which works best with a tightly fitted fan, the Newbold duct gave best thrust with the fan running completely unenclosed! No provision for induction was made, except open wheel wells, open cockpit and small scale-type induction holes in the wing roots. 3 1/4 oz., .049 thrust.
The centrifugal pressure-jet, as exemplified by the author's Blowbug (Air Trails, August '53) was an attempt to achieve extreme simplicity through a different approach, the use of high internal pressure and very high velocity jet. This system is not as efficient as a ducted fan, but it is extremely simple to throw together, no particular skill being required to produce a model that will fly quite well. The thrust of this job varied from 1.75 to 2 oz., depending upon how well and tightly the hatch fitted.
The fourth sketch shows an attempt to tie together the best angles of Purcell and Newbold, with a few of the author's findings. We called this the "Meld" Duct and ran it quite exhaustively through a series of bench tests. With a "war weary" Infant Torp .020 this job produced a static thrust of over 2 oz. When in motion it appeared to pick up somewhat more push from ram effect. We knew then we had something.
The Clough Duct with appreciation to Purcell and Newbold). This produces in the neighborhood of more than 4 oz. thrust with a Space Bug .049. Construction of the airframe is now much easier.
The "Alouette" duct is basically this arrangement, cleaned up a bit, with the engine turned around, a by-pass installed and given a new fan of improved performance.
Theory: Accelerate the greatest mass of air possible to the greatest speed possible for any given power.
Method: Air is picked up by direct ram (in flight) and is accelerated down the induction tube by squeezing it between a cone choke and the duct walls. At the engine location the air is suddenly allowed to expand, increasing its heat soaking from the engine and the air pressure in front of the fan; that is, adequate cooling is insured and the heat recovery helps the fan. Now, right at the place where the air is set in motion by the fan there is a bypass slot which allows external air to siphon into that low-pressure area, further increasing the reaction-mass. As the air moves into the tailpipe it is choked down, increasing its velocity and reducing the back pressure under which the fan operates, and, as it flows by the stator vanes which support the cone, rotation energy imparted by the fan is recovered which further increases the thrust and ejection rate at the nozzle.
And that is the reason that Alouette can take off and hold the lines tight at altitude - with the engine four-cycling!
Construction of this model is the result of considerable ducted fan experience - an all-out attempt to eliminate in one fell swoop the structural bugs which have dogged these machines in the past. Note that all major masses and highly stressed components have been tied rigidly together with a central anchor beam: exit endless maintenance grief.
Instead of building up the ducts from edge-cemented 1/32" sheet after the usual fashion (it invariably cracks and buckles out of shape), we used art paper. This may be purchased at any stationer's and is variously called art paper or "construction paper." It comes in 9x12" sheets in two weights and assorted colors. Use the heavier weight for the ducts and light stuff for cones; always bend with grain.
Complete building details on full-size plans.
Full-size "Alouette" plans are part of Group Plan #954 (Hobby Helpers. 770 Hunts Pt. Ave., NYC 59; 35c.)
Jaunty Alouette Plans
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Posted November 29, 2014