PatMc

Sadly Brian I'm not too surprised by your response. You have completely ignored the anomally of how a lower recorded voltage can result in higher rpm.

The pics below show two test runs that I mentioned yesterday. The tachometer (top of each pic ) shows rpm to the nearest 100, I think the Wattmeter readings are self explanatory. IMO no deep knowledge of how an ESC works is needed, beyond that the battery is connected to it at one end, it then controls the motor rpm by varying the RMS volts that it feeds to the motor via 3 wires. Note that in both images the wattmeter displays higher volts during the 1/2 power run than during full power but the tacho displays higher rpm at full power. Bearing in mind that the RPM is proportional to the voltage & Kv, it's obvious that the Wattmeter must be displaying the battery terminal volts not the RMS voltage being fed to the motor. Note that in each case Watts is taken to be = Volts x Amps from the respective display. In summary the above indicates that the wattmeter can only give a true indication of the power being consumed by the motor when the throttle is at full power. Mr Kirchhoff can rest easy in the knowledge that his laws are being obeyed. The voltage may vary in our series circuit but the current is constant from end to end. BTW IIRC, Delta connected windings produce root 3 x the power of Star connected windings. I can't think of any reason we would want a motor to be Star wound for our applications but I believe there are few.

Happily my explanation was spot on 👍 In fact I did some practical testing with a model yesterday using a wattmeter & tachometer with results that demonstrate what I've explained. I haven't had time to post the results but should manage later tonight or tomorrow. BTW you are completely wrong regarding the current & voltage levels at different points in the circuit. In any series circuit the current is constant throughout but the voltage varies at different points.

In the US ?

Why you should always put contact details on your aeroplanes (AKA airplanes). Link

I don't think Albert specified a cg position for the Jnr 60, I believe it was left up to the builder to trim the model to suit himself by trial & error. 😉 Post Keil Kraft kits - by Flair & Ben Buckle show cg positions on their plans but IMO they were overcautious & didn't take into account the generous tailplane area & longish separation between wing & tail. I have the Flair version built around 1986 then refurbed & electrocuted 2012/13. Originally I went with a cg a little further back than the plan then after the refurb I started with it at the same point but removed the ballast incrementally over a number of flights until there was none. The model is well behaved with the cg 3.5 cm behind what's shown on the plan. Result is a much better glide & greater speed range. PS which version is yours - the Ben Buckle / early version or Flair later version with wider fuselage ? PPS here's a link to the refurb thread

Last & only flying model Spitfire I made was from a KK kit around 1952 but I did help my son paint a plastic non flying one in the 1970's. 😁

Andy, I was specifically dealing with the power limiting factors of the motor. You're right that the BEC current limit is also a factor varying with the number of cells if it's a linear device, it also dependant on the type/current draw & number of the servos. However surely it's the users responsibility to check this from the ESC's instructions.

The police, prison officers, firefighters, ambulance crews, bus drivers ...

Nobody watches unless there are 22 people (at least at kick off) kicking the ball around. 😉

Can you give a link to the max current for the other Kv rated motors in the table please ? The true power readings can only be taken at max throttle, if you have restricted the throttle to when max rated current is reached the power reading is not true, it's too high. I don't doubt your wattmeter's accuracy in the manner it's designed to be used. However none of our wattmeters are able to measure the power input to the motor except at full throttle. This is because they measure the voltage at the ingoing battery side of the ESC which is not the RMS voltage that's being supplied to the motor via the 3 wire outgoing side except at full throttle. The current measurement is accurate as it's common at both sides. The wattmeter derives the wattage by multipling the voltage & current readings it detects.

That would mean that the power of all motors are being restricted to 2/3 of there potential. In fact there is no reason that more than 3 cells could be used so long as the max current is not exceeded & a practical size prop can be used.

The motor data chart is missing the most important parameter, namely the max current. There is no such thing as a max power figure unless the applied voltage is given. i.e. the max power using a 3s battery is 50% higher than it would be using a 2s battery. BTW the way our wattmeters are connected means that you can't measure the watts unless you have the throttle fully open. Which means that the 340W you measured is higher than the true figure by the fraction of throttle that's applied. If the throttle is 75% then the power taken is 255W..

The same way that a servo know the stick limits ?

The ESC overheating due to extended periods of part throttle may only have been a problem with older versions in pre lipo days when full or near full throttle was needed for most of a flight. But most flight times were usually much shorter due to limited battery capacity so any such problem were rarely encountered by most of us. I certainly never experienced it. First (& only) time I heard of the problem was watching Brian Collins & Nigel Hawes on TV preparing for, then flying the first electric model aeroplane across the Channel. IIRC they unexpectedly burnt out an ESC at low throttle during the preparations but the manufacturer later supplied them with a modified version that could withstand extended part throttle settings.

I don't know if this only applies to some older type's but not all ESC's need to be calibrated to recognise the Tx throttle limits. I have some old Jeti ESC's that state "The position of full throttle will be adjusted automatically" - i.e. once the initialisation beeps are complete each time the ESC is powered up.

The Unlimited after power conversion & later wing re-cover job. Igor 2 function, Solartex covered poly wing, RTF weight 950g. Close up shows TE notched to fit. As shown it would climb vertically & reach 200m within 30 secs then typically 320 - 330 sec glide.

I'm curious about some of the details of your model which I think have been modified from the original design - E.G. your model has an AMT but the OP one has elevators, it also looks like it has a ply-wood servo mount that's glued to the fus & of course the motor is an "enclosed" outrunner not a brushed one as I'd expect. BTW is the motor fitted to a moulded plastic mount ?

I think you'll find that the wing chord is 7.25" /18.5cm. I have a pair fuselages of an early version from a deceased clubmate's estate. They were from a full Protech Unlimited 160cm span aerobatic model plus one fuselage & tail only. Both fuselages were badly bashed about but useable & the complete model had a brushless motor & had been flown with 7 cell nicads. First thing I did was a brushless inrunner + 3s lipo conversion, then reinforced the front end with 3mm ply. The kit's plastic motor mount was later discarded & a new set up fabricated from birch ply & epoxy glass board to take an outrunner. I did a similar nose job & power conversion to the spare (most battered) fuselage & used a tatty 6 ft polyhedral wing that was a repaired left over from another model to make it a 2 function glider. The wing chord was a little oversize so the trailing edge was notched to make it fit. Climb & glide performance exceeded my expectation to the point that I'm eventually going to make a replacement wood fuselage & recover the wing. That version has been named Igor (Terry Prachett fans should understand) I'll post some pics of the models later & some details of the brushless conversion if you're interested.

If the signal to the Rx was being intermittently blocked wouldn't that be easily picked up by the telemetry record ?

Peter, I don't think we are disagreeing, I think we are considering different scenarios. I have only cast doubt on the "spraying with bullets whilst prop hanging" quote.

You're right, of course, Martin. Complete woolly non thinking on my part. But I think the idea of a D7 prop hanging & spraying enemy aircraft from below with machine gun fire is total fantasy. Even the slowest likely target aircraft, e.g. the much loathed R.E.8, would be travelling at at least 60 mph, making only one or two rounds even likely to hit it during the time available. The odds of a vital strike would be pretty poor compared to the conventional already proven tactics of the time. Back on track - Chris, one book that details an aerobatic display with a Spitfire is Sigh For a Merlin by the well known test pilot Alex Henshaw, The display being for the benefit of Winston Churchill who was hosting a group of, IIRC, US politicians & military officers. OTOH I think it was Pierre Clostermann in his book The Big Show that makes the point that any pilot doing aerobatics in a war zone is making himself a potential easy target. With this in mind I think if a war plane is in clean factory fresh condition aerobatics would be in order (though IMO not precise model aerobatic contest standard). But if the model is presented dirtied up to represent an in service operational machine the flying display should be more just tight turns & zooming around plus of course standard take off, procedure turns etc

Surely if the attack aircraft is prop hanging it will be at the mercy of the wind speed & direction.

That's about 2.5 - 3 times the power of a Cox 049 which would give that power on 6 x 3 or 6 x 4 prop.

Only if the target aiircraft is flying almost exactly in an up or downwind direction or there's virtualy no wind. I go with the stall turn/wingover as being most likely.