Author Topic: Airco DH2  (Read 40496 times)

Happy Days

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« Reply #15 on: January 20, 2012, 19:27:29 PM »
Quote from: "johnfireball"
Yes Keith dh2 was powered by a 100hp gnome rotary engine
   


They’ve always fascinated me, those Gnome engines. They had no way of regulating how much fuel/air mixture was sucked into the combustion chambers. So they regulated the engine speed by switching the ignition on & off. :shock:  You can hear that happening in the You Tube clip.

The problem really became evident when trying to land. If the pilot left the engine switched off for too long the spark plugs would get wet with un-burnt petrol and refuse to spark when the ignition was switched back on. So the engine wouldn’t re-start.
Of course if the pilot left the engine switched on for too long, while trying to land, the plane would have too much airspeed to be able to land!

I’ve often thought how much workload there must have been trying to land a plane with such an engine. Not only would the pilot be having to consider all the normal matters, wings level, airspeed, glide angle, distance from runway, but also having to think about whether he should switch the ignition on for a couple of seconds to clear the cylinders of fuel so that should he need to go round he would be able to start the engine to do so!

And people talk about the “Good old Days!”………. Good riddance I say!! :lol:

Keith
Try not to run out of airspeed, altitude and ideas....... all at the same time.

johnfireball

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« Reply #16 on: January 20, 2012, 20:40:55 PM »
I have the body of an 18 year old.........I keep it in the freezer

andrew wallace

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« Reply #17 on: January 21, 2012, 19:58:08 PM »
:clap: nice video
sounds like the pup has a bad case of wind  :oops: on the landing  :lol:
i\'m an ordinary man nothing special nothing grand

Happy Days

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« Reply #18 on: January 21, 2012, 21:12:04 PM »
No Andy, that’s not the sound of the pup.

It’s the sound of the pilot suffering from a bad case of wind while landing :lol:

 Frankly I wouldn’t blame him!

K.
Try not to run out of airspeed, altitude and ideas....... all at the same time.

johnfireball

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« Reply #19 on: January 28, 2012, 02:00:24 AM »
Hi All,
       Love this model, I have cut and chopped this kit to make it more scale and am really enjoying the build. Sorting out the wire rigging(there's an awful lot). Control surface cables are ready for connection as per full size ie on the outside. The aileron control was a wire led along the leading edge of lower wing to a pully to turn 90 degrees to lower aileron horn then both ailerons connected at trailing edges by cable  and connection from top aileron horn to leading edge of top wing by spring on top wing aileron. Should work ok I'm using fishing trace coated wire for the control wires and painted fishing line for the rigging. I dont like the nose on this kit as I think it's way off scale and may modify it to suit. The model is getting a bit heavy but I have tried some runs in my living room under power and I think she's gonna fly albeit just like the real thing (barely). Just what I'm looking for.
     

I have the body of an 18 year old.........I keep it in the freezer

billscottni

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« Reply #20 on: January 28, 2012, 12:01:03 PM »
Meautiful job. I wish my building was half as good :(

johnfireball

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« Reply #21 on: February 03, 2012, 01:00:50 AM »
Hi all,
      I had some fun with my DH2 today. It's not finished yet but is flyable so I decided to give it a test run to check out the power output and just as well I did. It needs about twice as much thrust as I'm getting so I'm guessing I need an outrunner powered with a 3 cell lipo. The weight of the model without battery is 240 grms.
http://www.hobbyking.com/hobbyking/store/__5428__AX_2306N_2000kv_brushless_Micro_Motor.html
http://www.hobbyking.com/hobbyking/store/__5430__AX_2308N_1800kv_Brushless_Micro_Motor.html
http://www.hobbyking.com/hobbyking/store/__5427__AX_2306N_1300kv_Brushless_Bell_Motor.html
http://www.hobbyking.com/hobbyking/store/__6784__ZIPPY_Flightmax_800mAh_3S1P_20C.html
http://www.hobbyking.com/hobbyking/store/__6457__Hobbyking_SS_Series_18_20A_ESC.html

These are my choices as they have the beam mounts and will fit with little mods required. OK what do I choose? I want to run an 8" prop for scale reasons and I don't really want a revving noisy fecking thing. The model at present has a 10 amp esc fitted in it's bowels and if possible I would like to leave it there, but if necessary will replace. Prop suggestions? Perhaps running with 2 cell lipos and less rpm. I'm new to this electric and perhaps need a little guidance. I broke the undercarriage today and I am  now redesigning for strength which is difficult due to its gangly nature.
John.
   



I have the body of an 18 year old.........I keep it in the freezer

billscottni

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« Reply #22 on: February 03, 2012, 08:15:07 AM »
I'm not up on electrics John, but I would suggest due to the draggy nature of the airframe it's going to need something with plenty of torque, but that you don't want something which will make it hurtle round the sky in an unscale manner.

Have you had a look at the calculator, (now FREE) on RCAdvisor?

http://rcadvisor.com/free-calculator

andrew wallace

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« Reply #23 on: February 03, 2012, 21:29:18 PM »
lovely model john all the best for the maiden flight  :clap:  :clap:
i\'m an ordinary man nothing special nothing grand

Happy Days

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« Reply #24 on: February 04, 2012, 08:52:31 AM »
Hey John,

You say that the esc is bedded into the bowls of the model, you need to be aware that it requires a cooling airflow to stop it overheating.
Another thing I’ve thought about relates to that dummy engine. Is it balanced? and is it strong enough to withstand being rotated at around 10000rpm? which is the sort of speed you’re heading towards with an out runner.

Of the outrunners you are thinking of getting, probably the 1300Kv model would be most suited. I see it draws 8 amps current, so to be safe I’d be thinking of using an esc able to handle around 15 amps. (Personaly I'd stick to a two cell battery.)

Here are some facts about props:

As for prop sizes; changing the diameter of the prop will vary the rate of acceleration and rate of climb i.e. changing the the diameter doesn’t change the top speed of the model.

Changing the pitch of the prop varies the overall speed of the model but also varies it’s rate of climb and rate of acceleration. Confused yet? :!:

Look,…….If you have (say) an 8” diameter prop with a 4” pitch. The thrust it produces at (a given rpm) will cause the plane to climb at a certain rate, and have a certain top speed.

If you maintain the 8” diameter and reduce the pitch size, the plane will climb faster but it’s overall top speed will be slower. (Like using a lower gear in a car.)
If you want to maintain the same top speed and increase the rate of climb you would have to increase the diameter.

All of these variations however will effect the amount of current being drawn by the motor, esc and battery, because they all form part of what's called the ‘Power Train’

( At a given rpm, doubling the propellers pitch will increase the power requirement by 50%. Maintaining the same pitch but doubling the diameter of the propeller will increase the power requirement by 100%)

As converting this plane to electric flight has never been done before there are no known facts for what would be the best power train for this model, so you are going to have to do a little bit of ‘suck-it-and-see’ John.


To ensure you don’t over load the power train you really need a “Watt Meter” to connect between the esc and the motor as you try different props. This will display a host of information about the different voltage on the battery, current being drawn, current used, and power being consumer at different throttle settings and with different props.



Don’t get disheartened John, Smile! This is all part of the fun of experimenting with model flight, 8)  (so I’m told! :roll: )

K.
Try not to run out of airspeed, altitude and ideas....... all at the same time.

johnfireball

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« Reply #25 on: February 05, 2012, 00:32:52 AM »
Hi Keith,
           Me disheartened! never. I relish a challenge. Yea with the higher rpm I think I will not turn the engine but perhaps try for a little friction freewheeling. I'm going with your motor recommendation which is what I also felt would be best and have ordered 8"x4" and 9"x6" props and also an 18amp esc. I ran the current motor flat out until the battery died and there was very little to no heating of the esc, I realise the new setup will be different due to higher loading so will test. If v/i=r then running on lower voltage(2 cell lipo) means pulling less amps so maybe 10amp esc will suffice. The worst part is waiting for the bits to arrive from Hong Kong but I am now getting stuck into the restoring of my dads old glider, it's an electric powered precedent from the mid ninties. The 1300mah nicad battery weighs more than the glider.
John.
I have the body of an 18 year old.........I keep it in the freezer

Happy Days

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« Reply #26 on: February 05, 2012, 08:08:29 AM »
Oh, very impressive John! Personally I think you’ve made a good decision vis the dummy motor. :D

Now I don’t recall ever seeing “ v/i =r ”  being quoted on this forum and before I start sounding like an electronics expert, which I’m not, (in fact I’m not an expert on anything,…….except, perhaps, making spelling mistakes, thank you Dave! :roll: ) So for your info, in case you don’t already know this, I’d just point out that when the motor is swinging the propeller it’s creating mechanical power.  That’s obvious I know, but think on a little………

Power (measured in Watts) = volts x amps. Let’s suppose the prop needs (say) 100watts to spin at a given rpm.
Working the above formula ‘backwards’ 100watts divided by 7.4volts = 13.5amps

So what would happen with a 3cell battery?…..let’s see :-

100watts divided by 11.1volts = 9amps

So higher voltage batteries actually mean less current being drawn, for a given power requirement.

One more thing that, again, you might already know, but just in case you don’t…….
Li-Po batteries die if the voltage on them drops below a certain minimum level. Most esc’s have a “Low Voltage Cut out” to protect the battery from going too low and dying.  (As the esc senses the lower voltage, it cuts off the power to the motor.)I assume your esc has an LVC or else you wont be able to recharge your batteries.

The point I’m coming to it that, for safety’s sake I suggest not to run the battery right down to the LVC level every time you fly,……..just to be on the safe side!

Oh, and one more thing.......Li-Po's don't like being stored at full charge. Best to keep them at around half charge.

I hope you're making notes on all these matters John, there'll be an exam at the end of term!!! :lol:


Keep up the good work John.

Keith.
p.s. Been out on the bike lately? :P
Try not to run out of airspeed, altitude and ideas....... all at the same time.

johnfireball

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« Reply #27 on: February 05, 2012, 22:06:39 PM »
Hi Keith,
          I think my only worry will be the  maximum amp value.
Supposing the max power is 100watt with a 3 cell lipo at 11.1 volt = 9 amp
Max power with a 2cell will be approx 66watt (2/3 of 100) at  7.4v = 9 amp

So yes for a given achievable power output the lower voltage does draw more current but both at full power will draw the same. Does this look correct?
John.
I have the body of an 18 year old.........I keep it in the freezer

Happy Days

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« Reply #28 on: February 06, 2012, 00:22:40 AM »
Hhmmmmmm,……………I have a feeling that this is going to be a long post, so just hang on while I go and put the kettle on……………

I’m not too sure where you are coming from with your question John. As it stands your assumption is incorrect.
 Let me give you some facts and hopefully you will be able to put together a concept of how the system works in your head.
I have no idea what you already know and don’t know, so I’ll assume you know nothing. (Forgive me if I’m wrong on that point)

For the sake of ease of understanding I’m going to generalize. So let me state first and foremost that some of the things I’m going to write might be only 95% correct. i.e. there may be some exceptions to what I say.

The maximum power that any battery can produce at any given moment is equal to the voltage of the battery x the total current it can produce,…. at a given time.
Lets talk about Li-Po’s and look at the following picture.


Here is a little two cell Li-Po of the type you already have.
The markings show that it’s basic current capacity is 450mAh. Just to remind ourselves what that means………it means that the battery could discharge 450milli amps of current over the course of one hour. This is known as it’s 1C rating. (‘C’ standing for capacity)
However the marking “28C” tells us that this same battery is also capable of discharging up to 28 times  it’s 1C rating of 450milli amps, (but for a much shorter period of time of course.)
So, for a short while this little battery could discharge 28 x 450mA.
That equals 12600 milli amps, or 12.6amps. (This figure is also shown on the battery.)

“This is all very well,” I hear you say, “but it’s only referring the battery’s current,  how much power will it produce?”
Well, we know that power (measured in Watts) = VxA
The nominal voltage of this battery is 7.4volts. The max rate of discharge is 12.6amps, so it’s max power output is 7.4 x 12.6 = 93.24 watts

Therefore,……………oooppss the kettle has just boiled, wait while I make a cuppa!
(Slurp) Ar! that’s better. :D  You can’t beat a nice cup of Earl Grey. It’s so refreshing! 8)  
Now where was I?…… Oh yes, we were doing some maths weren’t we.
So the maximum amount of power in the “Power Train” system that this battery is connected to can never be more than 93.24watts.

Now let’s suppose we were using a 3cell Li-Po (11.1volts) which could only supply a max current of 8.4amps. How much power would it produce? Surprise Surprise 11.1 x 8.4 = 93.24 watts
So both batteries produce the same amount of power, but one of them does it by way of using less current. (But a higher voltage)
So it is possible to derive higher power amounts without drawing more current. The trade off is that the system has to be able to handle higher voltages. (Hang in there John, we're nearly home :P)

If you have managed to stay awake through all this I’d be very surprised, but just in case you have here comes the final bit.

The more amps a device uses the hotter it gets. So there is always an incentive to keep the current down. However as we’ve seen the esc and motor have to be able to handle higher voltages.

There, well done John, you can go to sleep now. ZZZZzzzzzzzzzz
BUT just in case you’re still awake here’s another Li-Po.


It says it can produce 240amps. How is that so when it’s only a 4000mAh battery. And what do you think it’s max power output is? (Answer = 5.328Kw but how do I know that?)

Sleep tight!

K
Try not to run out of airspeed, altitude and ideas....... all at the same time.

johnfireball

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« Reply #29 on: February 07, 2012, 00:51:59 AM »
I'd rather be a hammer than a nail, yes I would, if I only could, I surely would. Yes I've played guitar all day to get this electrickery out of my head. Anyhow this is my pilot( back to basics), just have to carve him now, will try foam and see how I get on. Will post pics good or bad.
John.
   
I have the body of an 18 year old.........I keep it in the freezer