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Jef Ott

electric free flight design

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Jef Ott

Hi guys.

Could anyone please let me know (ballpark) static thrust figures for a KeilKraft Pixie (23" span, quite draggy, cabin model), for it to maintain a gentle climb?

The one I am building has an easily adjustable electronic throttle, so I want to get it set somewhere near right, before the first powered flight. 

Probably won't be complete until the New Year, and yes, totally free-flight.

Thanks in advance and Season's Greetings!

Jef

 

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Guest

Hi Jef,

What is your target weight for the Pixie (cw power unit)?

Are you running a brushed or brushless motor?

Is this FF or RC?

Does your 'electronic throttle' output a logic level signal?

Which 'electronic throttle' are you using?

Regards Ian

 

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Jef Ott

Target weight.... I Would think <60g, but don't have any experience with the model prior to this. Model is made with original Keilkraft wood. Built using PVA, tissue covered, very little deviation from original.

Brushless motor. 2S LiPo, 240mAh.

FF

What is a logic level signal?

KP Aero 35 sec timer.

Thanks for your help.

Regards,

Jef

edited_1447810234614.jpg

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Guest

Hi Jef.

First off ‘logic level’ This is what the output of a DK electronic timer might be, a pwm signal for input to an ESC before it goes thru a FET ‘power’ stage.

Now if you use a (10gm) brushless say capable of 6 amps and a 6 amp (5gm) ESC with a 2s 180mah Lipo (13gm). Throw in a wiring loom and 2mm bananas (2gm) and the DK controller (3gm) you have a very heavy 33gm power unit that will deliver 40 Watts, thta you do not want,  be a so and so to package and put your Pixie into orbit. This is not a good idea.

The PMFC have been messing about with the development of a small field E20 and have been using the 12mm GWS motor CN12-RXC with a FET timer as described below:-

http://www.peterboroughmfc.org/membersmodels2014/01-E20.htm

This system would also be ideal for the Pixie but run with a single 1s 180-200mah LiPo (6gm) and a FET timer set up for 35s motor run. Power unit weight would be a modest 20gm but you would have to control power via propeller size as there is no load control on a FET timer. Start with a GWS 4 x 3 which ought to give 1.2 amps 5W and a gentlemanly climb into the sunset on a perfectly windless day (optimist mode). This set up will be easy to package in a Pixie and sub 40gm ought to be achievable.

I can deliver a CN12 with FET timer for £10  post paid if you want to go this route as I cleared Steve Webb out on GWS motors some years ago and have dozens gathering dust supporting the PMFC E20 initiative.

Regards Ian

 

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Jef Ott

Sorry Ian,

The picture is not up to date, I have already fitted the motor, ESC and batteries, all I want is a ballpark thrust figure. Will post a more accurate picture after work.

If a figure cannot be given, with the meagre info I have supplied, can anyone offer a rule of thumb? E.g. one watt per Ounce, or whatever, or 20% of the AUW, or anything to give me something like the correct oomph/whoosh.

Thanks,

Jef

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Hi Jef,

Whoops.....you have a surfeit of power. You have 40W to play with and you need about 5W.

My Tomboy_E flew on 15W

KK Competitor on 20W

Tom Thumb on 3.5W

BUMP_E on 2.2W

For less than 15W a brushed motor is still my preferred power unit for low mass and easy instal.

Regards Ian

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PeterT

Jef

The trouble with measuring watts is that it's 'power in' and doesn't take into account prop efficiency. As the saying goes, you can bolt a 6" wooden ruler to the front of the motor and see loadsa watts.............but b*gger all thrust:D

As a practical start point I can tell you that a GWS 6x5 SF prop turning at 5000 rpm (static) will pull a 30" span 100g model away in a steady climb. That size of prop sounds about right for the Pixie but you might want to turn up the wick a bit on the K&P timer to say 5200-5500 rpm static if the Pixie is particularly draggy (not that my E30s aren't:rolleyes:), also the wing loading is likely higher.

By the way, if you intend to do any glide testing (even over long grass) I would recommend replacing the prop with a bit of blutac of the same weight, as an 'arrival' can stress the prop at the junction of the blades and the hub - not a financial problem at 99p a pop, but if it's the only one you've got on the field.:rolleyes:

Hope that helps.

Cheers

Peter

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Phil_G

I've been asked to develop some electronics for E30, I wondered, what electric setup is everyone using, is there a concensus or does everyone do their own thing?
I was thinking since its so low power, and simplicity is key, that a brushed 1S setup would be ok, mosfet switched via a pic timer which could also do the DT?
Any thoughts appreciated!

Cheers
Phil

 

 

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PeterT

Phil

Excellent news, if you decide to do it I would certainly be in the market for some (as I'm sure would be many of my clubmates. At present, from what I've seen, most E30 competitors are using either the Atomic Workshop Voodo, Derek Knight's E30 timer, the Peterboro' MOSFET one, or a clockwork timer + microswitch (all with brushed motors).

If you've not been given the BMFA spec for the power train, it's 6" max dia prop, 70mAH 1S lipo, and up to 60 sec motor run. My disconnected thoughts/questions are:

If you go down the mosfet route, please, please, please don't put a low voltage cutout in the circuit to protect the lipo. The Voodoo & the DK timers both have this (lv protection) facility to suit the sport & scale flyers. For those flying E30, on a cool day or with a degrading lipo it's possible (easy) to draw sufficient current such that the cut out operates after 10 secs or so motor run - I speak from experience :-( Typically a 60 sec run in the air takes about 25% of the lipo capacity so protecting the cell isn't an issue. I know that warming the lipo helps but it's not foolproof.

A servo DT would be nice, but using a separate Tomy clockwork DT won't be a problem for most of us as weight isn't an issue. My only thoughts relate to whether or not the timer circuit continues to run after the motor run/DT cycle and how quickly any residual consumption might take the battery below its safe low voltage - if it's days, not a problem.

I have no strong feelings about programming methods and only suggest that it's possible to programme a shorter run/DT time for trimming flights - also bear in mind that the motor run is timed to prop stop and thus the timing granularity is sufficiently fine to allow say 57-59 secs to power off.

The normal sequence for starting most electric FF timers is press and hold button to start the motor and the timer starts when the button is released.

Sorry  I've rambled on and please shout if you want to clarify anything. Once again, thanks.

Cheers

Peter

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Jef Ott

Ian,

As promised, pics of the Pixie, now that I am home from work.

Current weight is around 45g, so the <60g target AUW should be achievable, despite the rather over-engineered running gear, as the only things still to make, are the undercarriage, the cowling and the nose block. Once the wings are doped, and the fuselage is covered, doped and varnished, that may add a gram or two, too.

Not aiming for a model that will naff off into the stratosphere if a cow farts in the next field, just something to show my grandchildren and maybe spark some interest, so durability and reliability are it's main design parameters, despite the traditional build.

Sorry for hijacking my thread back. ;)

Jef

Pixie Progress 5.jpg

Pixie Progress 6.jpg

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PeterT

Jef

Looks good, and now I'm tempted to build one for FF. RCME a few tears back had a free plan for an enlarged RC version - so much to do so little time. I had a look back at my testing notes & also the Scorpioncalc thrust calculator and both show static thrust of about 60g with the 6x5 at 5K rpm. Thus you might not need any extra rpm to start with, as a 1:1 thrust/weight ratio gives a nice climb anyway.

My apologies for being part of the thread hijack:blush: I've slapped my wrist!

Peter

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Jef Ott
6 hours ago, PeterT said:

My apologies for being part of the thread hijack:blush: I've slapped my wrist!

My comment was made tongue in cheek - please feel free to carry on talking about free flight design here. Thoroughly enjoying it!

I know what you mean about too many projects and not enough time, I am building an 84" Radio Queen for RC concurrently. As one project stalls / glue is setting / time is needed for thinking about the best way of tackling the next step, work can continue on the other.  

By building a big one, it saves having to manufacture a box for the small one - as components of the Pixie will fit inside the RQ quite happily. Is that called thinking outside the box?

putting it in perspective.jpg

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Jef Ott

Ian,

Just checking the specs on the brushless motor in the Pixie, you were quite close on your estimates.

Weight is 9g, it can handle 10A, max wattage 30W.

8 hours ago, boffin said:

Now if you use a (10gm) brushless say capable of 6 amps and a 6 amp (5gm) ESC with a 2s 180mah Lipo (13gm). Throw in a wiring loom and 2mm bananas (2gm) and the DK controller (3gm) you have a very heavy 33gm power unit that will deliver 40 Watts, thta you do not want,  be a so and so to package and put your Pixie into orbit.

And yes the (6A) ESC is 5g.

Being an rc thug, I still can't get my head around how a complete 33g set up, capable of flying all day on one charge, is "heavy" !

Thanks for the help.

Jef

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Hi Jef,

Waddya mean ‘RC thug’…….not so long ago you and the Ott Eng’g Dept were responsible for the first use and development of the pull string torsion bar dlg rear end, subsequently adopted in toto by the masses. This gave a rear end weight reduction of decent proportions and sets the scene for considered , elegant, design solutions on all else. You have just bolted the equivalent of a Mills 75 on the front of a Pixie!

Will it fly?…probably.
Is it heavy….absolutely…40gm target
Will it Excocet all over the place?…probably
Is it worthy of the Ott name?……hell no.

Just one further issue……..PeeTee’s E30
This runs a 60s motor run on a CN12_RLC 3.5 :1 gearbox and GWS 6 x 5. My static data on a 70 mah Eflite gives an average of 3.3v at an average of 2.2a over the motor run. So his model weighs 100gms, produces 55gm thrust on my thrust rig and consumes an input of 7.2W and climbs away at 30 degs. It is  obvious that his model has an active climb on a thrust to weight ratio of 0.55:1. A recommended thrust to weight of 1:1 is excessive on a sport FF Pixie, and I will be fascinated to watch the trimming phase of this adventure.

If you want thrust/speed data for your calcs then try http://www.flybrushless.com/prop/search
Then Boucher for the unloading in the air.

Grumpily, Ian

 

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Jef Ott

So, Ian my old chum,

0.5:1

= (assuming circa 57g AUW) 28.5g thrust. 

Should give a sedate climb on the porky Pixie?

I will aim at an ounce of thrust.

Thank You All.

Jef

PS Like I said, this is not supposed to be a competition machine (unusual, I know), just a hint at enthusing the youngest generation of our family, and the way to do that won't be glide angle, height and retrieveability/ "Grandad do it again!"  might be more relevant.

  

 

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PeterT

I see that Ian is giving me a bit of a slapping again:D:D I'll have to set his clubmates Rod & Bert on him!

I'd be surprised if my E30 actually climbed at 30 degrees, and I normally attribute any sprightly altitude gain down to impeccable air picking - if you believe that you'll believe anything!

Jef, if you really want to play around with thrust rigs and different props then the choice is obviously yours. However, if you'd prefer a simpler solution Scorpioncalc says that the GWS 6x5 gives 38g thrust at 4000rpm, 30g at 3500rpm and 25g at 3200 rpm.

Happy flying

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Phil_G
17 hours ago, Jef said:

Sorry for hijacking my thread back. ;)

yeah sorry Jef my fault I thought the thread was for electric FF design in general :)

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Jef Ott

Unfortunately my tongue in cheek comment has come across as a telling off. Please do continue to use the thread for general electric free flight design discussion. Enjoying the enthusiasm here!

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Jef Ott

If the Pixie flies at all, and if the torque reaction is not uncontrollable, as well as to capture the imagination of my Grandsons, it will be the tug for aerotowing the West Wings Swallow, which will follow it onto the building board. 

The Pixie has settable ailerons. 

Not sure if anyone has done free flight aerotowing before. To make life easy for myself, I will have rc in the Swallow, on tow release and rudder. 

 

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18 hours ago, Jef said:

 

= (assuming circa 57g AUW) 28.5g thrust. 

Should give a sedate climb on the porky Pixie?

I will aim at an ounce of thrust.

Thank You All.

Jef

PS Like I said, this is not supposed to be a competition machine (unusual, I know), just a hint at enthusing the youngest generation of our family, and the way to do that won't be glide angle, height and retrieveability/ "Grandad do it again!"  might be more relevant.

  Jef,

An ounce of thrust just will not do ......... we were jolly well metricated in 1975 and 28gms of thrust it ought to be.

Now that we have got to a respectable thrust for a gentlemanly climb, a CN12 will do 30gm thrust on a GWS 4.5 x 4 on a 1s. You save 13gm power unit weight, simplify the instal and impress young Grandson no end. I guarantee that this will not be a competition machine and Grandad mend it might very well be Grandad do it again. ....I doubt if you will acquiesce to my suggestion but now I have to build a Pixie CN12 to show you just how much better it flies for the first OW in 2016.

Peter, 30 deg climb?..I was being generous or was that Rod's (Nats champ) E30?

Regards Ian

 

 

 

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