Vic Smeed Cherub

Anything with a propeller
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Wayne_H
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Vic Smeed Cherub

Post by Wayne_H »

One of our Club members was having a clean out recently, so I was fortunate to aquire a model that upon examination, appeared a worthy candidate for S/C conversion. Bernie wasn't sure of the design's origin, it having been a design that at the time had simply appealed to him for 3 channel use.

Anyway, I started searching Vic Smeed designs since the model just had 'that look' about it. It didn't take long on OZ to identify the mystery model as VS's Cherub. First 'mystery' solved!! 8-)
"Cherub" as obtained, but with as-built nose squared off
"Cherub" as obtained, but with as-built nose squared off
original motor & cowl, but with as-built nose squared off. Note the packer at the wing TE.
original motor & cowl, but with as-built nose squared off. Note the packer at the wing TE.
Second 'mystery' was to identify the extent of Bernie's departures from the original F/F design.
  • Rudder & elevator control surfaces were obvious. But as built, the rudder in particular, was huge, being >50% of the fin area. Probably not so bad for full propo & adjustable travel but far too big for bang-bang S/C :x . Not surprising that Bernie said it had been a "bit of a handful". Fortunately, my Mule II.4 tx with Martin's terrific encoder has adjustable throws.
original.rudder.jpg
original control surfaces.jpg
  • Another control problem was the default control surface throw of +/- 45 degs. Moving the connections at the servo to the innermost holes and to the outermost holes on the control surface horns significantly reduced the rudder & elevator deflections, but the throws are still excessive for S/C. Adjustable throws will have to handle any further reduction, as I don't have any longer control horns :( and home-made horns would need to be of a length that they would be prone to breakage :cry:
  • the wing incidence had been reduced by a 6mm packer at the trailing edge. I removed that straight away.
  • Since the Cherub uses an under-cambered profile and a lifting tail plane, the original CG is >50% MAC. I've had some not so good experiences with similar F/F configurations when converted to R/C without any changes to the CG & rigging angles - the worst being a violent nose dive if the glider exceeded a certain (relatively slow) flying speed. My research at the time indicated movement of the Centre of lift to behind the CG as the speed increased. Once in the nose dive, the recovery was down to good luck rather than good management. Consequently, my strategy is to increase the longitudinal dihedral by packing up the tail plane TE, and moving the CG forward to about 30% to get a more R/C friendly configuration
  • Before I could pack up the tail plane TE, I had to un-glue the tail plane :cry:. Thereafter, I think I'll use dowels and rubber-bands, to accommodate fine trimming and transportability.
  • As received, the nose had been changed to try and get the motor, and hence the CG forward. Bernie had even turned up a heavy brass prop nut, about 2-3 times larger than the normal ali spinner-nut.
While thinking about the number of tasks still to do to get the Cherub converted to S/C, I thought about alternative, more simple approaches:
a) Convert to GG instead? Or,
b) Make another fin & tail plane with smaller control surfaces? Or
c) ???

After much deliberation (aka procrastination), I settled on:
  • A probiscorectomy :shock: - a longer nose to get as much gear, hence mass, as far forward as possible while making the rear end as light as possible.
  • Fin + rudder surgery to increase the fixed fin area by reducing the rudder size
  • Return wing incidence to original design
  • Make tailplane removable and pack up tailplane TE 3mm initially
It's still a work in progress, so more to follow.....
Cheers,

Wayne
Once a Retrobate, always a Retrobate............ ;)
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PaulJ
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Re: Vic Smeed Cherub

Post by PaulJ »

Wayne, it's a nice project but if I may....... "My research at the time indicated movement of the Centre of lift to behind the CG as the speed increased. Once in the nose dive"...... Yes, it may have been due the the centre of lift moving aft as speed increased but it will also have been due to the lifting tailplane taking over which is why I have never been a fan of lifting tails. Your "probiscorectomy" is certainly a good idea and I also like the idea of new (lighter) tail feathers but making the new tailplane a flat plate section, as Vic smeed did use with some of his r/c designs :)
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Wayne_H
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Re: Vic Smeed Cherub

Post by Wayne_H »

Hi Paul, apologies for any confusion - I agree re the lifting tailplane taking over. Rather than dive too deeply down the aerodynamic "rabbit hole", I recommend that anybody who does have such an interest try and get hold of a copy of Model Aircraft Aerodynamics by Martin Simmons - https://www.amazon.com/Model-Aircraft-A ... 1854862707

Meanwhile, a bit more to report on the Cherub's progress in my Kiwi chop-shop :D

Nose Job
I utilised some "bush engineering" to graft on a new nose. First, I removed the packer block & original motor mount and cut off the angled fuse sides just in front of the canopy where there was extra thickness from the original doublers. I then squared off the fuselage sides by placing the fuselage nose-down on a sheet of sandpaper on the kitchen bench, alternately sanding and periodically checking squareness by eyeing the fuselage off against the window frame. When satisfied with the squareness, I then traced around the fuse onto a printout of the plan, then drew up two new nose sides incorporating a 15mm longer nose (measured to the back of the motor firewall) as well as a little side and down thrust.
eyeball engineering to square off nose_1.jpg
eyeball engineering to square off nose_2.jpg
before adding the new nose
before adding the new nose
Using the tracing, I then cut out new sides and doublers and the new firewall. The new sides were glued to the squared-off fuse, then the firewall added. After checking the alignment, top & bottom sheeting was added. When dry, everything was sanded, sealed & painted white to match the existing plastic covering.
new nose.jpg
cooling outlet & access to motor connectors
cooling outlet & access to motor connectors
Motor
I swapped the original motor to a 2208 1100kv out-runner, slightly heavier than the original motor but with better torque on 2 cells.
if I still need to move the CG further forward, I'll probably replace the 2208 with a 2212
if I still need to move the CG further forward, I'll probably replace the 2208 with a 2212
motor_mounted_2.jpg
Tailplane
Since I don't have any covering film (other than doculam) or a covering iron with me, my options for fixing(?) the tailplane were limited.
- A new flat t/plane, out of either foam or balsa, would need covering (coloured tape or doculam)
- a foam t/plane would be heavier than the existing t/plane. Even an extra 5 gms presented challenges.

In the end, I opted to add a 3mm thick subspar under the existing spar and a 1.2mm subspar at the LE. Balsa sheet was added between the 3mm under-spar & TE, then sanded to provide full seating for the tailplane at the altered incidence. Once the rest of the underside was sanded & covered, it made a crude semi-symmetrical section. The clear doculam still gives the appearance of coloured semi-transparent solarfilm with the original covering on the underside still visible. Dowels were added front & back to allow attachment by rubber bands, and ultimately, shipping back to Aus when my NZ gig is finished.
removable_tailplane.jpg
new rudder.jpg
Dowels were added to the fuse to rubber band down the tailplane.
removable_tailplane_in place.jpg
The CG came out about 5mm ahead of the original position (but still about 5mm behind where I was aiming for :( ). Initially, the Tx I've set the Cherub up with is the Sankyo Tx I converted a few months back (viewtopic.php?p=11749#p11749).
finished_with.Tx.jpg
Now that it is finished, all I need are the weather and time planets to align :P
finished_1.jpg
finished_2.jpg
finished_3.jpg
Cheers,

Wayne
Once a Retrobate, always a Retrobate............ ;)
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Wavemeter
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Re: Vic Smeed Cherub

Post by Wavemeter »

Looking good Wayne

Be sure to take some long rolls of floating Gossamer with you for that Maiden - all the flying Cherubs I’ve seen have stacks of it tucked under their arms or between their legs ;)
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Wayne_H
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Re: Vic Smeed Cherub

Post by Wayne_H »

Finally got to test fly the Cherub today.
looks good in the air.JPG
The weather was overcast & breezy with passing rain. Plenty of power from the 2208/1100kv - the Cherub doddles around very nicely at less than 1/2 throttle :)

It needs the CG moved further forward and some more rudder throw, but it's almost there.

Video is up on da tube -

I had to mute the sound, as I was holding the mobius camera in my mouth :cry: and my breathing sounded like I was about to expire......... :lol:
Cheers,

Wayne
Once a Retrobate, always a Retrobate............ ;)
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PaulJ
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Re: Vic Smeed Cherub

Post by PaulJ »

'Nice one Wayne! :P You don't need to go all the way to Oz to get weather like that though! :roll: (or are you in NZ?)

Paul
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Wayne_H
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Re: Vic Smeed Cherub

Post by Wayne_H »

PaulJ wrote: 05 Dec 2021, 09:10 'Nice one Wayne! :P You don't need to go all the way to Oz to get weather like that though! :roll: (or are you in NZ?)

Paul
Thanks Paul.

The NZ weather has been wet, windy & changeable, even more so than usual according to the locals :o . Still in NZ at the moment, heading home to Oz for Christmas & New Years with the family - it's been a looong year away from them :cry:
Cheers,

Wayne
Once a Retrobate, always a Retrobate............ ;)
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PaulJ
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Re: Vic Smeed Cherub

Post by PaulJ »

I will never forget the Christmas we spent in Coffs Harbour..... We wanted to make it as different to a UK Christmas as we could so we had BBQ'd Kangaroo steaks on the beach!...... :P

Paul
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Wavemeter
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Re: Vic Smeed Cherub

Post by Wavemeter »

It’s flying beautifully Wayne - I did wonder why there was no sound :lol:
Just can’t picture this as NZ in my head it’s permanent blue sky down there
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GarydNB
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Re: Vic Smeed Cherub

Post by GarydNB »

You're allowed OUT in New Zealand????
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