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A little Cheetah!

quorneng

Well-known member
#1
My Lidl RC conversion as a powered glider flies really quite nicely but it does not really look the part with rather chunky wings.
FoldProp1.JPG

I then saw the Cheetah. A bit smaller (and cheaper at £5.49 free postage!) than the Lidl.
Orange.jpg

I certainly looks a bit more like a high performance glider but with significantly less wing area could it really be turned into a sub 250 g RC motor glider. Indeed there were comments on some other forums suggesting it would not be a practical proposition.
That immediately attracted my attention so I ordered one and waited for the inevitable slow boat from China. ;)
Parts.JPG

Well moulded (actually better than the Lidl) with reasonable 4 mm thick trailing edge rather than the 6 mm on the Lidl.
With no reinforcing the wings are pretty flexible a situation not helped by the 'clip together' two piece wing.
Although the control surfaces are marked out in the mouldings everything is completely solid so to fit the RC gear space will have to 'excavated' from the foam.
As the wing is already flexible at its 119 g at anywhere near 250 g flexing could be a real problem and even if reinforced the wing loading would be doubled.

My proposal is to make the Cheetah look even more like a high performance glider by adding a substantial centre section with the existing foam wings as outer panels.
If the centre section added 1/3 more wing area the weight could rise to 170 g (possibly an achievable target) without increasing the wing loading or the bending on the existing foam wings.
So now its just a case of turning a proposal reality.
I might just as well as with the current Corona virus 'lock down' in the UK I can't fly anyway!
 
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quorneng

Well-known member
#2
As this is intended to be a sub 250 g I reasoned than a simple wing centre section skinned in 2 mm Depron would be strong enough without any further reinforcement. All it would need would be ribs and a shear web at the point of maximum thickness.
As my stock of 2 mm Depron is limited, irreplaceable and as cutting out ribs tends to be rather waste full of sheet I decided to print them as well as the intervening shear web.
RibShearWeb.JPG

Together the 'unit' weighs 0.75 g. It is just repeated to give the required span.
The ribs and shear web glued to the Depron lower skin.
Center2.JPG

Although the webs are intended to to resist only shear forces when they are all glued together in a line they should provide some additional 'spar' strength.;)
 

quorneng

Well-known member
#3
As the aileron servo wires would have to run inside the centre section only the top skin was only added back to the shear web. The wing was then plenty stiff enough to be lifted from the board.
The existing RH foam wing was then glued on.
Center3.JPG

Next the trailing edge extension are added and the aileron cut out.
Extensions1.JPG

The extensions add about 10% to the wing area and provide a fine trailing edge as shown in this before and after picture.
TrailingEdge2.JPG

I don't know if the fine edge actually makes a significant aerodynamic difference but for this plane the added wing area will be useful.
The full span shown in relation to the fuselage and tail plane.
FullSpan1.JPG

It now spans 53.5" (1360 mm) compared to 1000 mm of the original
 

quorneng

Well-known member
#4
Next to start excavating foam from the nose and canopy.
Cockpit.JPG

I even kept all the foam to find out how much weight it saved.
Just under 3 g so far but it may need more 'excavating' depending on the size and position of the battery.
The canopy is retained by a little printed peg and socket at the front and small magnets (in printed holders) at the rear.
CanopyPlug.JPG

CanopyMagnet.JPG

And it survives the upside down test.
Secure.JPG

Took hours to do but with the Corona virus 'lock down' in the UK, so no flying, doing this sort of thing helps to pass the time!:eek:
 

quorneng

Well-known member
#5
Next the tail plane and fin trailing edge extension are added and the elevators cut out
.
TailExtns.JPG

The problem is the elevator hinge line axis are different so they cannot be joined through the fuselage by a simple tube.
After much thought I elected to join them using a printed universal joint.
EleJoiner.JPG

This type of joint has limited angular capability but is fine for the 10 degrees between the elevator hinge lines.
The joint installed.
EleUniJoint.JPG

The fin extension is fixed as the Cheetah will be flown 3 ch AET or bank and yank.
 

quorneng

Well-known member
#7
With the aileron servos fitted, the wires run through the centre section to exit through the leading edge and the wing skin completed the wing is permanently glued in.
The Cheetah is now in its final configuration.
WingOn.JPG

It now certainly looks 'glider like' although I am now beginning to worry about the adequacy of the tail volume. :unsure:
The next stage was to confirm the thrust available from the Racestar BR1504 motor with the recommended 5x3 prop.
The Corona virus lock down means there is no rush so I 'printed' a suitable test stand.
TestMount.JPG

Took hours to do but it is cheap and helps to pass the time! :D
On a 3 s it draws 4.8 A showing 49W on the Watt meter. At the Cheetah's expected 200 g all up that will give over 100 W/lb.
In its final form it will also have a spinner and folding prop.
 

quorneng

Well-known member
#8
My next concern was cooling the ESC. Not that it is likely to produce much heat but if inside the cockpit it would be exceptionally well insulated and would 'share' any heat generated by the battery.
After some consideration the most practical solution was to place it on the outside but as a concession to aerodynamic drag it would be in a recess with its shrink wrap removed.
ESCmount.JPG

The printed mount is glued in permanently where as the ESC is a 'tight fit' into the mount. The ESC wires feed directly through the cockpit walls into the cockpit.
With some ballast in place of the 370 mA 3s battery it should be possible to determine a suitable position for the elevator servo to achieve a suitable CofG without resorting to ballast.
 

quorneng

Well-known member
#9
Tests showed the weight of the elevator servo was not required to achieve a good CofG which mean it could be placed somewhere under the wing. The position chosen was so it was just far enough back so its pull/pull wires would not interfere with a hand launch.
EleServo.JPG

A double sided elevator horn was printed and connected to the servo with 5lb mono filament fishing line.
ElePullPull.JPG

Simple, direct and very light.
I know mono filament is considered "stretchy" but under the sort of loads the elevator of a sub 250 g 'foamy' is ever going to need the degree of stretch is insignificant.
 

Jackson T

Well-known member
#10
Tests showed the weight of the elevator servo was not required to achieve a good CofG which mean it could be placed somewhere under the wing. The position chosen was so it was just far enough back so its pull/pull wires would not interfere with a hand launch.
View attachment 165671
A double sided elevator horn was printed and connected to the servo with 5lb mono filament fishing line.
View attachment 165672
Simple, direct and very light.
I know mono filament is considered "stretchy" but under the sort of loads the elevator of a sub 250 g 'foamy' is ever going to need the degree of stretch is insignificant.
How did you get both pull pull lines tightened properly?
 

quorneng

Well-known member
#11
It actually one piece of line tied to one hole in the elevator horn then fed through the servo arm holes and back to the other elevator hole. It is then simply tied off with adequate tension.
The line is still free to be pulled through the servo arm to allow the elevator to be set to neutral with the servo in mid position. When happy with everything the line is 'locked' in position with a dab of POR at the end of the servo arm. When dry POR is very sticky so prevents the line from moving but as it never sets really hard it can be removed if the elevator needs repositioning or to reset the line tension.
ElePullPull2.JPG

I find that with a sub 250 g converted foamy "just good enough for purpose" works fine as long as it is light.;)
 
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Jackson T

Well-known member
#12
It actually one piece of line tied to one hole in the elevator horn then fed through the servo arm holes and back to the other elevator hole. It is then simply tied off with adequate tension.
The line is still free to be pulled through the servo arm to allow the elevator to be set to neutral with the servo in mid position. When happy with everything the line is 'locked' in position with a dab of POR at the end of the servo arm. When dry POR is very sticky so prevents the line from moving but as it never set really hard it can be removed if the elevator needs repositioning or to reset the line tension.
View attachment 165682
I find that with a sub 250 g converted foamy "just good enough for purpose" works fine as long as it is light.;)
That's a good idea, I'll have to remember that!
 

quorneng

Well-known member
#13
Slowly moving on I printed some clips to retain the 370 mAh 3s
ESCwires.JPG

Note the minute JST ZH connectors that are required for the micro rx.
The battery is offset in the cockpit to counter the weight of the ESC and to leave adequate room for the wiring.
BattHolder.JPG

To install the motor the nose is cut off at a suitable point to be replaced by a printed bulkhead that includes holes to accept the motor mounting screws as well as extensions to carry a circular former to match the spinner of the folding prop.
MotorMnt.JPG

Depron planking is used to make good the required shape change.
Fold5x3.JPG

The motor is now completely built in. It drives a light weight 5 x 3 folding prop.
At this point with a temporary rc installed it was possible to test the controls.
Sorry about the shaky hand held video but it is getting there.
 

quorneng

Well-known member
#14
As I has broken one of the HK 5x3 folding prop blades I found one in Thingyverse that could be printed.
6 x 3.5.jpg

Not the right root thickness or shape for the HK hub but nothing a bit of work with a file could not sort out.
Trimmed to 5" diameter. A suitable rounded shape spinner is also printed.
PrintFold1.JPG

It folds neatly
PrintFold2.JPG

With the extended 'lock down' in the UK no maiden for at least 3 weeks. :(
 

quorneng

Well-known member
#15
Just to complete the build part of the story I did an extended power test but after just 20 seconds the motor was getting very hot so much so that the heat transferred through the mount, into the screws which softened the PLA bulkhead and pulled out! :eek:
Of course PLA starts to soften at only 80 Celcius but in doing so and terminating the test probably prevented damage to the motor.

Obviously a 5 x 3 is simply too big for the motor on a 3s. I suspect the prop would have to be 3 inch or even less.
To keep a reasonable level of thrust in what is going to be a very slow speed plane the solution is to keep the 5 x 3 but drop to a 2 s.
As the bulkhead had to be replaced anyway I modified it to include some positive motor cooling ducts.
An air inlet above the spinner as well as HK 5 x 3 folding blades that are more effcient.
AirIn.JPG

And 'cheek' outlets on either side.
AirOut.JPG

The cheeks do restrict the blade folding a bit and with a 500 mAh 2s the motor only takes 20 W but the Cheetah is now a shade lighter at 182 g all up. This gives it a power loading of a modest but hopefully adequate 50 W/lb. It went through a 30 second full power run with no problem.

Now only the lock down is in the way of the maiden. :(
 

quorneng

Well-known member
#19
Glad you both like it but as this blog testifies it was a remarkably long conversion with far too many issues to overcome.
My feeling is it really will have to fly superbly well to even begin to justify the effort which is not that likely given it humble origin.
It is worth noting that the cost of all the RC bits was at least 5 times the cost of the air frame but then I did it for my own satisfaction rather than for reasons of practicality. ;)