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Author Topic: Kenny Penny Kit by Jiri Kalina Models  (Read 1534 times)
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piecost
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« on: May 01, 2014, 08:05:02 PM »

I Purchased the Kenny Penny Limited Pennyplane kit from Jiri Kalina models. I have not built such a model before, the nearest being a few NoCal models.

The kit contains:

Complete propeller 12” diameter by 28.5” pitch (estimate), P/D=2.36
Motor stick 0.360” x 0.117” with Aluminium propeller bracket and wire rear hook mounted <7 lb/ft3
Tail Boom  0.196” x 0.119” tapering to 0.094” x 0.117” over 210mm length, 8 lb/ft3
6 x Balsa strips for Wing Spars : 0.067” x 0.085” x 320mm, density = 8.1 lb/ft3, Stiffness Coefficient = 60
3 x Balsa strips for Tailplane Spars: 0.066” x 0.078” x 320mm, density = 5.3 lb/ft3, Stiffness Coefficient = 115
1 x Balsa strip for Fin: 0.067” x 0.067” x 320mm, density = 6.1 lb/ft3, Stiffness Coefficient = 70
Wing Ribs from 0.032” Pre-cut, density = 10 lb/ft3
Tailplane Ribs 0.033 Pre-Cut, density = 6 lb/ft3
2 x 0.076” Diameter Balsa Rods, density = 17 lb/ft3
3 x tissue tubes
Carbon steel razor blade
3mm Super-Sport Rubber
Plan (no instructions)
Plan in Czech for different model
Condensor Tissue 7.2g/m2
Film 6.0g/m2

I weighed all the components and performed a Euler buckling stiffness test on the balsa strip. I understand that it is not critical on this class of models, but is for my own interest.

I was impressed with the quality and completeness of the kit. In particular; the propeller was complete and assembled. The only downside is that I do not know the form used to generate the blade twist and camber. Alternative covering options of Condensor tissue (7.2g/m2) and plastic film were given (6g/m2). I planned to go for the lighter plastic, being lighter and less prone to warping due to moisture absorption.

Strangely, a second plan is included – by the Czech manufacturer with copious instructions – but I cannot read Czech. Therefore, a quick web search revealed a scan of the original Aeromodeller plan and article. The kit plan was re-drawn with minor changes in some wood sizes, presumably for ease of manufacture.

The Aeromodeller article recommends 7lb/ft3 wood for the wing and tail spars. So, the wing spars are potentially overweight at 8 lb/ft3 and of poor stiffness, the tailplane spars are under the specified weight 5 lb/ft3 and very good stiffness and the fin is under-weight 6 lb/ft3 and poor stiffness. The wing ribs are from heavy balsa (10lb/ft3) and the tailplane ribs are ok (6lb/ft3)

The lovely propeller is built as per the Aeromodeller plan with round tapered spars fitting into tissue hubs, hopefully allowing future pitch adjustment. Unfortunately, the form from which the blade twist and camber were set was not specified and so a replacement blades will require some guesswork. A crude measurement of the pitch gave 45º at 75% radius, the Aeromodeller plan gave 40º. A similarly crude measurement of camber, at the same position, gave 10%.

There was confusion between the Aeromodeller specified rubber; it recommended 1/8” rubber with a lower pitch from the plan – but does not give the length or mass! The kit was supplied with “3mm” Super Sport rubber, which measured as 0.125” x 0.042”.

The following uncovered weight schedule was given in Aeromodeller. I estimated the weights of the components based on measured weights.

                                    Aeromodeller       Kit - estimated
Wing                                  0.55g                  1.12g
Tailplane                            0.30g                  0.31g
Stick                                   1.20g                 0.79g
Boom                                  0.21g                 0.30g
Propeller                             0.80g                 0.71g
Total                                    3.06g                3.23g

I estimate the film covering to be 0.55g, giving  an airframe mass of 3.78g, which is 0.69g over the BMFA minimum of 3.09g.

So, it is clear that the wing would turn out too heavy.

So, I will replaced the wing spars with 6 lb/ft3 1/16 Stiffness Coefficient=100 sheet cut to 0.085” depth. This was a smaller cross section than the kit wood, but I hope that the higher stiffness will compensate.

The Boom was also be substituted with 6lb/ft3 wood. The lovely round rod at 16 lb/ft3 seemed rather extravagant; I made a replacement from 6lb/ft3 wood and it broke straight away, so I used the supplied component.

By careful weighing of the kit parts and substitution I was able to keep the airframe weight to the target.

Completed Mass

Fuselage                         1 .13g
Wing (with carbon bracing)    0.88g
Tailplane                         0.42g
Propeller                         0.73g
Total Airframe                 3.05g note that the numbers don't add up. The measurements are within the accuracy of the scale

The wing turned out undermass so I added some 0.25mm carbon rod bracing.

In conclusion; the kit allows a perfectly nice model to be quickly built. But, to make a model to the target weight I had to replace all components except the motorstick, wing posts and propeller!


Flying

I fly under a 24' ceiling and plan to compete with the model under a 50' roof so a stint was used. The kit supplied motor was used:

Rubber 3mm x 200mm loop (40% motor) 1.39g with a 60% Stint 2.09g and achieved a best time of 2m36s

Next,  I reduced the motor width to Rubber 0.093" x 200mm loop (40% motor) with a 60% stint and recorded I got 3m20s

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piecost
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« Reply #1 on: May 01, 2014, 08:05:25 PM »

picture of kit parts
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piecost
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« Reply #2 on: May 01, 2014, 08:12:34 PM »

I next try making a couple of flaired propellers copying Oldbills shape, but using a 3/23" bass spar. I wonder if I have ruined the critical part of his design in using a stiffer material for the spar!

One set of blades was formed on a plastic bucket of over 9" diameter and another on a 7" vase - this looks too twisted and will be redone.

The model climbed to the ceiling with the existing 0.093" motor, perhaps 0.080"/0.085" will be better.

As usual I split the propeller blades and am considering adding condensor tissue patches to reinforce the spar position. Is it normal for propellers to break ever few flights? I don't see tissue reinforcement on photos of other peoples models.

Further tests await.

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ykleetx
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« Reply #3 on: May 01, 2014, 08:36:53 PM »

Hi, piecost,

No, it is not typical for the propeller blade to break every few flights.   The propeller will some times break at the spar if the model hits an obstruction and dives to the ground.  On contact with the ground, if the model dives with excessive speed, the propeller may get damaged.   It is also not typical for the model to dive to the ground after hitting an obstruction.

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piecost
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« Reply #4 on: May 02, 2014, 12:18:29 PM »

Thanks for that ykleetx. I must resist the temptation to unnecessarily reinforce components. The real answer is to more carefully avoid the walls and ceiling.

Can you tell me if the flaired propellers rely on the blades deforming, or should the spar be torsionally flexible as well?
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ykleetx
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« Reply #5 on: May 02, 2014, 01:32:35 PM »

It is better for the spar to flare/rotate than the blade  This is more easily done with a Penny Plane because the prop can be made relatively stiff and strong.  On a lighter model, like a EZB, the flare will come from a combination of the blade and the spar.

Olbill has a really good flaring system for the Penny Plane.  He uses very stiff blades and a hub/spar reinforced with carbon rods, and the flare is limited to the spar.  Look for his description in one of the LPP threads.
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piecost
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« Reply #6 on: May 02, 2014, 04:44:26 PM »

Thanks again
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ykleetx
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« Reply #7 on: May 03, 2014, 12:05:48 AM »

Look here if haven't found it:  http://www.hippocketaeronautics.com/hpa_forum/index.php?topic=3619.msg42077#msg42077

Replies #15-#18.

Very very clever hub/spar design by olbill.
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cvasecuk
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« Reply #8 on: May 05, 2014, 09:21:35 AM »

Through last winter CVA ran a comp for the Kenny Penny at our monthly meetings in Cornwall. Several people built the Jiri Kalina kit. All were impressed with the value. Finished models varied from 3.2g to 3.8g. Our site is 25' ceiling and about 5min was achieved regularly. Winning time was 6m 12s (lots of lucky bounces!) but this was not from a kit.
Ron
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Maxout
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« Reply #9 on: May 05, 2014, 12:30:22 PM »

One of the things to consider here is that the covering included is, according to the first post, not substantially lighter than C-paper. That makes it pretty heavy. I've covered LPP's with C-paper before, and you have to build lighter than is wise to meet the 3.1 g minimum. I'd recommend substituting polymicro film, which works just fine with the wood sizes and densities cataloged in the first post. Polymicro allows you to build Pennyplanes quite strongly, and it's still plenty strong.

Overall this looks like a great value for the money, especially with a preformed prop that is of reasonable pitch. Most beginner indoor planes have the prop pitched way too low, guaranteeing that the beginner will not get very good times with the finished product. It can be quite discouraging.

I'm gearing up to produce LPP kits on a limited basis myself. The prop won't be pre-finished, but a jig for setting it up correctly will be, for what it's worth.
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piecost
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« Reply #10 on: May 05, 2014, 06:35:37 PM »

Maxout,

Regards the covering, I agree that the kit film didn't offer much weight saving so I substituted it with Ultrafilm. Using the lighter covering; the major issue is with controlling the weight of the adhesive; I had problems using thinned spraymount in that it didn't thin too well with lighrter fluid. Next time I would simply spray it onto the wood.

I didn't know that the kit's prop was pre-made, but that must be a good selling point of the kit since building a prop likely puts off prospective first-time builders. For me; the main problem is in forming the blades to the correct twist. I soaked the blades in boiling water and taped round a plastic bucket. Sometimes the blades take up the shape, sometimes they do not. Will your kit include pre-twisted blades?

I rebuilt my propellers with Oldbill's hub system using 0.018" carbon robs. The propellers are more flexible, but are rather heavy at about 0.9g each. There was a gap between the rods and the blades towards the centre which I filled with Duco - I should have been more sparing with the glue.

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piecost
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« Reply #11 on: May 05, 2014, 06:39:23 PM »

A picture of my Kenny Penny in flight.

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Olbill
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« Reply #12 on: May 05, 2014, 07:03:30 PM »


I rebuilt my propellers with Oldbill's hub system using 0.018" carbon robs. The propellers are more flexible, but are rather heavy at about 0.9g each. There was a gap between the rods and the blades towards the centre which I filled with Duco - I should have been more sparing with the glue.


Those props look great! My first LPP props were over 1 gram so you aren't doing too bad at .9g. I think my current props are over .8g. If you need to get some weight off trying sanding the blades. Duco adds very little weight so don't worry about how much you used. (if we were talking about EZB or F1D then you DO need to be careful with the glue)
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Maxout
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« Reply #13 on: May 06, 2014, 09:40:28 AM »


I didn't know that the kit's prop was pre-made, but that must be a good selling point of the kit since building a prop likely puts off prospective first-time builders. For me; the main problem is in forming the blades to the correct twist. I soaked the blades in boiling water and taped round a plastic bucket. Sometimes the blades take up the shape, sometimes they do not. Will your kit include pre-twisted blades?

I rebuilt my propellers with Oldbill's hub system using 0.018" carbon robs. The propellers are more flexible, but are rather heavy at about 0.9g each. There was a gap between the rods and the blades towards the centre which I filled with Duco - I should have been more sparing with the glue.


I only use a pair of .020 rods on mine instead of Bill's four. Since I started building decent LPP's, I haven't flown one against him, so I don't really know how the performance compares. I did build a couple of balsa/carbon composite spar hubs with wide blade props, and they came out around 700 mg, but I used .020 A-grain for those, so that might be the weight difference right there.

I'm still trying to decide whether to include pre-formed blades in my kits. It'll require a new prop design to do that, since my personal LPP designs all use pretty wide blades which definitely won't ship well once assembled and formed (2-3 piece blades, grain running chordwise). I've had reasonable success with Don Slusarczyk's LPP prop design, though, so might go with that one, which is of a more reasonable size.

As Bill said, your props look very nice and you'll be quite happy with the performance. Some things that might also help: Learn to cover as neatly as possible. My flight times have gone up in response to better efforts at getting the covering evenly tensioned and set with as little slack as possible. Also, for low ceiling flying, the optimal camber seems to be around 5.5-6%. I've been having consistent success with camber in this range, gaining a noticeable reduction in power required to maintain level flight. The high point needs to be around 60% of chord.
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Olbill
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« Reply #14 on: May 06, 2014, 11:33:20 AM »

There are lots of ways to do mylar covering that work well. Each method has its own advantages/disadvantages and people who either love or hate it. I've been pressuring inexperienced people to try the wet method that I use most of the time because it is nearly foolproof and absurdly easy.

To start you need some 3M77 and some naphtha or some "premium" lighter fluid. If you use lighter fluid you can squirt some on a hard surface and see if anything is left after it evaporates. If there is anything left then don't use it.

Then take a small bottle and squirt enough 3M77 into it to have about 1/8" of liquid in the bottom after all the bubbles go away. Then add about 1 1/4" of naphtha and mix. This gives a 12:1 mix but don't obsess over getting it exactly right. Cap the bottle tightly and you can use it for a long time. If the level goes down add more naphtha.

Crinkle your film several times by wadding it up in a ball. Smooth it back out and stick it to your covering frame with lip balm or vaseline or liquid soap. Put your wing or stab frame on top of the film and adjust the film until there are no wrinkles and the wood is touching the film everywhere you want to glue it.

This is the most important point - you don't attach the film to the wing or stab until everything looks PERFECT. If you follow this plan there can be almost NO REASON to get a bad covering job.

I've covered this in more detail in other posts but in simple terms you just use a small, soft brush to apply the liquid to the mylar on the outside of the wing or stab frame. When a drop of liquid comes in contact with the wood it will spread and adhere an inch or so of the wood to the mylar. After it's all attached and dry you can cut the film with whatever device you prefer. I use a 15 watt soldering iron.

As I said in the beginning there are lots of acceptable ways to cover. When a beginner has mastered one method then they can try different ways to reduce weight gain or get better results or just be different. I'm just putting this out as what I've found to get excellent results with a very low probability of problems. And it's also what I use.
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piecost
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« Reply #15 on: May 06, 2014, 06:16:25 PM »

Thanks for the advice guys - I really appreciate it. I spend allot of time reading the various threads and trying to absorb as much information as possible. I will certainly build another LPP for the start of the indoor sessions in the autumn.

The covering advice is of great interest since a friend had better luck than me with thinning of spraymount. so I must scrounge his lighter fluid off him. Is the crinkling of the film for aerodynamic reasons or does it help the film cover with less wrinkles?

My blades were made from 5lb/ft3 0.030" balsa thinned to 0.010" towards the tips and edges. Regarding the large blade chord; am I correct in thinking that the large blade chord is to cause the increase in pitch at high torque? Or would the large chord be useful on a non-flaired propeller?

Maxout, I felt that the propeller in my kit was very close to being split since it was not protected in any way. The danger being the packet of rubber moving about. If you include pre-formed blades in your kit then you might get complaints of them being damaged unless you protect them in some way.

I spent some time poking around this forum trying to decide which model to build when I came across the Kenny Penny kit at a model show. I decided to build it whilst making up my mind on the choice of a more advanced model. Since I am intending to compete with this model in the UK nats in June; I  have run out of time. The competition is held in a wooden dome with 50' clearance to horizontal bracing wires and a central boss.  Some models fly above these, but it is rather risky. Being a dome the diameter at height is reduced which requires a relatively tight turn. I attach a photo of this interesting site.

Can you guys offer any advice for competing under such a ceiling? I am practicing under a 25' ceiling, employing a 50% stint, simulating a 20" full motor. I only have a few more practice sessions before the competition, so no time really.

I started keeping meticulous records, wind, launch and landing turns and torque etc. I try to follow Bill's advice and Fred Rash's spreadsheet about back-off/landing turns to optimise the motor. I also found approx time to the ceiling an interesting measure (not sure how useful yet) but I expect the flaired propellers to significantly increase this.

My background is in scale models and for years I didn't think that I'd find performance models very interesting, but they have really caught my enthusiasm!
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Olbill
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« Reply #16 on: May 06, 2014, 07:28:22 PM »

Wrinkling the film removes the static and makes the film more flexible. Not everyone does this but I always recommend starting out with crinkled film. I ALWAYS do it.

I'm not sure what spraymount is. Are you using 3M77 or something different?
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piecost
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« Reply #17 on: May 07, 2014, 04:11:52 PM »

Bill, Spraymount is a 3M product sold in the UK. I have seen 3M77, but I think that is different stuff.
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Olbill
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« Reply #18 on: May 07, 2014, 08:02:08 PM »

The thinning recipe I gave you is for 3M77. Something else might work as well but I have no experience with other products.
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mkirda
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« Reply #19 on: May 07, 2014, 08:53:31 PM »

The thinning recipe I gave you is for 3M77. Something else might work as well but I have no experience with other products.

1:5 to 1:10 starting for rubber cement/thinner. I have used the Best-Test rubber cement and Bestine with good success.

Regards.
Mike Kirda
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piecost
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« Reply #20 on: May 09, 2014, 09:29:42 PM »

I tested the propellers tonight. One was terribly wobbly, but the other worked like a dream. The session was brief and after adding yet more rudder to keep away from the walls I had a single flight without terminating early or hanging up.

I used a 50% stint and a 0.110" x 10" loop of Super-Sport weighing 1.55g. I wound to 650 turns (0.6 oz.in), backed off 20 turns launching at 0.5 oz.in. The model reached 21' or so in a minute, the motor running out of turns at about 5' and landing at 3m25s.

From Frash's spreadsheet tool it is clear that I was not pushing the rubber and could go down in thickness.

Best still; I didn't brake a prop! This is a new experience to me. So, I am very happy with the propellers and have more work to do in finding the the optimum motor.
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piecost
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« Reply #21 on: May 09, 2014, 09:30:23 PM »

A nice picture of the Kenny with Oldbill's propeller design
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