Electrifying free plan models

[Swissflyer]

Hello KC,

You asked me to explain pitch speed and Rex gave us a perfect definition, thanks Rex, so I assume you are now OK on that. I would like to share with you, & anyone else interested, why I think it would be useful for modellers to think more about pitch speed. It may also help in converting Peter’s models to electric at a "market attractive cost"

First a sad but true piece of history. In order to relieve hard pressed Malta, the aircraft carrier HMS Eagle twice made eastwards dashes along the Mediterranean from Gibraltar & flew off reinforcements of 50 Spitfires, once within extreme flight range of Malta.

On one such launch a pilot set his prop to coarse pitch (cruising speed) instead of fine pitch (thrust). The Spitfire went over the bows of HMS Eagle into the sea and both aeroplane and pilot were lost.

I observe that many modellers do not consider the basic definitions of force (thrust), speed, work and power that my physics master drilled into us at the age of 14 (when there was still a Welfare State in the UK )

In simple terms, for a given amount of power you can trade-in speed for thrust or thrust for speed. However, the “Juju of Power” won’t let you have both.

The problem is that the “Juju of Drag” will (above a certain flying speed) consume your power at the square of your flying speed (double your speed & the Juju takes four times the power)

So if you set your pitch speed too high, the Juju of Power will happily consume it for you but won’t even give you the thrust you thought you were going to get – you go over the bows of HMS Eagle.

It is better to decide the pitch speed you want your model to have according to your ambitions for the model and then use the rest of the power available to either:

-         Have more thrust available when you need it (on the uplines)

-         Have longer flight times for a given fuel load

-         Carry less fuel (lighter LiPos)

In the olden days (Speed 280-700 and NiCads) we had so little power that we had to allocate it perfectly & do our sums, well some of us did! This photo is from RCM&E January 1986

 

Today we have vastly more power available and it is still best to use it well.

Please note there is nothing new or theoretical here, it is old knowledge that many of us have forgotten or neglect.

For Peters’ models, the Drag Juju lets us estimate (engineers do that, then we test by flying) that in straight & level flight his model will consume:

2x as much power at 45mph as when flying at 32mph

3x as much power at 55mph as when flying at 32mph

4x as much power at 64mph as when flying at 32mph (64mph was Peter’s max airspeed)

And most interestingly for me

2.5x as much power at 50mph as when flying at 32mph

That also says only 60% of the 64mph power will be needed to fly straight & level at 50mph (2.4 / 4 = 60%)

This last number interests me as I like to have an aerobat set up with a flying speed of 50mph (it handles reasonable winds OK, I can use the rest of the power for thrust, it doesn’t get out of sight too quickly etc).

Even more interestingly, I need only 60% of the LiPo capacity at that speed as compared with 64mph. Earlier on we discussed a 4’400mAh pack on 3S to “fully power” Peter’s models.

By deciding not to overfeed the, very greedy, Juju of Drag it looks like a 3S 2’650mAh setup may be quite OK for my Feugray, if I set it up accordingly and accept 50mph as maximum speed in straight & level flight as OK for me (it is)

 

 

So very concretely, this is one configuration I will test on my Feugray. With luck (after building some furniture) I will be able to start building it this weekend.

BTW I used exactly this technique 2 months ago to get a Ripmax Spitfire set up with a 55mph straight & level speed and the rest of the thrust available for verticals.

It is most impressive to watch the Spitfire pull up into a vertical climb and just go up at the same speed as straight & level as you gently feed the power in – you can fly big smooth constant speed loops

The LiPo is 2’200mAh but gives 12-15 minute flights with reasonable power management.

Mark

[Peter Miller]

I have just had an email from someone building a plan from another magazine. The usual question "What does he need for electric power conversion"

 

Further questioning elicited the response that he flies with a club where 60% of the models are electric powered.

 

Surely someone there could have answered that question.

[Erfolg]

Peter

 

I guess as the designer, who often will receive feed back from users, your knowledge of the various power trains installed will be second to none.

 

Even if the design has remained IC, you will probably been told of how small on power you can go and the consequences. Likewise how heavy or light aircraft can be and be viable, relative to power.

 

In my opinion, the biggest issues are, where and how to both lighten and rearrange to suit electric propulsion. Many seem to take the view, remove IC engine and replace with a electric motor. I am convinced you should look at the installation as an electric, with ESC and Lipo and not as single items which are not related. Just as you do with tank and throttle servo etc. 

 

Although you down play your electric knowledge, I am certain that you will know much more about the characteristics of your own designs than anybody.

 

With electrics, it can be made to seem difficult, I just the rules of thumb generally known. They work, at least sufficiently to allow fine tweaking. As stated they work.

[Swissflyer]

Posted by Peter Miller on 04/11/2010 08:39:36:

 

Surely someone there could have answered that question.

Peter, straight question, answered by another one!

The “question” rarely comes up (these days) on French & German Forums but always crops up in the UK – Why?

I think there are two answers:

1. The British Aeromodeller has been very poorly served by the British Aeromodelling press in this regard. As an example the French magazine Modèle Réduit Avion published a special supplement on electric power in December 2002. It told the French aeromodeller all he needed to know.
   It is awkward to say this here, but no British Aeomodelling magazine has come anywhere near to that level of quality in the last 8 years so all “knowledge” has to be scrounged in other ways!
   The “knowledge” was similarly well spread in Germany
   
   
2. The second reason is that some British aeromodellers like to save money and of course Chinese Websites are published in English (not French or German)

I rarely see the question “Can I fix this high quality motor to this model?”

What I see a lot of are plans where the designer has specified a good quality electric motor & someone is trying to buying something cheap (and sometimes very nasty) on ebay or a Chinese Website.

Bell motors are a case in point, when my friends started reporting that these had around 50% efficiency, I couldn’t believe it!

I bought one from a supplier, who is frequently discussed in RCM&E, as I wanted a datasheet. The supplier told me they never come with datasheets! When I measured its efficiency I found my friends were right, it was only 50-55% efficient.

By definition a 50% efficient motor can only deliver half the power you supply it to the prop. That means that in my photo, one of the LiPos (172g / 6oz) will be used to give useful power to the model and the one is just 6 oz of dead weight!

 

 

In contrast if you fit an efficient AXI 2820-10 you get close to 80% efficiency from 20-35 Amps (on 3S)

80% means that if you carry one 2’200mAh Lipo to fly the model, your “deadweight” LiPo is only 550mAh (1.5oz)

 Clearly the higher quality motor is going to give the modeller:

-         Higher power output per oz of motor & LiPo payload

-         Far better performance (he can fly a lighter model with no power penalty)

-         Lower running costs (les Lithium to buy)

 Did our modeller really save money?

Of course some Chinese motors do achieve better efficiencies (typically 70-75%) and they are fine if you know exactly how to set them up and operate them.

Erfolg clearly has some and the two distributors mentioned earlier on this thread seem to filter reasonable quality into their offer.

Mark

 

[Erfolg]

Swisslyer

 

We are in danger of going of at a related tangent, but feel compelled to reply.

 

The vast majority of the Chinese motors are capable of achieving a similar level of efficiencies as some of the big labels.

 

What they do not have, is

 

a) The same build quality, in bearings and often balance of rotating components.

b) The magnets are not as high quality, in that although starting of with similar magnetic qualities, suffer an attenuation (reduction, but I liked the sound of the word) of field strength, particularly if subjected to heat.

 

What should be recognised that efficiency quoted even for expensive motors is a max. These to can be propped to achieve low efficiencies. I have seen a plot for a Hacker motor, which was all but identical to its clone.

 

The main issue with a lot of Chinese motors, is how frequently they change the models, a massive range of essentially similar motors, which are produced for what appears to be a twinkle of your eye.

 

For small motors, the CF2822 etc. take some beating at a 82% max being found.

 

Unfortunately Dr Kiwi, no longer seems to test budget and expensive motors. I guess publication of data that some may dislike is fraught with risk.  

 

I am not sure that there is a issue of lack of knowledge at club level in the UK, as the club I am a member of, all manage quite well. Though I do believe it is useful to exchange information.

[Steve W-O]

50% efficiency?

 

Yes I agree if it was the case only 50% of the power from the batteries would go to powering the plane, the other 50% would go in heat (apart from a little for friction and air movement)

 

That amount of heat is likely to destroy the magnets, and end up with runaway without special cooling arrangements.

 

I only use motors from the east, and have had none which exibit such low levels, I would accept a few percent difference between an expensive motor and a budget motor, and I accept that the quality may not be as good (but still better value for money), but I would not accept 50% efficiency without seeing it for myself!

 

There may be the odd one that was a "Friday afternoon" job, but in general I think it is a very pessimistic value.

 

I don't see the problem with electric motors, the information is there and not hard to follow, and for people like the OP, Turnigy have motors marked in "glow" numbers which would answewr his problem without any working out

[Swissflyer]

Erfolg,

Most of the points you make are spot on. I especially like the observation that you can prop a high efficiency motor for low efficiency – I have seen a lot of that and I will admit, with a red face, that I have done it myself

The trick is to know how choose & prop the motor for exactly the performance that you want. Then efficiencies can really peak. Nigel Hawes' review of the GWS Formosa is being showcased on this site right now. It is an excellent example of very careful matching of motor, prop, pitch speed airframe etc. The result is reasonably good aerobatic performance on 50Watts per lb with a 60% efficient drive system & a heavy battery.

Now how did GWS do that?

What if Peter’s models could be persuaded to do the same?

I do dispute the point that the knowledge is well spread, if it was that same old question, the one that Peter complains about, would not come up!

Instead the modeller could have answered his own question in less time than it takes to post a question!

I do agree with your earlier post that Peter’s models need a total think through before electrification. I have finished that process for the Feugray this week and made 1:1 copies of the plan last night. I look forward to cutting out former templates, ribs etc during the weekend

Mark

[Steve Hargreaves - Moderator]

Are we in danger of drifting too far towards theory as opposed to practical everyday results here.......efficiency will be subject to many different parameters such as speed, loading etc. Obviously efficiency is important but hard to measure for the average club flyer & lets face it you rarely hear someone saying they wouldn't buy an SC motor because it uses more fuel than an equivalent OS engine.

 

I would agree with Erfolg about the level of knowledge within UK clubs.....my own clubmates seem to manage electric flight very successfully &  we all had to learn..... as the saying goes "Good judgement is the result of experience & experience is the result of bad judgement" Many of us, myself included, "stand on the shoulders of giants" by following the advise of those who have gone before......

 

& if that doesn't work there is always this forum....!!!

 

Some of us like to experiment....many of us like to follow the advise of those who know.....all of us like flying a well trimmed & powered model..... Each will choose how he gets there!!!

[Tim Mackey]

I also feel somewhat dissapointed that swissflyer complains that no UK magazine comes close to doing a good job of trying to educate people about electric flight - I may be biased.....but I thought RCME had done quite a lot of exactly this sort of stuff over recent years

[Peter Miller]

Dear Erfolg.

 

No one bothers to tell me what they used in the versions of my design that they build. In fact, apart from this forum I don't get feed back, just questions.

 

Now back in the days of Speed 400, 500, 55o and 600 motors I did design a few models for electric power but that was pretty simple, we knew roughly what the motors would fly. True, there were variations in wind and the number of cells but that was it, we also had the extra power of wet magnet motors etc.

 

Now if I was interested in electric power I expect I could find out what combinations would suit my designs but you see, I am not. I like engines. I like the enginering that goes into them, the simplicity. I buy engines to try out. I may buy a Wankle engine just to fly it because it is different. J.Perkins list the new version of the SC 25. I will buy one to find out how it compares to the current one.

 

I design my models very much on rule of thumb. I know what works with what engine.

 

Now if someone else wants to scale one up for i.c power I can advise them because I have a good idea of what will work.

 

If someone wants to convert to electric power then they can ask an electric power expert and from what I have seen on this thread there are an awful lot of theoricians but no one seems to have said "For this model you want this power train." I wonder why?

 

 And that was the basic  question asked at the start.

 

 

[Richard Wood]

Dead right Tim.

RCME has done numerous articles to electrically enlighten people, with

stuff on batteries, motors, ESCs - the lot.

 

 

 

[Steve W-O]

Posted by Peter Miller on 04/11/2010 12:41:34:

 

If someone wants to convert to electric power then they can ask an electric power expert and from what I have seen on this thread there are an awful lot of theoricians but no one seems to have said "For this model you want this power train." I wonder why?

 

 And that was the basic  question asked at the start.

 

 

   The reason why is quite simple, the answer was not wanted. What was wanted was to hear from someone who had already converted one of the stated models.

 

The OP said this several times, the only info he wanted to hear was from onversion experience already done.

 

Requests for model details that would allow an answer were rejected, because that was not needed to answer the question from someone who had already done the conversion.

 

There are several tools that will allow an electric seup to be given, and they will give a good working setup.

 

There is no mystery to it, you can just put the numbers in,or you can do some working out. 

 

But you do need the figures to put in if you want a good suggestion.

[Tim Mackey]

Exactly.  I also confess to not bothering to recommend a power system for each and every model listed as frankly, I am getting a little tired of doing so - there I said it.

Now if someone asks to suggest a typical motor for a particular model, I am usually happy to help ( very many of my thousands of posts do exactly that ) but to just list a lot of models, and ask for answers from people who have actually done it, is, IMO both lazy, and constricting, and I for one haven't done any of those models, and perhaps others haven't either.

Despite this, I DID go on to suggest a simple "one size fits all" solution which addressed the OP.

You couldnt ask for a simpler answer, but it seems this was either ignored, or still unsatisfactory.

I give up, and shant be bothering to post any  more on this matter in this particular thread.

[Erfolg]

Steve

If the originator of the tread wishes to build the PM Cassutt, there are two that have certainly been built.

 

There is Tommy's and my own.

 

He would be told that they both fly well. Though my own is not finished, as no UC is as yet fitted.

 

He could have details of the motors and batteries etc. What is interesting that Tommy and myself went two separate routes. I went for bags of power, which is not needed and Tommy less power but apparently equally as successful. I have only test flown my own, prior to full finishing.

 

Again Tommy has a slightly different approach to lightening to my own. We also have had slightly differing approach to accessing Lipo. Yet both concepts work.

 

 

 

 

[Swissflyer]

Posted by Tim Mackey - Administrator on 04/11/2010 12:32:19:
I also feel somewhat dissapointed ...

 Tim,

Please take my comment as gentle feedback. I would love RCM&E to be my reference source for such information. I have loved the publication since I built single channel super regen receivers & "hacked" the digital proportional system from the designs back in the 1960’s

It is still the only model aeroplane publication I have a subscription too

Mark

PS thanks to all on this thread as thanks to their contributions I will start building an electric Peter Miller design this weekend. I would never have considered that without all these very useful discussions.
Back in the 60' s there were no threads so that could not have happened

[David Ashby - Moderator]

It goes without saying that if there's any aspect of electric flight folks would like us to feature/re-visit etc. then please let me know either here, via a PM or in the feedback section.

Edited By David Ashby - RCME Administrator on 04/11/2010 13:59:18

[Tim Hooper]

Posted by Peter Miller on 04/11/2010 08:39:36:

I have just had an email from someone building a plan from another magazine. The usual question "What does he need for electric power conversion"

 Peter,

 

It's all part of being a victim of your own success!

 

You design your own models.  You write for a magazine.  Therefore you know everything there is to know about everything.

 

Even I get asked the occasional question;

 

"Do I need a full-range receiver for a thermal soarer?"

 

I kid you not.....

tim

[Peter Miller]

Hi Tim.

 

"You design your own models.  You write for a magazine.  Therefore you know everything there is to know about everything."

Oh! Really? I wish!

 

 

I never mind answering questions but I do make a point of never answering a question when I don't know what the answer is, Much better than giving a duff answer.

 

I sometimes thing that the more I learn about modelling the less I know.

 

In passing, my favourite definition of an Expert.

 

An expert is someone who learns more and more about less and less until they know absolutely everything about nothing at all.

[Myron Beaumont]

Well I must say that Timbos mag features inspired me to try lekky stuff . Well explained for a dumbhead like myself (although I do know about electricity) . Someone mentioned about efficiency & we all know about the 100 watts per pound weight as a good guide . This is obviously as measured on a Watt. Meter Whether or not the power drain from the battery is 100% converted into power or not is irrelevant . It's the figure you use to go by the criteria that we accept ie The only way of measuring the necessary power output rightly or wrongly .

Don't get down hearted Timbo --I for one enjoy & have learnt from your articles  & by the way I have been reading Aero- modellisme (french) for several years

[kc]

Well we have finally got this back on subject after an interlude for humour.  I do thank the people who obviously put a lot of time into giving comprehensive replies. ( notably Swissflyer -thanks )

I will bring this right back to basics........I need a reasonably aerobatic, plan built model which will use the 3S2200 lipos I have already.  I reckon I can build as light as most people.  I am looking for a sure fire solution, not to experiment!  Something over 40 inchs -big enough to see though.

So what should I build and what motor/ ESC/ prop?

 

I think this question is the one many other IC fliers would ask too, so its not unreasonable to ask again.

 

Note.  No ARTF !  and its not worth buying plans at the current ridiculous prices so I am looking through my collection of 'free' plans. Ryans Daughter is a candidate ( daft name puts me off! )   Swizzle Stick seems another possibility.

Edited By kc on 04/11/2010 19:53:12

[Steve W-O]

Posted by kc on 04/11/2010 19:51:32:

Well we have finally got this back on subject after an interlude for humour.  I do thank the people who obviously put a lot of time into giving comprehensive replies. ( notably Swissflyer -thanks )

I will bring this right back to basics........I need a reasonably aerobatic, plan built model which will use the 3S2200 lipos I have already.  I reckon I can build as light as most people.  I am looking for a sure fire solution, not to experiment!  Something over 40 inchs -big enough to see though.

So what should I build and what motor/ ESC/ prop?

 

I think this question is the one many other IC fliers would ask too, so its not unreasonable to ask again.

 

Note.  No ARTF !  and its not worth buying plans at the current ridiculous prices so I am looking through my collection of 'free' plans. Ryans Daughter is a candidate ( daft name puts me off! )   Swizzle Stick seems another possibility.

Edited By kc on 04/11/2010 19:53:12

 Two come to mind, Peter Miller's CAP21 from the August 2007 issue, and the Maricardo. (April 2004)

 

Though I like the Maricardo, I would be inclined to go for the CAP 21 as it is lighter to start with.

 

I also wouldn't worry to lighten it any more. F1 would need some minor mods to allow the battsry in, as would the top nose deck (or the bottom) for battery access.

 

It gives a 6oz fuel tank on the plan, which at a guess would weigh around 6ozs full, if omeone could give me an approximate weight of the motor, then could give the motor/prop combination.

 

I would assume you want aerobatics, not 3D power.

 

I've had this plan out for a while looking at it as a possible winter project.

[Steve W-O]

A 3542 940 motor with a 11X8 prop would be fine, but flying time would only be 4-5 minutes, and you would have to ensure your batteries are 25-30C.

 

If you put two of your batteries in parallel, you would get a reasonable flying time, and the wing loading would be about 5-6% higher than the original, so should not be too bad.

 

If you needed vertical performance, you could go for more power, but I limited it to the batteries you stated.

 

It would be best to fit a 50A ESC to allow some headroom.

 

With two batteries, just flying around, you would get around 15 mins flight time, and around 8 on full throttle

[Erfolg]

The thought that the equivalent electric model would have a higher wing loading than an IC version, does not fit with my own experience.

 

There have been a number of kits for electric I have one which indicates the difference between an electric and IC model.

 

the original specification as a 600, Nicad model is as follows:-

 

SPECIFICATIONS
Stock Number: GPMA0165
Wing Span:49 in (1245 mm)
Wing Area: 401 in² (25.9 dm²
Wing Loading: 17-20 oz/ft² (51-61 g/dm²
Weight: 3-3.5 lb (1360-1590 g)
Length: 36.3 in (920 mm)
Requires: 4-channel radio with 3 mini servos, 600 motor (reverse rot.), gearbox, ESC (30A min.), battery, charger, 9x8 prop and 2+ rolls of MonoKote®

 

If you go to some other USA web sites, a number have been buit as Brushless Lipo models. The weight as been reduced to below the 3lb level, using a motor of typically 28 dia, drawing about 260w.

 

When I build my kit this will be the aim.

 

There has also been a Great Planes Park Flyer kit of the Ryan Rv -4, which was braodly similar to the Ryan, yet lighter still. Not the current ARTF model.

 

Any of PM's designs can be electrified, they should be lighter, if done properly. It is worth repeating, there is no vibrating motor, which is putting a lot of forces into the airframe.

 

So with Peters designs, I would be looking to loosing probably a pound in weight. There need be no compromising the strength. Which would would allow a motor of typically 28-35 dia. It is not difficult, eminently doable, even a 68" span Gee Bee R1/2 has been electrified successfully.

 

I would just pick the model i want and start building. How much would I worry about the motor, I would not worry at all, there will be a lot of motors suitable, then buy the Lipo to suit. If the Lipo is the atarting point, it provides the duration that it provides..

 

[Steve W-O]

First of all, the OP asked for ideas from the free plans, there are of course many more, but the two I mentioned I happend to have out of the file on the desk.

 

As to the higher wing loading, I am going from the magazine article figures, and I took 6oz off for the fuel tank and 8oz of for the 25 motor (guess, but I doubt if it is too far out)

 

Then to get a longer flight time with the batteries the OP already has and wants tmake use of, i suggested using two in parallel. That takes the wing loading up to 21oz /sq ft., the original was 20.

 

No doubt the design can be lightened, but to loose a pound will take quite some doing.

 

Admittedly there is no vibrating engine, but I have yet to see a model vibrated apart by the engine, they usually come apart due to unplanned contact with the ground.

 

Keeping the build the same will sacrifice a bit of power and duration, but it will also keep the flying qualities of the original, and its abilities to fly in wind etc.

 

Keeping the build the same will enable an answer to be given to the OP's question, if he wants to build lighter, all well and good, but he will have to ask his question again when he has completed it and weighd it.

 

Yes, there are lots of ifs and maybes, I tried to answer the question as given, without changing it and giving an answer based on yet unknowns.

[Erfolg]

Hi Steve

 

The weight loss is possibly best based on a real model.

 

When I started the PM Cassutts, it was on this basis, that even now, the power density for electric power is not as good as even a Glo. So if a poor flying model was to avoided, weight must be lost, the plus was the drastically reduced vibration/forces.

 

Peters model came in at 2.5lb, my own model at 2lb and Tommy's less still. So on a small model 1/2lb could be lost.

 

How?

 

In my case I actually increased the span to 36", which increases the weight slightly, but reduces wing loading (that was the intention).

 

My first decision was with the UC, PM, uses torsion bars and distributes the forces via the sides and an area of the bottom of the fuz. I decided that in the event of a poor landing I was prepared to loose the UC. So I have a ply plate, when attached the UC will be fastened on by nylon screws. Onto a ply plate, with some triangular stock reinforcements.

 

My next decision is that I use 3 off 9g servos, 2 are placed in the wings, using short push rods.

 

In the case of bulkheads, these are 1/8 light ply, where ply was used.

 

The sides will have been made from 1/8 balsa, with 1/32 ply facing to behind the the wing seating.

 

To ensure strength all spars are spruce, I normally, place spruce longerons along the bottom corner of the fuz and similar longerons as high up as possible that permits a continuous run front to back(this is aimed at forming a light weight box beam with the spruce taking much of the forces, the balsa acting as shear webs).

 

To save weight further the shear webs on the wing are made to form an !I" beam, rather than a "U" on its side.

 

All planking or turtle decks are made from rolled 1/16 balsa. Every gram saved adds up as Tommy pointed out.

 

All block work is replaced with "Blue Foam" skinned with glass and WBV.

 

My biggest mistake is an overly powerful motor, which is significantly heavier than Tommy used.

 

By doing simple things, grabbing a gram here, saving a gram there, I lost 20% of the original build.

 

PM has done the hard work, you just have to do the weight engineers job.

 

Edited By Erfolg on 05/11/2010 11:46:28

Edited By Erfolg on 05/11/2010 11:49:04