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Old Oct 06, 2010, 12:56 PM
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Choices, choices

Hi Renatoa,

I see that Dr Kiwi, very well respected on RC Groups, also put data on the KEDA TR2837 / 08 into DriveCalc. It is not as efficient as an AXI but should give you a healthy 640g of static thrust at a pitch speed in line with WebOCalc using a 2S LiPo.

The KEDA TR2837 / 10 will give you a pitch speed of 95 km/hr with an APC 7x5 on 3S, that would be throwing a lot of energy away in the slipstream when you only need a pitch speed of about 68km/hr

Of course it is your choice but do read the wise comments of Chuck & FB on this thread.

I would love to know how it flies

Mark
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Old Oct 06, 2010, 01:19 PM
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Even for the foldable 3 blades you think the 1500kv shouldn't be more appropriate ?
Is 2s choice a personal preference, or derived from these computations ?
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Old Oct 06, 2010, 02:56 PM
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More choices

Quote:
Originally Posted by RENATOA View Post
Even for the foldable 3 blades you think the 1500kv shouldn't be more appropriate ?
Is 2s choice a personal preference, or derived from these computations ?
Hi Renatoa,

Sorry I don’t know anything about foldable three blades. Frankly I am not sure you need that kind of prop for this model.
It certainly flew OK 25 years ago on a 2 bladed prop with the heavier motors & batteries that were available then. So I would expect a modern lighter, more powerful system to fly it perfectly well with two blades, when you find the right system

What drives the computations (for me) is to get the maximum amount of energy coupled into the air from the prop to fly your Aerofly.
If your prop is spinning too fast the pitch speed is too high relative to the flying speed of your Aerofly (think of motor bike spinning its wheels in mud and not going forward very fast)

A rule of thumb for pitch speed is Max straight & level flying speed = 80% of pitch speed.

So if you put a power system that creates 95km/hr pitch speed on your Aerofly it would be fine if you expect the maximum straight & level speed to be 75km/hr.
Looking at your model, I don’t think it is designed to fly that way!

If you prop it to have a pitch speed of 71km/hr (the other calculation) then one may expect a maximum straight & level speed for your model of about 57km/hr.
That would seem far more reasonable to me looking at the shape of the Aerofly.

So for me this comes down to what battery, rpm/V motor combination will give you good thrust at a reasonable pitch speed. A 2S LiPo and a 1’800rpm/V motor then drops out of the calculation.

If you really want 3S then 2S/3S * 1’800 = 1’200 rpm/V and there is a KEDA TR2837 / 12 -1250 (1209 rpm/V) which is also validated by Dr Kiwi (see screen shot below).
Your Aerofly should fly nicely with that motor, a 3S LiPo and the same APC 7x4 prop.

All that said, I don’t know your model! So do keep asking plenty of questions until you feel sure that you have all the key points clear in your mind.

Have fun

Mark
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Old Oct 06, 2010, 05:19 PM
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Quote:
Originally Posted by RENATOA View Post
Even for the foldable 3 blades you think the 1500kv shouldn't be more appropriate ?
Is 2s choice a personal preference, or derived from these computations ?
I think the main difference between you and Mark is now a philosophical one - he's looking for setups that will let a big floaty glider fly around gently, you're looking for a way to get 1.3 kg of thrust (about twice what Mark is going for!). And the airframe is going to fight you - it was designed in a way that makes it very difficult to achieve your goal without serious compromises (wing-shredding top speed, lousy efficiency, lots of prop noise, etc).

Back in 1985 electric power systems were fairly horrible, with heavy NiCd batteries with small capacities, heavy brushed motors, and no gearboxes. So you had high weight, low battery capacity, poor motor efficiency, and poor propeller efficiency, all at the same time. Back then people were usually happy to just get an electric powered model in the air, never mind lots of performance. Power had to be low, otherwise those tiny NiCd's would give you very short flights. I think Mark is looking at power systems that would give you "better than 1985" performance, but because of the constraints of the airframe, not the kind of performance many models have today.

As we discussed earlier, it is hard to get 1.3 kg of thrust from a 7" prop without also getting too much pitch speed in the process. More blades and a lower rpm is one way to move a little in the desired direction.

APC makes a 7x4 Thin Electric as well as a 7x5 Thin Electric, and the 7x4 would probably be a better choice if you go with 3S and too much power - a 20% reduction in propeller pitch will be a step in the right direction, helping you reduce the excessive pitch speed a little.

If that was my glider, I would be tempted to either accept that fact that it was designed to waft gently around with very little thrust, or else hack off the nose and stuff a motor and big folding propeller there, or make it a twin, with two big propellers mounted a few inches out on the wing...

-Flieslikeabeagle
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Old Oct 06, 2010, 05:30 PM
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A note: the 1.3kg trust is from webocalc !
Without this indication, I had not had an exact idea what to look for. All I want to have is the maximum power available to be able to fly with a little of wind, I am not a big fan of dead calm.
Today I learnt a lot, thank you all !
I think the unavoidable way is to sacrifice a bit of original look and rise the engine a bit, to allow me at least an 8" prop...
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Old Oct 07, 2010, 03:24 AM
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Focus

Quote:
Originally Posted by flieslikeabeagle View Post
I think Mark is looking at power systems that would give you "better than 1985" performance, but because of the constraints of the airframe, not the kind of performance many models have today
Great mindreading, great summary

Quote:
Originally Posted by RENATOA View Post
All I want to have is the maximum power available to be able to fly with a little of wind, I am not a big fan of dead calm
Glad, yes really glad, that you have a clear mission defined. I think that is key in enjoying any model aircraft (or any other activity)

Quote:
Originally Posted by RENATOA View Post
Today I learnt a lot, thank you all!
I think the unavoidable way is to sacrifice a bit of original look and rise the engine a bit, to allow me at least an 8" prop...
That would be more comfortable.

I wrote to another RC Groups member yesterday & thanked him for some insights he gave me. Learning is a nice part of this hobby.

I still want to know how it flies

Mark
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Old Oct 07, 2010, 04:57 AM
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Weight Loss

A German friend of mine remembers that these flew with Speed 600 & 7-10 cells (FB guessed right). Assuming they used 1’700 mAh SC cells the weight budget would have been:
Then:
1'700SC @ 54g * 10 = 540g
Speed 600 = 200g
Power system weight = 740 g

Now.
Keda TR2837 = 80g
LiPo = 120g

Power system weight = 200g

So 500g saved, which is huge.
That is why those models were large enough to carry the payload!

From the screen shot below you can see that although the Graupner 600 was flying the model, the pitch speed was high but the thrust comparable to what we have been talking about.

That makes me confident your model will fly on a KEDA with a 7inch prop & pitch speed of around 70km/hr (2S or 3S as you prefer) because:
- Thrust will be similar
- It will be much lighter
- Stall speed will be lower
- The pitch speed will be better matched to the airframe

Of course an 8 inch prop will be better if you can make space easily.

Must stop now

Mark
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Old Oct 07, 2010, 03:12 PM
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Quote:
Originally Posted by RENATOA View Post
A note: the 1.3kg trust is from webocalc !
Well, there you go, you just demonstrated again that human beings are much smarter than my dumb computer program!

WebOCalc, of course, does not know anything about RC history, or the dismally bad electric power systems typical in 1985. It just uses basic physics to figure out what sort of thrust, speed, power, etc is necessary to provide the sort of performance we expect today from our electric RC models.

My first electric model was a Wattage Lite Stik, same thing as the GWS Pico Stick. It had a tiny 6-cell NiCd 150 mAh pack powering a tiny 12 mm brushed motor in a gearbox. The motor drew a little over 2 amps from that 6-volt pack, so about 12 - 13 watts into the motor. Running at 50% motor efficiency, probably about 6 watts made it to the propeller. The propeller itself was probably no better than 50% efficient, so maybe 3 watts of actual propulsive power made it to the air! The Lite Stik would barely maintain level flight at full throttle - no climb at all after the first one-second burst on a full pack. And that 150 mAh pack would be empty in three or four minutes of "flying".

I think WebOCalc would probably say "not enough power to fly" if you put those numbers in, and I'm not sure the program is wrong...you could barely call it flying! Today, you could put a cheap 15 gram outrunner and a little 2S 500 mAh lipo pack on that model and fly around effortlessly for half an hour at a time.

I think Mark's analysis of your model, circa 1985, compared to the equipment he suggested for use today, is very good. A Speed 600 motor is an incredibly heavy thing. Shedding half a kilogram of weight will transform the way the model flies, for sure! It will probably soar on light thermals and float around really slowly at the lighter weight.

-Flieslikeabeagle
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Old Oct 15, 2010, 11:54 PM
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Thank you

This is great. You have done a real service to beginners. I have been reading all the "guides" and "all you need to know" stuff I could find but I just couldn't see a systematic approach to figuring all this out. I even previously looked at Webocalc but didn't know where to start.

This makes a mystery very straightforward. This thread should be referenced in the Beginners Forum to make it easier for us new guys to even know it is there.

Great job and thanks again.
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Old Oct 15, 2010, 11:58 PM
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Quote:
Originally Posted by saintk View Post
This is great. You have done a real service to beginners. I have been reading all the "guides" and "all you need to know" stuff I could find but I just couldn't see a systematic approach to figuring all this out. I even previously looked at Webocalc but didn't know where to start.

This makes a mystery very straightforward. This thread should be referenced in the Beginners Forum to make it easier for us new guys to even know it is there.

Great job and thanks again.
Thank you very much for the kind words! A note to the moderators to petition that this thread be made a "sticky" would be all that it takes to put it in the Beginners section for all to see.

Thanks again!

Chuck
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Old Oct 16, 2010, 01:46 AM
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What do you think about this plane and these choices:
http://www.hobbyking.com/hobbyking/s...idProduct=4909
and
http://cgi.ebay.de/Klappluftschraube...t=RC_Modellbau
or
http://cgi.ebay.de/Klappluftschraube...t=RC_Modellbau

Please explain me the difference between wide and narrow blades.
Thank you
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Old Oct 19, 2010, 01:49 AM
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Thoughts

Quote:
Originally Posted by RENATOA View Post
What do you think about this plane and these choices:
When I saw the 3kg of thrust you set up in Webocalc I was worried you were going to create a monster by using 5 times more than design thrust.

Potentially you could take the wings off with that amount of thrust, which would create 2kg of falling fuselage & LiPo that might kill someone and/or start a fire.

In fact your motor set up will create about 1kg of thrust so might not be too over the top. That said you will have to cantilever your motor mount (to make it longer) so you are now putting a poor quality out runner with high vibrational moments at the end of a cantilever.

I would be concerned that such a set up might shake the motor mount to pieces on launch and take your eye out.

Quite frankly the power set ups we discussed earlier were offering you the power as specified by the original designer (a pro) but with a 500g weight saving.

I think most experienced RC modellers will tell you that saving 500g on a 2.5kg model is huge and really boosts the performance.

So why not go with one of those set ups until you know the model much better? Add more power later if you need it

Mark
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Old Oct 19, 2010, 02:56 AM
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Nooo... ths is a different plane, of a club fellow, not the Aeronaut Aerofly, this has prop in nose ! It's a Multiplex Alpha 27.

Why you say about this "monster plane" ?
Isn't the same setup recommended for a Cularis / Ventus / DGxxxx / whatever 2700mm glider class ?
Again, the thrust is recommended by webocalc, should I trust it or no ?!
I think its scenarios list is too limited.
I don't see where motorised gliders falls in any of the categories, so I pick one quite random.
Some page ago I got a response that the props list is limited and can't be upgraded. Why ?
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Old Oct 19, 2010, 04:52 AM
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Why not

Quote:
Originally Posted by RENATOA View Post
Nooo... ths is a different plane, It's a Multiplex Alpha 27
Ah, wish you had said that before.
Old German posts talk about a geared Speed 600 so your set-up should do fine

Check out this comment too

http://www.hobbyking.com/hobbyking/s...idproduct=2128
KD36-12L Charles Smitheman - I have installed this motor in my multiplex alpha 27 glider using a 12*6 graupner CAM folder, a 60a fly opto controller and a TP2200 11.1v pack. It draws 37amps.

Ask FB about WebOCalc model range for gliders.


Mark
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Old Oct 20, 2010, 01:47 AM
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Quote:
Originally Posted by RENATOA View Post
Again, the thrust is recommended by webocalc, should I trust it or no ?!
The thrust calculated by WebOCalc will give you the performance level you selected. That part is pretty simple physics. I have found the results quite accurate on a wide range of airframes - I use WOC to pick the power system for every one of my RC planes. You can read through this thread (and various other WebOCalc threads on this forum) to see other peoples findings.

The speed calculated by WebOCalc is usually very close to reality, but there is more "slop" than there is in the thrust. That's because "fast sport flying" means different things to different people. One persons "fast" may be another persons "not fast enough"!

So thrust and speed are going to be pretty accurate, at least they have been in every case I've encountered.

However WebOCalc cannot possibly understand everything that is in the mind of the pilot, or of the airframe designer. Some airframes, like the Easy Star or many flying wings, have physical constraints on the propeller size that WebOCalc does not know about. (Both the Easy Star and most flying wings suffer from very inefficient power systems because of this.)
Quote:
Originally Posted by RENATOA View Post
I think its scenarios list is too limited.
I don't see where motorised gliders falls in any of the categories, so I pick one quite random.
You know, there are a few commercial RC calculator programs you can buy and try out, perhaps you might like one of them better. For instance, you can buy MotoCalc for $39 (USD). It is much more complex and hard to use, but perhaps that will suit you.

There are already eleven "flight missions" choices in WOC! Adding more would clutter up the selection list and confuse the user (eleven is already pushing it, a good software interface design rule of thumb is to limit selections to no more than five choices).

My experience is that the eleven available choices cover most sport RC aeromodels, if you understand how to use them properly (I'll explain this a bit more in a moment).

There's no need for a random selection - a little more knowledge on your part will go a long way to helping you get the most out of the program. Basically, WebOCalc is trying to find out how fast you want your model to fly at full throttle, and how much thrust you need for the model to perform the way you want it to perform. To make it easy for beginners, I wrote several "wizards" that make helpful suggestions and fill in numbers for you.

But if you know what you're doing, and you want something other than what the wizards suggest, you don't need to depend on the wizards. You can type in any numbers you like into the "Desired top speed" and "Desired thrust" boxes, and WOC will still try to help you to find a power system that gives you what you want.

I suggest using the wizards as a starting point, and then fine-tuning the numbers to suit your preference. That way you won't put in numbers that are completely unsuitable for the airframe.

So, how do you intend to fly this other model? What sort of maximum climb angle do you want? Do you want it zoomy and fast, or thrusty and slow? If it's a typical thermal-soaring glider, you may want plenty of thrust for a quick climb up to altitude, but you don't want much speed. "Slow Sport Aerobatics" will give you fairly steep climbs and is probably a good choice for "flight mission". "Slow unlimited aerobatics" will give you enough thrust to climb straight up, which is probably overkill for a glider (remember more power also equals more weight from the bigger motor, battery, etc).

Or perhaps you (or the models owner) simply wants a nice lazy relaxing model. Steep climbs not necessary. The owner plans to enjoy watching the model slowly circle up to altitude, after which he'll shut off the motor and enjoy watching it slowly glide across the sky. In this case, pick "Gentle scale flight" for the flight mission, and go from there.
Quote:
Originally Posted by RENATOA View Post
Some page ago I got a response that the props list is limited and can't be upgraded. Why ?
I'm not sure I understand you exactly - you know there are hundreds and hundreds, maybe thousands, of different RC propellers on the market, right? Also new brands and models appear all the time. So there is no way any software could possibly contain every single possible RC propeller in its internal database.

Before I can add any propeller to WebOCalc, I need propeller constants - numbers that tell the program how much power it takes to spin that propeller at a specified rpm, and how much thrust it will make in the process. RC prop manufacturers never give us this data, so everything has to be carefully measured by someone with a motor dyno and thrust stand.

I was able to obtain data for most of the APC Thin Electric and Slow Flyer propellers, as well as for GWS RS, DD, and HD propellers. The data comes from several sources, who were kind enough to allow me to use the results of their hard work in WebOCalc. WebOCalc interpolates between the available data and makes decently accurate guesses for other propellers in the same families.

In the end, I was able to obtain enough data to make usably accurate predictions for ALL the two-bladed APC Thin Electrics, APC Slow Flyers, and GWS propellers - one hundred and fifteen propellers in all. Every one of those is contained in WebOCalc already. Those are also the most widely used RC propeller brands for electric flight here in North America, so they work for most RC pilots here.

I know some other propeller brands are very popular in Europe, such as Aeronaut and Graupner. If complete motor constants for those are found, they could be added to WebOCalc. Keep in mind, though, that one of my major goals with WOC is to keep it simple and uncluttered; if it suggested twenty different propellers every time, it would be overwhelming and confusing to the user. Adding more propellers would require other software design changes to keep it simple and usable.

One last thing - I created WebOCalc in my spare time, and made it free and open source as my gift to the worldwide RC community (it pleases me to notice that you're in Romania, Mark is in Switzerland, Chuck is in the USA, and I've heard from other users in Australia, New Zealand, and various other countries). I put in hundreds of hours working on WebOCalc over the years from 2004 till 2009. In late 2009 I burned out on RC flying, and diverted my creative energy in other directions. I also began to have RSI (repetitive stress injury) problems whenever I used a computer, which is one reason why I've not been very active on these forums in 2010.

So, to let my hands heal, I have not worked on WOC since late 2009. I gave it a good five years of my life, and I don't know if I will return to it in the future or not. There's a good chance I will, but no guarantees (I had version 1.7 almost ready to go when I quit working on it).

But one of the beautiful things about open source software is that anybody is free to take all my work and build on it and improve it. Long after I'm gone, WOC could survive and thrive, if it finds someone with the skills, understanding, energy, and willingness to keep it going. Maybe that could be you!

-Flieslikeabeagle
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