Mig 15 (oz14269)

 

Mig 15 (oz14269) by James Ruffell 1994 - plan thumbnail

About this Plan

Mig 15. Radio control scale model for (home-brewed) electric ducted fan.

Quote: "Try this neat scale ship by for electric or PSS. MiG 15, by James Ruffell.

The thought that an electric ducted fan powered aircraft might actually be possible, had been lurking in the back of my mind since I became involved in electric flight, some 4 years ago. It was while reading about some of Dave Chinnery's experiments in 92 that I finally decided to at least, experiment with some earthbound fans, but where to start?

Firstly, I reckoned we all knew the formula for producing a viable electric semi-aerobatic sport model, powered by 6 or 7 'sub c' cells. This dictated a static thrust of 350 grammes minimum and an all up weight of 2 lb 8 oz max, the wing area needed to be around 2 sq ft, (smaller areas worked but tended to fly uncomfortably fast) and one of the less hot buggy motors, direct driving a 7x4 or 6x6.

It was clear from the beginning, that to match the performance of current IC fan powered models, huge amounts of power would be required. However, using calculations applied to IC fan models and substituting Watts for BHP, (see 'Ducted Fans for Model Jets', by David James) revealed, that with a modestly efficient fan of reasonable diameter (3 inch), static thrusts in excess of 400 grammes, should be possible with 150 Watts.

While this would not provide the sort of ballistic performance that we would normally associate with Ducted Fans, it should at least fly.

Several fans were constructed, using a technique pioneered by Marcus Norman (see 'Radio Control Scale Aircraft', by Gordon Whitehead), this consisted of a plywood hub turned up on a lathe and then slotted for the fan blades by using a simple jig and saw. Numerous configurations of numbers of blades, pitches and motors were evaluated, culminating in the design detailed on the plan.

Various tests with a simple fan and duct strapped to the top of a conventional small aerobatic model, seemed to indicate control difficulties, possibly attributable to the fan exhaust passing over the tail. The Mig was then developed, out of the need for an airframe to test the fan in flying.

Apologies for choosing this much modelled aircraft, but it is easy to see why this subject is favoured, a tube with wings, reasonable wing area and does not require too much tinkering with the outline to get the wind in and out. The model was scaled from the fan diameter and is very much 'Sport Scale' with the wing area increased to maintain a loading of less than 25 oz/sq ft, so hand launching would be possible. Interestingly, the model is close to 1/12 scale and the wings are less than 10% oversize.

The model features full access to fan and motor for servicing, the fan being removed via the front of the duct. This then gives access to the motor mounting screws using a long screwdriver. The rear duct is constructed from drawing film and may be collapsed within the fuselage and withdrawn, allowing the motor to be removed through the battery access hatch.

The radio gear will fit internally between the duct and fuselage walls with the AMT servo positioned under the canopy. The flight pack is situated in the bottom of the model and is free to exit via a shear catch on the access hatch during rough arrivals. If you are worried about planking, this was my first attempt, an absolute doddle and surprisingly quick.

The flying performance of the model is best described as similar to your average electric powered sport model using 6 or 7 cells, standard 540 motor and direct drive 7x4. The model will loop and roll from a shallow dive in dead air, with a roll rate of 1-1.5 sec. Elevator response from the AMT is fine, I wonder what the full size was like with that short moment and tiny elevator?

The motor: The prototype has flown with two motors, the first used was branded 'Trinity' and titled 'Oval Man'. This motor sported 'wet' magnets, ballraces, and was double wound with thirteen turns. The motor turned the fan in the model at typically 26-27000 RPM while consuming 22A from a standard seven cell pack.

Initially, this motor suffered excessive brush wear which the buggy boys told me was to be expected and for a while I pondered this problem, 10 flights from 1 set of brushes was not promising. On replacing the brushes for the third time, the motor was stripped and inspected and heavy wear was apparent on the commutator. The commutator was then re-cut, this process revealed that it was also distorted, so it was concluded that the motor was running too hot. The prototype originally had a streamlined fairing over the rear of the motor, totally enclosing it. This was discarded and brush wear is now negligible with this and a second motor.

The second motor used was branded 'Schumacher' and titled 'Flash 4'. This motor was similarly specified to the first motor but with aluminium end bell and brush heat sinks and performance seems identical. These motors tend to be expensive for buggy motors, typically about £40 but shop around as I found both motors for less than £30.

The fan detailed on the plan is matched to this type of motor, namely 13 turns and wet magnets and if you wish to use a different wind motor, then the fan will need to be changed in pitch, to maintain the overall performance of the model. I strongly suggest that you do not use any motor with more than 14 turns, as it is unlikely to develop sufficient power at the RPM's required. Both the motors described above claimed 35000 RPM with no load at 7.2V.

Fans and ducts: The fan hub and motor mounts are best turned on a lathe though you can probably improvise something suitable - ask around the club, someone will help. Make up a jig (see photograph) to cut the slots in the fan hub, draw index marks spaced at 72 degree intervals and bolt the hub into the jig, with an index mark aligned in the jig. Hold the jig in a vice and saw the slots using a tenon saw and repeat for the remaining slots keeping the depth of slot similar. It will probably be necessary to enlarge the slot to accommodate the blades, I did this with a rat tail file which was not terribly accurate but it seemed to work..."

Direct submission to Outerzone.

Quote: "Hi Mary & Steve, Please find attached plan and article for the MiG 15, designed by James Ruffell. Details as follows:

Name: MiG 15
Span: 36.3" (922mm) - measured from plan as no figure is published
Designer: James Ruffell
Publisher: Silent Flight (No. 16)
Date: Jun/Jul 1994
Description: 3 Channel Scale Electric Ducted Fan or PSS

Kind regards,
Ian Salmon."

Supplementary file notes

Article.

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Mig 15 (oz14269) by James Ruffell 1994 - model pic

Datafile:
  • (oz14269)
    Mig 15
    by James Ruffell
    from Silent Flight
    June 1994 
    36in span
    Scale Electric R/C Military Fighter
    clean :)
    all formers complete :)
    got article :)
  • Submitted: 12/12/2022
    Filesize: 1071KB
    Format: • PDFbitmap
    Credit*: Ian Salmon
    Downloads: 1441

ScaleType:
  • Mikoyan-Gurevich_MiG-15 | help
    see Wikipedia | search Outerzone
    ------------
    Test link:
    search RCLibrary 3views (opens in new window)


    ScaleType: This (oz14269) is a scale plan. Where possible we link scale plans to Wikipedia, using a text string called ScaleType.

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