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Member postings for PeterF

Here is a list of all the postings PeterF has made in our forums. Click on a thread name to jump to the thread.

Thread: Lipo advice
19/02/2019 21:13:18

The bulk of the charge current passes through the main battery leads so they need to be used. The charger then bleeds in or out small currents through the balance leads to bring all cells in the pack into balance. This is because the Bmax type can put a current into the pack that is higher than the thin balance leads can handle. Your original basic charger probably had a low current rating so could work through just the balance leads.

19/02/2019 19:41:57

The 2200mAh will give more duration, 50% more than the 1500mAh roughly, although the plane will be slightly heavier, meaning higher throttle for same airspeed, which will mean that the increase in duration is not quite as much. You also need to be sure that the heavier battery can be positioned to give the correct CoG.

The ESC rating is 30Amp, so if you have a 2,000mAh battery = 2Ah, the current rating is 30 / 2 = 15 so a 25C would be OK, no need for 60C for this current draw.

Thread: Motor / prop calculator?
18/02/2019 23:39:48


Thanks for your point, I had alluded to the motor offloading somewhat but not explained it very well, I know it occurs, just didn't get it over very well. Yes, as the model speeds up, the propeller thrust does reduce as the drag increases, but I believe that the drag increases faster than the thrust reduces, so the motor remains under a high load.

What I was trying to explain was the conundrum Peter B had raised between my assertion that motor power rises with the cube of the rpm and his view that motor power rose then diminished. I did not clearly answer that, but your point has got me to the answer in my head.

To Peter B's

You describe power from the motor rising from zero to a peak and then declining to maximum rpm fully unloaded. However, to get a motor to follow this curve requires a variable load with rpm. With a propeller, this is a fixed device whereby the power demand can only increase as the rpm increases. Therefore, what you describe is the maximum potential power that the motor can supply at any rpm and I describe the load placed on the motor bythe propeller, these are different. The motor can only supply what is demanded of it.

Graphing both of these illustrates the difference.

motor power.jpg

I have plotted the maximum power deliverable by the motor over the rpm range (red) as the torque (blue) falls per your description. I have plotted the power required by a static (green) and a flying (purple) propeller. In our case, as the propeller is fixed, the motor may be able to deliver more power at 5,000 rpm than the propeller demands from moving air, and if the throttle is fully open, the difference in power not required to move air will be used to accelerate the prop to its peak rpm. The peak rpm achieved is less than the unloaded rpm and occurs where the potential power delivery from the motor matches the power demand from the prop. When the prop unloads in flight a small increase in rpm occurs. Typically for our models, we operate with the motor rpm around 80 to 90% of the unloaded rpm. Thus, any higher power demands during propeller acceleration are transient and have little effect on motor temperature. What motor rating is all about for our models is the steady state condition because this is where they spend a lot of their time.

Hopefully, I have better explained the differences in our thoughts.

Once again, this is simplistic.

18/02/2019 21:44:26


Power being proportional to speed cubed is one of the basic fan laws, I have worked on large fans throughout my career. This strictly relates to a stationary fan and you are correct that a moving prop will have some unloading. Your view of the motor current being peak at zero revs due to peak torque is in our terms as model fliers somewhat academic in my view and is only of transient concern as the prop is accelerating to full rpm and not something we would be able to measure. We are interested in steady state performance, and once the prop has reached full rpm, its aerodynamic load from moving all the air around is at its peak.

Consider a plane on the runway with the motor off. The throttle is opened fully, the prop rapidly accelerates to maximum static rpm and takes a certain amount of power to do this, which is largely irrelevant. At the peak static rpm, there will by a high power delivered to the prop as it is now moving the maximum amount of air from zero velocity to lets say pitch speed. The model starts to accelerate, takes off and reaches it maximum airspeed. The power required to overcome the drag of the airplane increases rapidly with its airspeed, again by the cubic law. Therefore, the planes maximum power occurs at maximum airspeed. The propeller has to deliver this maximum forward thrust, therefore the motor can not fully unload. It will unload some amount but never a lot, because for us, the motor essentially has a load that increases with prop rpm.

The cubic law is involved in nearly all turbulent fluid flow phenomena. If you have a pipe and you blow a certain amount of air through it with a fan, you will have a certain pressure drop. If you double the air flow (by doubling the fan rpm) the pressure drop will increase by a factor of 4, in other words 2 squared. The power required is the pressure drop times the flowrate. Therefore, doubling flow requires 8 times as much power.

This is simplistic but hopefully useful.

Thread: Electric setup advice
18/02/2019 21:13:39

Some ESC have auto calibration, I am not sure exactly what this means, perhaps on these if the throttle range is reduced on the Tx, then the ESC auto adjusts to give full power at whatever max throttle it sees, so setting it low would achieve nothing. Others can easily be recalibrated by certain stick movements at power up, if you restricted motor power by reducing throttle range after calibration and then inadvertently recalibrated the ESC it would then give full power at the reduced throttle range. Therefore as others have said, set the proper throttle range and choose an appropriate prop.

Thread: Motor / prop calculator?
18/02/2019 10:12:11

Yes, a luggage or fishing scale can be a useful addition to the test kit. However, when dealing with large kit where a good prop can cost £20 or more for a scale 18 x10 or larger then the buy a few props and test them out is not really viable, I wan't to choose the prop with some confidence before I buy. Then I test it out after wards which is usually a confirmation. This becomes even more the case when choosing a set up from new. The suck it and see method is fine for smaller planes where props are less expensive and if the plane does not fly as expected, changing the prop is a small(ish) matter.

17/02/2019 23:36:01

The biggest issues I have found is the actual rpm achieved when checked against the motor KV times the measured voltage, assuming minimum voltage drop over the ESC. Normally with a motor running a prop within the motors capability, the measured rpm will be 80 to 90% of that calculated, and ecalc makes a good guess at working this out. However, I have some motors with the prop running at 70% of the calculated rpm where the propeller is in proportion to the motor, so the motor has a KV 10% lower than spec. I also have some recent measurements on a 2.5kW motor in a 2m aerobat, with a 17 x 8 prop taking close to 2kW of power at 8,460 rpm where the calculated rpm from volts x KV was 8,410 rpm, i.e. there was no reduction in motor rpm for load. I can only assume that in this case the actual motor KV is about 10% higher than spec. Therefore, motors as delivered can have a +/-10% variability in KV, and as motor power is proportional to rpm cubed, this can result in 30% discrepancies in any calculations. I have to admit that in these extreme cases I am quoting the results of cheaper motors, for my more expensive motors I find that they tend to operate closer to expectations and the calculations by ecalc and I would not be without ecalc.

Thread: Electric setup advice
17/02/2019 23:14:49

That seems to be as expected, I gave an ecalc estimate on page 1 of just over 37 Amp on a 11x6 and the 10x7 will be similar. It was only an estimate as I did not know all your details. This should fly a 5 to 7lb trainer OK. I think you might find the 11 x 8 towards the limit of the motor, so if you test it, open the throttle steadily and watch the Wattmeter to make sure you do not pull too much current.

Thread: Airco DH2 - Microaces
15/02/2019 18:51:53


I will keep an eye on this, I have recently completed the Microaces SE5a and it flies really well and I have been looking for another one. I like the DH2 but have shied away so far, so I will see how you go.

Thread: Mini/micro lathe
14/02/2019 19:56:27
Posted by kc on 14/02/2019 17:04:41:

The Peatol lathe has been around for decades ( remember them at the Model Engineer Ex and being advertised in RCME ? ) and is much cheaper than others especially if you use your own motor ( ex washing m/c etc ) and might be the cheapest satisfactory lathe.

I have a Peatol lathe with compound slide and milling attachment, normal chucks and a collet set. I have found it very useful for a lot of small items, spinner nuts, scale details, pulleys etc. I have cut aluminium, brass, carbon steel and stainless steel using a small set of replaceable tip carbide tools. It really came in really useful when completing the scale detail on my quarter scale Tiger Moth.

Thread: Digital incidence meter
14/02/2019 07:32:22


I have bought one of the ebay inclinometers, slightly different to the one you linked to, see here.

This has a magnetic base as do many of them, so I fastened a small steel sheet (galvanised) onto the top of the measurement rail with epoxy. It works a treat. Set up all the flying wires on my Tiger Moth with this.

I did initially set my Tiger Moth rigging up with my android tablet using the "Clinometer + bubble level" app, but found that the weight of the tablet was significant. I also found that over a long period of use as the tablet warmed up, the zero drifted and it was not repeatable.

dsc07380 (custom).jpeg

dsc07379 (custom).jpeg

Thread: Good lipo charger
10/02/2019 19:01:59

I use the icharger 106b, I have a couple that are 6 or so years old and have been reliable. They are at the upper end of your price range though. Based on my good experience with these when I started to fly large electric I stayed with the icharger brand and bought a 4010 duo.

Thread: Electric setup advice
03/02/2019 13:53:16

If it is the Hornet noted above, then the set up I gave above with an 11 x 6 prop for starters. The recommended motor has a kV of 650 so would spin the prop more slowly than your 800kV motor, hence keep the prop size down a bit to limit the power output.

For ground clearance, you can measure from centre of spinner to ground, but as the nose gear is often sprung with a coil in the wire leg, you need to allow a good 2" of clearance, perhaps more for a heavy landing when the nose wheel bends quite a lot to absorb the shock.

Also, if it is the one shown then you will not need to make alterations to the model for the electric conversion as it already has a hatch for the battery between the wing and the firewall.

03/02/2019 12:56:13

Running the set up through e-calc, an on line electric set up calculator, the Ripmax Quantum 40 motor on 4S battery as you have shown will give an rpm of around 10,000rpm. Run this with an 11 x 6 prop and you will get a current draw of about 37A (well within motor and ESC specs and battery capability) giving just over 500W, which will be good for a 5lb weight trainer. If you then develop your skills, the prop could easily be increased to a 12 x 7 which would draw 52A (still in specs for motor, ESC and battery) giving 700W and aerobatic performance. As others have stated, you can not use the program card to limit the motor output, you do this by changing the prop size. One thing with electric is that the motor tries top turn at the same rpm whatever the prop that is fitted, so if you fit too large a prop it draws too much current / power and blows the motor or the ESC. Likewise, in your case fitting a smaller propeller does not allow the motor to rev faster, if you fitted too small a prop on an IC engine then it would scream.

Thread: Forum members' new models: Let's see them.
02/02/2019 12:10:26

Love the alternative decals, was it an inspired choice given the clack and yellow colour or is there really a Yak with JCB logos. It should fly like a rocket with the 30cc engine in it, hope you have lots of fun with it.

Thread: Wing walkways
29/01/2019 18:37:49
Posted by Manish Chandrayan on 29/01/2019 12:45:34:

I made mine by mixing micro-baloons with flat black paint and brushed it on. Idea was shamelessly borrowed from one of the threads on this forum

I used paint with micro balloons for the walkways on my Tiger Moth, see my build, **LINK**

Thread: BMFA News Feb 2019
26/01/2019 23:15:10

I participated in Archery before I became involved in model flying, and even in the late 1980s early 1990s archery maintained a level of continued competency for coaches, there was at the time no equivalent of examiners. Coaching certificates were valid for 3 years and at the end of that period of time you had to be able to demonstrate by means of keeping a log that you had been active in coaching and attended workshops etc. If at the end of the 3 year period you had not demonstrated that you had been a practising coach, then your certificate was not renewed. I gained my basic coaching certificate after a series of around 6 2 hour sessions and then an assessment by a regional / national level coach. When I stopped participating seriously in archery my coaching certificate automatically lapsed and if I wished to regain this then I would have to go back to going through the full cycle of coaching workshop sessions.

26/01/2019 21:53:01

See page 51, column 2, para 3, questionnaire will be sent out to individual examiners once the data is entered into the Go Membership system, but this does not mention a date, merely that it will happen after the data is processed.

Edited By PeterF on 26/01/2019 21:53:43

Thread: What does your wife or significant other half think of your hobby?
26/01/2019 16:15:15

I am well set with my wife, she grew up with her father and brother flying RC and it was those two fine gentlemen who got me involved in model flying. Unluckily we live away from my in laws so getting to fly with them is an infrequent event. She has her hobbies too, gardening and paper / card crafting and that equally racks up the pounds, I am always amazed at how much quality paper and card stock costs as well as all the dies and embossing stamps. But then she is amazed at what balsa wood costs. What has turned out to be a useful cross fertilisation is she has a CNC cutter and I found I could use it for all the lettering and paint masks on the quarter scale Tiger Moth I completed last year.

Thread: Heated Lipo Bag?
26/01/2019 10:21:59

I have 3 of the same Turnigy ones as linked by John in the post above. They work well and I would not be without them.

For example I have an EDF model that pulls 100A from 5000mAh batts and launches direct from a ramp, no bungy. Summer time on a calm day it rockets off. Winter without using the heating bags, on a calm day, it sags after leaving the ramp and almost gets to the grass before enough speed has developed, nervous times. Also, the low voltage telemetry alarm is sounding until the packs warm up. Winter when using the heating bags = summer.

I can also notice a difference in other power critical applications, or ones such as F3A style where accurate throttle management is vital, the throttle stick has to be much higher in the winter, hence pre-warming the batteries gives a much more consistent throttle throughout the season.

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