Ducted Fans! -- Scale-Like R/C Planes
March 1962 American Modeler
As with most other aspects of aeromodeling, ducted fan propulsion systems have come a long way since the time you had to build the entire unit yourself. Modern prefabricated ducted fans are molded glass-filled nylon or carbon fiber that has been engineered, manufactured, and optimized in every aspect of strength-to-weight, thrust, and dimension for the intended application and powerplant. It is like the difference between early radio controlled helicopters that were built on hand-drawn plans and built on a lathe and end mill, and the models you can buy now that practically fly themselves right out of the box. If you do, however, relish the thought of designing and building your own ducted fan or if you just like learning about how the pioneers developed the science into what it is today, then this article is for you. |
The ducted fan unit to the right is E-flite's EFLDF15 electric motor powered unit. It costs a mere $30 from Horizon Hobby. Using the purpose-built BL15 brushless motor ($65) on a 3-cell LiPo, you can expect 1.7 pounds of static thrust at 31,000 rpm. Add another cell in series and increase the thrust to a whopping 2.8 pounds at 40,000 rpm.
Ducted Fans - Scale-like R/C PlanesBy P.E. Norman
Having been an ardent flying scale fan since I built my first model plane back in 1925, it was with mixed emotions that I watched the rapid progress in jet-plane development after World War 2, spelling as it did the beginning of the end of the military prop-plane era. No longer would there be such a wide choice of interesting new prop-driven aircraft from which to choose new subjects for my scale activities.
Author-experimenter P. E. Norman, son Marcus, friend, 5 R/C ducted fan models.
But as the years went by and the post-war skies of Britain became more and more the domain of jet-planes, these beautiful winged projectiles soon cast their spell. I determined to build flying replicas and join the Jet Age myself! This was in the early "Fifties" when very little work had been carried out with model jets. As far as I could ascertain, the ducted-fan appeared to offer the most practical source of power ... especially for my own particular style of model built very rugged to withstand plenty of wear and tear as well as frequent contact with trees, buildings and Mother Earth!
Intake and efflux (outlet) sizes were an unknown factor, but extensive bench tests with tubes and cones of various diameters and tapers, showed that they would have to be of generous proportions. Restricting the opening at either end caused a substantial drop in engine rpm and "blow" from the rear end. So from the start all of my D/F models featured monocoque fuselage construction to permit the largest possible size fans to be fitted. Observation of other D/F craft have confirmed that those equipped with small fans (in relation to maximum fuselage cross section) usually have marginal performance. Most seem incapable of ROG fights or of gaining much height after being launched from shoulder level.
With all my models I have always tried to approach something like scale speed, so they usually fly fairly fast and are heavier than the average F/F scale type. To continue the same kind of operation with D/F models, I would have to squeeze every possible ounce of thrust from the powerplants. Which led me to the next unknown factor - the actual fan. My first fan of bent aluminum was discarded at an early stage of testing when a blade flew off at peak rpm and just missed my foot.
Boulton Paul 121.
Fan Design ... Among the materials I tried for fan blades were 1/16" plywood (quite good), tufnal, cut-up, plastic bottles (too flexible) and finally fiber. Fiber turned out to be ideal, so I have used it exclusively with blade thicknesses varying from 1/32" to 3/32". Fan blades should be cut slightly oversize and secured in the hub slots with resin glue and small brads through the hub face. When dry, a disc is bolted to the front of the fan so surplus portions of the blade tips can be trimmed off. Blades should be bent gently into a slight camber and then given several coats of fuel proof dope for protection against warps and oil soakage.
I have tried dozens of different fan designs, with 3 to 24 blades of various width and pitch setting, in combination with different hub diameters. Best results have been with 6 and 8·bladed fans (1" to 1 1/8" width). set at 40· to 45° pitch in reasonably small hubs. Fan hubs are cut from 3/8" thick plywood and accurate blade installation obtained by using a simple jig for aligning hub-slot locations correctly. I make such a jig from a 1" wide strip of 1/16" aluminum. A hole of the same diameter as the engine crankshaft is drilled close to one end of the strip, which is bolted firmly to the plywood hub (already marked with the blade positions on one face). The strip is bent over at right angles (so that it rests flat against the circular edge of the hub) and cut off about 1/2" beyond hub's rear face. A 40° or 45° slot is cut in the bent-over portion of the strip with a hacksaw and the hub rotated until the first penciled-in blade location is lined up with the strip-slot. Then the assembly is secured in a vise and each hub-slot cut to the desired depth.
A carefully assembled fan is important, since perfect balance is essential for maximum fan efficiency. Originally I used "diesel" (compression ignition) powerplants, but changed to glo-engines when I found the latter gave much more thrust due to high operating rpm. A large Volume of air moving through the duct at great speed is desirable - with as little restriction as possible in the area behind the fan and at the efflux (rather than a large Volume of air being drawn into the fan and then being compressed and exhausted through a restricted efflux).
Fuselage Design ... My first ducted-fuselages, built almost exclusively of balsa, consisted of 1/8" x 1/4" planking over 1/8" sheet assembly formers (later removed). Upper and lower halves were each made in this way then cemented together and covered with light silk doped in place (after first covering the interior with similar material). However. this method was prone to soak up fuel with a consequent reduction in strength and a marked increase in weight after a few flying sessions.
Obviously this wasn't the solution, so next I tried papier-maché (paper-pulp mixed with glue) fuselages, but this also suffered from similar defects. Fiberglass molded bodies proved fairly satisfactory, although they tended to come out on the heavy side and were likely to fracture upon a sudden heavy impact.
Eventually I turned to 1/32" plywood, soaked in water to make it flexible (again using 1/8" sheet assembly formers). Although this is more difficult to form (except in very simple curves), it proved the best all-round construction method since it is fairly light and highly resistant to oil soakage. So all my subsequent D/F models have had bodies of this type. The fuselage forward of the wing position should always be the strongest part of a D/F model - and you can afford the extra weight since nose ballast is usually required anyway. Resin glues (liquid or powdered) are used, extra strength is obtained in larger models by using two or three laminations of 1/32" plywood, reinforced by light nylon or nylon-chiffon attached with the same adhesive. All hinges, for powerplant, R/C equipment or other items, can be made from nylon ribbon.
Flying Surface Design ... Conventional building methods proved satisfactory for wing panels, stabilizers and fins, although I install 3/16" diameter reed along the wing leading edges and around the tips as with all my scale models. Full depth mainspars of hard 1/8" balsa, strengthened by gluing 1/32" plywood on one face, provide a very tough wing structure.
Thin, flat-bottomed sections should be used for the wings, installed at 1.5° to 3° to the thrust-line. High set stabs usually require a similar amount of negative incidence. Wing boxes are best made from 1/16" or 3/32" plywood, with hard 1/4" wide balsa spacers at the edges and the complete boxes securely bound and glued with nylon thread.
Knock-off wing panels and stabilizer are best, wings being retained by built-in spring clips, the stabilizer by rubber bands stretched over the top from hooks set in fin leading and trailing edges. The latter is for high-mounted stabs. When the stab is lower or on top of the fuselage, separate knock-off stab panels can be mounted in a similar way to the wing panels. It's usually best to mount the fin integral with fuselage. light silk covering, or preferably nylon-chiffon, is ideal.
Powerplant Considerations ... A good guide to the amount of power required is to aim for .060 cu. in., per 1-lb weight, per sq. ft. of wing area. This is assuming a well-streamlined clean design with a fairly low aspect-ratio wing. Keep in mind that long fuselage ducts have the disadvantages of increased air-friction and weight, so keep ducts as short as possible. Although air-flow straightener vanes (mounted at an angle of 30° close behind engine on a streamline balsa cone) do increase thrust slightly, they also make duct cleaning difficult. Down-thrust-vanes of 1/32" plywood should be installed in the extreme end of the duct efflux to permit power flight trimming.
Getting down to cases for typical model! powerplant combinations: 18" to 24" wing span suits .049 engines; 24" to 30" span for .09's; 30" to 36" span for .15's and 36" to 42H spans for .19''s. Most of my D/F flying with American powerplants has been carried out with Cox and Fox products.
Typical fan-diameter/powerplant combinations are 3" for .049's; 3 3/4" for .09's; 4" to 4 3/8" for .15's; 4 1/2" to 4 5/8" for .19's. For 3" dia. fans, 6 blades should be used - 8 blades on anything larger. Both 3" and 3 3/4" fans (1/32" thick blades) need 40° pitch - while the larger sizes (1/16" thick blades) can take from 40° to 45° pitch.
It's worth while making up several fans of varying pitch and picking out that best suited to a particular model, after flying tests. Although there is quite a bit of work involved in these fans, remember that the chance of breaking one in a crash (except in a complete write-off) is very slim. A range of commercial fans of this type are made in Britain by Veron - some U.S. mail order dealers carry them.
Intake area should be as close to fan-diameter as possible; efflux area should be from 70% to 80% of the fan-diameter area. Direct nose intakes or "elephant-ear" intakes (forward of wing) are far more satisfactory than wing-root intakes. The circular intake and efflux areas may be changed into elliptical shapes by cutting 1/2" wide strips of thin card and overlapping the ends to form circular rings, then squashed slightly to provide the desired section patterns.
Combined plywood engine/wing-tongue/fuel-tank mounting-plates must be absolutely rigid and well braced to the lower fuselage half to avoid any vibration. For .049 to .09 engines use 1/8" plywood; for larger motors, 3/16" plywood. Fan rings of 1/32" plywood (1" wide) should be glued to inside of duct, with a minimum of fan clearance, to avoid blade tip losses. Engine, fan and tank must be easily accessible for cleaning and inspection, since the oily duct interior retains an amazing amount of dirt and grass picked up during landings.
It's a good idea to fit engine exhaust stacks with very light aluminum shrouds, to prevent dirt from entering the cylinder head. Engine holding bolts should be locked to guard against vibration. All access hatches must close flush and hold securely, since a hatch opening in flight will most certainly result in a spin or dive-in.
Flying Ducted Fans ... Correct balance should be obtained by adding weight (I use empty cement tubes) to the nose. Before leaving the workshop, become well acquainted with starting your engine by means of a cord starter, find the best needle setting for maximum rpm and check the duration of the fuel tank. A D/F model without landing gear should push itself slowly across the floor under full power.
In addition, to your usual tool kit, starter battery, you should take along a spare starter-cord, glo-plugs and fan. Before starting engine, remove from intake front anything which might get sucked in and damage the fan or engine.
If possible, fly over reasonably long grass. Try to choose a time when a slight breeze is blowing as these models tend to have a fast gliding speed. Aim for a dead-straight glide, without any suggestion of a stall - correcting any turning tendency by small celluloid or plastic trim tabs attached to the rudder and one wing tip. Avoid maximum power for the first few flights, as slight adjustments may be needed on the downthrust-vanes (to prevent stalling) or to the trim tabs (to correct too sharp a turning tendency).
Types Built ... Since turning to D/F models, I've built and flown 11 scale and 13 scale-like F/F types - followed by 7 near-scale R/C types. These divide into four flying-surface configurations, as follows: #1) Swept-wing; #2) Delta wing; #3) Swept-wing with swept-stab; and #4) Deltawing with delta-stab. Of these, type #3 models have proved the fastest and most stable, with type #4 following close behind - they need no dihedral, provided the stabs are set high. All types have performed well and most models built are still in good flying shape.
Apart from the four general categories detailed, my D/F experiments have been concerned with three different series of models. The first set included models from 30'" to 36" span, usually powered with a .15 engine. My second series consisted of smaller models of from 23" to 28" span each with an .09 or similar powerplant. The third series covers most of my present D/F activities - R/C designs from 32" to 44" span, powered with an engine from .09 to .35 size.
Scale Ducted-Fans ... the first F/F scale D/F model I tackled was the MiG 15 powered by a .11 diesel (driving a 6-blade fan), It had a span of 36", weighed 27 ounces and was very stable, with a moderate rate of climb. This was followed by two more MiG 15's, of slightly smaller size (32" span), but with more powerful .15 diesels. One used conventional balsa construction (like the first MiG) and weighed 26 ounces; the other had a fiberglass fuselage and weighed 29 ounces. Both differed from the original MiG in that they featured integral fins and separate knock-off stab panels, instead of a single knock-off fin/stab assembly.
These two new models had greatly increased thrust output and really got up there in a hurry. They were soon followed by a string of similar proportion .15 diesel powered scale types - including the Boulton Paul P.111 and P.120 deltas. Ouragon Mystere and the Grumman Cougar.
R/C Ducted Fans ... Some of the most exciting flying I've ever experienced has been with R/C ducted-fans. They are beautifully smooth in flight and respond to control immediately. To date, I have only used rudder, but intend to try multi on future projects. The present models are fast (about 50 mph). Since they have little torque or gyroscopic effects they are quite easy to fly. They climb well and settle easily into a fast, flat glide, followed by smooth belly landings. Low passes downwind at a height of 2 to 3 feet with a 50-60 mph groundspeed stir anyone who ever nibbled cement off his fingertipsl
My first D/F type fitted with R/C was a .09 diesel powered delta-wing/delta-stab aircraft of 32" span and allup weight of 28 ounces. A 4.5 volt transistorized receiver was carried in the nose, pencells in the fuselage "spine" and a very small home-built escapement housed in a fin-blister. Initially flown in '59, this one took first place in an R/C spot-landing contest and also made un-assisted ROG fights, using a take-off dolly. A larger 36" span version of 350 sq. in. wing area was next powered by a Cox .15 Olympic; she weighed all-up 40-0z.
Subsequent R/C designs have all been swept-wing/swept-stab using a Max .15 then a Fox .15 (34" span, 36 ozs.), Fox .19 (39" span) and Fox .35 Combat (44" span, 3.75 lb) powerplants. The .19 and .35 models develop enough thrust for easy loops and wing-overs.
Scale-Like Ducted Fans ... Most of my F/F semi-scales have been based on well known British, American and Russian jet-fighters. Sometimes I borrowed features from different types to obtain characteristics that fitted in with the type of model size/powerplant/fan combinations I wanted.
Included among the more interesting semi-scale D/F types I've turned out is a .15 powered Douglas Skyray delta of 31" span and 30 ounces weight - built to obtain very large intakes and efflux openings - with a thinner-than-scale fuselage nose, which tapers sharply at the point it merges with the large intakes. It flies at very high speed with a rapid rate of climb, has remarkable stall recovery-the latter is an outstanding and rewarding feature of all delta and tailless types.
After many .15 powered models, I decided to tackle a series of smaller designs, with a view to closer scale speed performance. Although the larger ships were quite fast, they did not reach 60 mph - which would be the scale speed of a 36" span model when compared with a 36 ft. span, 700 mph-plus fighter. Since a 24" span model of the same type would have to obtain only 40 mph for scale speed, I decided to "think smaller" and this resulted in a delightful delta-wing/delta-stab D/F of 27.5" span and 21 ounces weight.
This model, dubbed "Javahawk," was a blend of the British Javelin and the American Skyhawk fighters. Fuselage and duct side profile consisted of a straight underside and gently curved top to give built-in down-thrust effect under power. It once turned in 7 minutes on the glide alone! A lighter version can be trimmed to perform consecutive loops until the fuel runs out. Both models definitely fly at scale speed!
Then there is a swept-wing/swept-stab job based on the Supermarine Scimitar. Powered with a .09 diesel, spanning only 27" and weighing 20 ounces, this one has ultra-thin 5% wing panels and is very, very fast. It belongs to the same "family" which culminated in the 34" span, .15 powered "Rapier" F/F (or R/C) design - plans of which are due to appear in a forthcoming American Modeler.
In a report of this type, I've naturally had to gloss over many points of design and construction. Should you want to get the old drawing board and start off with a D/F dream-ship of your own the "Rapier" plans will fill in most of the gaps. But you'd be wiser to build the "Rapier" first, as it incorporates the experience gained through thousands of flights logged with more than thirty D/F models during the past ten years.
Posted September 28, 2013