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Author Topic: 35 cm Entry level class in UK  (Read 1838 times)
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Phil9x9
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« on: December 12, 2012, 04:29:20 AM »

In the UK this winter we are having a postal competition using 35cm models(entry level specification) to encourage novices like myself to get involved in lighter models.

Normal 35 cm rules apply plus:
These are to be built to a minimum weight of 1g and must not use VP props ( but can be built up).
Half motors must be used. Motors can be any weight but a ballast spacer of at least equal weight must be used.

I have built a model to around  0.85g (no prop). My current prop gives an AUW of 1g,(prop is built up almost symmetrical 11" dia with 21"p)It has 8.5” between the motor hooks.
I have not flown on the half motor yet and am still getting used to the model.
My practice hall is 20ft to the first light fitting and I am now struggling to improve my time above 7min 30 sec no touch. Using a 9” loop of ,040” rubber 0.52g.
Advice on 35cm  prop design is difficult to come by and I would appreciate any advice on suitable props and a guide to a suitable motor, both for the half motor use and full motor use. Remember no VP.



Phil
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Maxout
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« Reply #1 on: December 12, 2012, 07:15:01 AM »

A flaring prop should help a good bit, but you also need to provide a photo of the model and prop so we can get an idea where you're coming from.

One of the best fixed pitch props out there for 35 cm is Tom Sova's design, though the prop off Coslick's Micro-B EZB would be a good choice as well. At that weight, you'll want all the prop you can get...
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Phil9x9
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« Reply #2 on: December 12, 2012, 07:55:15 AM »

Here is a shot of the model.

Correction to my first post on checking my notes. I actually finished the session on 11" of .055" rubber.
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Maxout
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« Reply #3 on: December 12, 2012, 01:01:07 PM »

Hmm...that power might actually be a little low for this weight. My advice is to build your model to be similar to an F1L. You could actually build an F1L and then just use a shorter, wider chord wing, and probably be very close to a viable 1 g 35 cm. Main issue is that you need a very strong motorstick. I'd allot 300-350 mg for the stick and build the rest of the airframe around that. Stabs can be very light--80-90 mg, especially if you use a rudder instead of tip plates. The rudder need not be large...a 1.5" diameter circle is plenty and can be built down to 10 mg covered.

Your propeller looks pretty good, but I think it could benefit from a little more twist--doesn't look truly helical. Further, since that is a symmetrical prop, you're not getting any flare at high torque, so you can't use as much torque and still stay off the ceiling as someone who has a flaring prop. My advice on the prop remains--build Sova's design. I would recommend using the largest prop you can--probably 12.5-13", by scaling his blade design as needed. Additionally, a long motorstick will help you get the most from your prop/motor combo. For a model of this weight, a 10" motorstick should give good results. To save weight, you can keep the tailboom around 8-10". It needs to be as stiff as possible.
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Phil9x9
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« Reply #4 on: December 12, 2012, 01:58:32 PM »

Thanks for your opinions on the design of a 1g 35cm. It is very hard to know where to go when all designs and info are aimed at  0.4g models.
I will be building a second model now I have some experience of this class of  model and will incorporate the suggestions you make especially about giving emphasis to motor stick stiffness and length, and reduction of weight elsewhere.
The design I have used is one being produced as a kit to promote this class, although the one I built was from scratch, and I thought it as good as anything to get a feeling for the class.

You are correct about my prop, it is not helical. I have been trying to locate info on producing a true form block and will continue searching.
I have looked at photos of  Sova’s model and get where you are coming from on his props. These do not carry as much flare as the current trend in penny planes , of which I have some experience.
I am interested in you suggestion to go larger on diameter, when I considered this myself I was concerned on the amount of rubber I would need to spin them.
Do you think I will have to go over 1g for the motor.
What would you consider a maximum weight of rubber on a 1g model.
Do you think a pitch of around 21" is OK for the bigger props.


Phil
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Hepcat
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« Reply #5 on: December 12, 2012, 05:39:28 PM »

Phil
I have a paper on making formers for propellers in the 2012 Sympo.  This shews calculations for helical or non helical blades.  The paper offers an 'Excel' spreadsheet that does all the calculations with a few button clicks.  If you would like a copy just let me know your email address.

John
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Phil9x9
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« Reply #6 on: December 13, 2012, 04:14:57 AM »

Yes please I have pm tou with my email address.

I was going to calculate my own using

(Dont know how to get greek letters)

Alpha = tan of (20/2pi r)

Would i have been correct.

I will build a new prop for our tuesday flight session and try and get a few flights in using the 1/2 motor where we have to add the wieght of motor as extra ballast!!! eg 1 gram motor will need 1 g ballast. That should kill it!!!!

Phil
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Maxout
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« Reply #7 on: December 13, 2012, 09:07:08 AM »

Ok, interesting stuff...you are correct about there not being much out there on models of these specifications. That said, you can find some very good info in INAV 118, which details Slobodan Midic's 1.2 g 35 cm models. Based on what he says, you can probably expect 14-15 minutes to be the absolute limit for these models in Cat I flying. On half motors, you may make it to 8 minutes if everything is just right. Considering Slobodan's scores in several indoor WC's, I expect he's getting pretty much everything out of his models that can be gotten.

A little more wisdom can be obtained from INAV 120, which featured a number of successful F1L designs. Of particular interest would be Danjo's Cat I model, which has a very good blade outline. I've used that one on all of my really good F1L flights. You can crank it up to 20-24" pitch and get really good performance from it. You'd need to shrink it a little for 35 cm, of course. A pitch of 18-20" would work, though you do need to be ready for some trimming challenges. I address most of these issues by having the left wing be washed out slightly at rest, and the right wing washed in slightly (maybe 1/8" each). When the motor is fully wound, the twist will be reversed to the normal setup, but not excessively so as it would be were the wing flat at rest.

Rubber usage on models like these is a little surprising. Bill Gowen is currently using more than 2 g of rubber on his F1L's, and my standard 35 cm's all use motors of at least the airframe weight. I think that a good starting point would probably be 1 g of rubber for a full motor and then work your way up from there. On partial motors, you'll want to land with no more than 5% of full turns.

Unless your contest rules specifically prohibit it, don't be afraid to hit the ceiling. As Nick Ray says, you have to hit the ceiling at least once each flight to prove that it's there. A number of various methods exist for not getting stuck. If there aren't a lot of really serious snags, a big propeller will usually bounce you free of ledges and lights. The key is to be climbing very, very slowly when you reach the ceiling, but to keep climbing as long as possible. In F1L, I aim for the model to be climbing at least half the flight, preferably 2/3 of it. Another trick is to put a finger on the front wing post that sticks up above the prop arc and is thereby the first thing to hit any obstacles, keeping the model from climbing into them.

And that's enough to keep you busy for a long time. Please keep us all posted on what results you get!
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Phil9x9
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« Reply #8 on: December 13, 2012, 10:47:17 AM »

Thanks very much for your input.

Phil
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piecost
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« Reply #9 on: November 21, 2016, 09:08:39 PM »

I found this old thread interesting as I built an entry level 35cm model about 18 months ago. This is my first model of about 1g mass. I had not flown it much so am determined to devote more time to it. I noted that the propeller from the plan was of low flair and with a low pitch of 20" and seemed to wizz round at a high rpm. I wonder if this was done for ease of trimming; the low torque to minimise twist on the solid motor stick.

So, I was delighted to find this thread and took Maxout's advice and built three Tom Sova 35cm propellers of 11.75" diameter with his original pitch of 24" and two more with 20", to match that of the original and an intermediate value of 22". I employed 0.010" sheet wood instead of the built-up structure of the original and a spar from #6lb/ft3 tapering 0.063" down to 0.025" giving props weighing from 0.240g to 0.260g. I employed the Hobbyshopper method of selecting half spars with matching deflections and was rewarded with three propellers that tracked true without wobble.

I noted that the model was not turning as it normally would do, when using either of the three new propellers. I put this down to the higher launch torque needed by new flairing props. Returning to the original propeller showed a consistent lack of turn, so it wasn't due to the new propellers. The air temperature was 14ºC with a relative humidity of around 70%. So, I wonder if the damp was reducing the stiffnes of the motorstick or inducing warps. However, an experienced flyer noticed that I had put the assymetric offset on the wrong wing! The wing-post tubes should have been inline with the offset centre rib. The model had trimmed OK with this in other sessions and turned well. I was a little upset that my last flight confirming the lack of turn with the original propeller ended up loosing the model on a roof. I was going to retire the model anyway and this was oing to be its last flight - honest.

So, I have decided to build a couple of new models employing motor tubes and booms and to employ the increased torsional stiffness and to take advantage of the higher torque launch capability of the Sova prop. I employed a helical pitch on the Sova propellers without giving it much thought. But, I note that many UK flyer employ significant washout (up to 6º) on the tips and even at the root of the blades.

So, I have a question;  Is this trend also used by fliers outside of Britian? What are the percieved benifits?


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mkirda
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« Reply #10 on: November 30, 2016, 08:54:32 PM »

So, I have decided to build a couple of new models employing motor tubes and booms and to employ the increased torsional stiffness and to take advantage of the higher torque launch capability of the Sova prop. I employed a helical pitch on the Sova propellers without giving it much thought. But, I note that many UK flyer employ significant washout (up to 6º) on the tips and even at the root of the blades.

So, I have a question;  Is this trend also used by fliers outside of Britian? What are the percieved benifits?


If you look at the top flyers in F1D and go by the published info, one thing you will notice is the wide disparity in the propeller size and pitch distribution used. Some use 35"P helical. Others use a Treger pitch distribution. One is rumored to be way lower and turned up in pitch. So things are all over the place and they still get excellent times.

In F1L, you work to match the rubber to the prop. You build a few and test a lot. One will stick out as a better flyer than the rest. Measure it and be sure to record it, then match rubber to that prop to give you the best times.
You should see someone like Larry Coslick when he opens up his F1L box. There are at least a dozen propellers.

Even when you make the blades symmetrical, the center of pressure is in front of the spar. There will be twist, or maybe I should say, there will be flare.
It therefore makes some sense to have a bit of washout so at higher torque the prop still functions with the flare.
But think how you can accomplish this: One way is to form at a higher pitch, then set at a lower pitch. Both ends (tip and root) of the prop are then at a lower AoA than the center.
Jeff Annis' Symposium paper in the 70's on VP props has a nice graph that shows how this happens.

So yes. And no. Try it and see how it works for you. I'll bet you can build five identical props and one will outfly the rest.

Regards.
Mike Kirda

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Maxout
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« Reply #11 on: December 01, 2016, 10:46:43 AM »


So, I have decided to build a couple of new models employing motor tubes and booms and to employ the increased torsional stiffness and to take advantage of the higher torque launch capability of the Sova prop. I employed a helical pitch on the Sova propellers without giving it much thought. But, I note that many UK flyer employ significant washout (up to 6º) on the tips and even at the root of the blades.

So, I have a question;  Is this trend also used by fliers outside of Britian? What are the percieved benifits?


I'm not going to get deeply into this, but here are a few things to ponder...

1. I use more or less helical distribution on F1R props. Seems to work ok. I've played with others and haven't seen much difference, but I fly VP and my primary problem is efficiently metering out the power on a VP prop. F1R's carry a huge power/weight ratio.

2. You definitely need the correct wing offset; don't be afraid to use rudder and thrustline offsets as well. This becomes particularly relevant with VP systems.

3. I have to note that things have changed drastically for me since my last post on this thread. F1R wasn't even recognized by the AMA back then, and now hold two records in the class and have set F1R records in three ceiling categories. Also, there are a significant number of F1R fliers using my hubs for this class. F1R is growing and there are *many* different viable approaches to flying them. For advanced fliers I think it's the best all around class there is because they have extreme performance without the annoyance of needing a big transport box. They don't have any real trim issues either.

4. Shameless plug: when you get the desire to really fly these seriously, take a hope over to jhaerospace.com and see if you'd like to buy a VP hub.
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piecost
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« Reply #12 on: December 01, 2016, 07:07:45 PM »

Thanks for the replies gents, it is appreciated
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Skymon
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« Reply #13 on: June 21, 2017, 06:47:32 AM »

Is anyone flying this restricted class in the UK?
I notice a plan is advertised on indoor duration UK site.
I don't believe it's an official FAI class like F1R

The simple spec and lack of VP prop makes it seem an ideal starter point.
The 35cm span is a nice step up from ministick.

So are they flown anywhere in the UK?

Si
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piecost
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« Reply #14 on: June 21, 2017, 10:38:03 AM »

si, Yes the class is flown. Contact Tony via the email address on the indoorduration-gbr website to find out where and when the indoor sessions are taking place. You could also enter the nations in September
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Skymon
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« Reply #15 on: June 22, 2017, 07:06:10 AM »

Many thanks for the info - I sent a mail to Tony
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