Please Help Me Out With The Arithmetic On This One Gentlemen!

[Former Member]

[This posting has been removed]

[Dave Hess]

Posted by David Mellor on 22/08/2018 20:00:51:

So.... I've been doing some research for the technical "nitty gritty" on what is going on here.

The operating voltage under no-load is indeed 5 cell lipo (21 volts), as I suspected.

The motors themselves are designed as very short burst, high current devices.

So high, in fact, that a 5 cell lipo stands no chance whatsoever of maintaining voltage under load. So they are "fed" from 10 cell lipos (as Mike mentioned) and the high voltage (42V) instantly sags down to 20 Volts under the colossal load imposed by huge "square" propellers (17 x 17 to 20 x 20 types).

This is where the physics goes to pot. The initial energy drawn by the motor does indeed accelerate the plane at a very high rate. For a brief but glorious duration.

So in the first few seconds the motor is converting most of the electrical power to mechanical power.

Thereafter that conversion to mechanical power fades rapidly as the motor generates increasing amount of heat and strong stray magnetic fields.

The current drawn remains high, of course, but it isn't going into turning the prop so much as heating the environment and generating stray mag fields.

But by then all it has to do is turn a, by this stage, 200 kph highly unloaded prop to offset airframe drag to sustain speed, and that now takes very much less of the apparent 6,000 Watts than it did to make the initial - and spectacular - acceleration.

The current is limited by the back emf when the motor is up to speed. Any motor will take a high current the moment you open the throttle, but when it gets up to speed, the current will reduce because the voltage driving it is the difference between the battery voltage and the back emf. As I understand it, the aim is to get the motor spinning at approximately 75% of the rpm calculated from Kv x actual battery voltage when up to speed. That's a rule-of-thumb point where you're likely to find the best compromise between efficiency and power. If you have a data sheet for the motor that shows the actual power curve, you can get the exact rpm to aim for.

[Erfolg]

Nigel

What you would do, is precisely what I would be watching and evaluating, does it all work as a package, as far as i can tell.

Personally I could not be bothered to put much effort in for one event, but that is just me.

What we do know, is that there is a slot time of 10 minutes (which is quite long for a electric model running flat out, in my experience) where as many laps as possible are to be achieved. What we know is that the present set up cannot run for 10 minutes.

Knowing how many laps the winner managed would be helpful, in just gauging how far from the pace, the model presently is.

For me it is a "no brainer", in that knowledge of what the current draw presently is. Just to know how much or little headroom there is.

Perhaps the biggest obstacle to winning, is being competent to fly optimally the circuit.

All the points made by the contributors play a part in the construction of a optimal aircraft system. Yes, weight needs managing, drag reducing as much as is possible. How all of the ideals are achieved and how significant each element will be debated add nauseam.

For my part, I would favor E205, because it can be flown slowly, it is capable of high speed with visibly less loss in height (with gliders). How does it differ from Clark Y, standard E205 is pretty much the same thickness as Clark "Y", although there are modified thinner versions about. Where it differs is that max thickness is further back, the nose radius is higher, leading to a lower camber. All seem trivial, but built with reasonable accuracy, it works very well.

With drag reduction, it is often about detail, although if cross sectional area can be effectively reduced, the formula does have the term as a factor, ridding the external elements of bits sticking out will normally improve the Cd value, another term. Sharp edges and pointy bits tend to degrade the Cd value, although they often look good. Radiuses are often much better. Perhaps the most obvious candidate for drag reduction is the UC. The real problem with higher speeds is that there is a V^2 term, which increases the drag force significantly as you go faster. The up side is that more lift is generated the faster you go.So the wing area etc can be tinkered with, although there is again limits to gains. But you do have to land.

All of the above the average modeler knows instinctively, does not need the formulas/relationships, although some knowledge of them can be insightful as to why.

I would guess that the majority of model Pylon racers know little of the relationships, do not care, but will build a faster model and certainly fly more competently than the likes of me. Experience matters a lot, observing what actually works, and what in practice does not really matter.

[David Davis]

Erfolg, the pylon race is only one of four flying tests or rounds in La Coupe and the only one which requires sheer speed. Each round lasts for a timed five minutes not ten and the Contest Director gives us all a count down before the timed five minutes starts. However, if an i/c engine fails in flight before the official start, you could find yourself bimbling about for a couple of minutes before the start. This could happen in any of the four rounds but only the pylon race would require me to fly at full power all the time. My model lost power just as the CD declared that there was one minute left before the end of the round, i.e the model spent a couple of minutes bimbling about as slowly as possible and four minutes at maximum speed using a 4S 2200 LiPo.

If I am to run an electric motor next year I feel the need for greater endurance.

My ambition is to improve on 18th place in the concours d'elegance and 53rd place overall next year.

Ben B, what are MR racers?

Must get on with doing something more productive!

[Former Member]

[This posting has been removed]

[Erfolg]

If slow speed is part of the required regime, I certainly like bobs idea. A second set of plug in wings. Generally a higher camber with a large well rounded LE helps the lower speed aspect. Although with a plank (parallel) chord wing stalls tend not to be an issue, a bit of washout makes sure, that controllability is maximized.

Again as suggested to keep the weight down, a low capacity Lipo could well reduce the wing loading and still provide sufficient energy to undertake that aspect of the event.

Concurs, well it is often about opinion. Spats are often seen as pretty. A Clean external line improves appearance. A fancy colour scheme is often drooled over. I guess a recover just for the event is in order.

I know that all of this is to much work as far as I am concerned, some one has to come last. If coming last saves others from metal traumas, I would feel good about myself.

[Erfolg]

Ah, just caught up, a second set of wings would not be permitted.

That is not a problem, I would consider full span flapperons. This can have the benefit of being reflexed, for high speed, in addition to drooping for loitering. Choosing the width of the surface carefully, can enable other sections to be mimic, the movements are generally very small. Yet again, it requires a development and familiarization Programme.

As again has been observed by others, I would give the winner and other models a good doze of looking at.

Becoming a winner comes at a price, that is measured in time, money and mental application. It all seems like hard work to me.

[David Davis]

Erfolg,it would appear that you have not read the post I put up at 18.54 yesterday where I attached the competition rules (in French) which specifically say that ailerons, and by extension flaperons, are banned or if fitted have to be disconnected and taped up. It's a competition for three channel trainers.

If you scroll forward to 21 mins 33 secs on the video of the event you will see the winner of the competition trying to fly through the obstacle inverted! He didn't succeed but flying a basic threee-channel trainer inverted takes a bit of doing. Beyond my skills that's for certain! **LINK**

[Erfolg]

DD you are correct.

i would suggest that design, in this context is about creativity, in establishing where the boundaries lie. Then getting up close and personal, with the boundaries, where an advantage can be gained.

The irony is that the biggest advantage is often in being able to fly the circuit efficiently. Unlike formula 1 where every single driver is pretty good, in our hobby most of us just bumble about.

You seem to take the competition seriously and want to win, it will take application and not being squeamish about interpretation of rules and applying them to your advantage.

[Erfolg]

OK the rules say you have to fix any ailerons. Also you can only use 3 controls.

An optimal fix, would be to have the training edge moveable. OK it must be fixed, it does not say they you could not adjust the trailing edge between rounds, does it? On that basis, I would drop the TE about 2mm (whatever is optimal) for the slow flying bits. For the pylon bit I would adjust the TE to a low drag position.

As others have suggested. I would be thinking about thinning the depth of the wing. I would be building a low camber wing section, that lends itself to an approximation to a high lift section with the TE dropped that tiny bit.

Is the wing area fixed?

The ultimate secret weapon though, is improving my Limboing and perhaps the harder bit, flying fast a pylon circuit. I am assuming you have flag men? It is the turns that can blow everything, to early a disaster, to late and time and power is wasted. Pull to tight and the speed will be lost, to wide and you fly to far.

[David Davis]

Erfolg the rules specifically state that you can reduce or increase the size, shape and thickness of the wing by up to 10% but that a Clark Y aerofoil must be used. **LINK**

The standard Baron wing rib is 3cms at its thickest point. Will 3mm make that much difference?

I was thinking of using the Super 60's thicker wing rib so that the model would fly more slowly in the first and last rounds which call for accurate flying rather than sheer speed. However, the Super 60's wing rib is 4 cms thick and that's more than 10% thicker than the standard Baron rib. I don't suppose that I'd be banned from the event if I turned up with a wing as thick as a Super 60's. It's not as if I'm going to win! I just want to finish in a higher position than the 53rd place I acheived this year.

For all of the events a judge from the local club stands behind you and counts the number of sticks you hit, the number of pylons you pass, the number of cuts you make and the number of times you fly through the obstacle in the limbo.

You can get three bonus points in any of the rounds for a touch and go. In the streamer cutting round, one contestant made no attempt to cut the streamer but spent the entire round doing touch and goes. It's pretty hard to cut the streamer. There were at least two rounds in which no cuts were scored at all.

Edited By David Davis on 26/08/2018 14:27:12

[Erfolg]

If you enter into the spirit of the rules and competition, the rules may state Clark Y the competitor thinks, I am cutting these wing ribs out, then assembling them. Hmm, how accurate am I? Looking at the wing, can you really see the difference between what most modelers can actually build, when compared to the co-ordinates of, well, lets say E205. I do not think so!

All of the ambiguity presents opportunities.

Will 3mm make so much difference. In thickness possibly. Raising the LE to reduce camber most definitely, in essence a different section, due to camber, move the max thickness back or forward by 10%, again potentially yes.

Being an ex glider guider, I believe, or some times will say, I know that sections matter. Clark Y is OK for those who stick a motor of some kind up front. Glider guiders, who have tried Clark Y know that many other sections really do perform better. That is PE is converted into time, distance and speed more effectively.

I do admire the competitor with the touch and goes, he or she analyzed how to max the points, not necessarily as the organizers envisaged.

There is another aspect we have not mentioned and that is propellor selection, particularly pitch. Not just from the point of view of max speed, but the ability to accelerate.

Personally i would just think about the wing, with emphasis on achieving slow controlled flight and then max speed. On second thoughts, i would minimise cross sectional area and eliminating or reducing the draggy bits, as a given, for any model, as a normallity.

In my case the flying would be the Achilles heel, that I would be spending proportionally more time on it.

[David Davis]

Posted by Erfolg on 26/08/2018 21:28:08:...In my case the flying would be the Achilles heel, that I would be spending proportionally more time on it.

Same here. I can do a variation on the touch and goes, it's called touch and crash!

[David Davis]

Are you saying Erfolg that if I were to build a wing with an E 205 wing section then not only would no-one notice the difference but that the model would fly better?

I have had a look at the E205 wing section, But how do I get the image out of the computer and increase it so that it has achord of 24cms which is the chord of the original model. **LINK**

[Erfolg]

I am certainly not suggesting no one would notice, as even building a Clark Y some one could challenge, if it is Clark Y.

I have printed out a Clark Y and a E205 using profili software. I do have the co-ordinates for E205 although a pretty big table, arranged probably in such a way to make plotting easy. I do not seem to have Clark Y readily to hand, from memory the data is presented very differently, for graphical manual plotting, then finished with French Curves.

I am not sure that I would push the boundary to the extent, that I would deliberately use a E205 plot. Given that an allowance of 10% is allowed, I would be tempted (more than tempted) to use the 10% to recreate the features that probably make E205 so successful.

I plot using a piece of software called Profili, which was free, I am not sure it still is. It allows you to printed off to whatever chord you wish. For the version that is purchased you can design the whole wing I understand.

In my case i just draw in spar locations etc and draw out the wing.

Edited By Erfolg on 30/08/2018 15:33:21

[Nigel R]

I used a naca 2412 on my RM Trainer. Very docile. Stays up on low power levels a treat. It is essentially a Clark y with half the camber.

DD I am sure someone on here could print off a bunch of templates if you needed them and could not sort the it yourself.

Profili can print and is still free I believe. As erfolg says spars and so forth can be added manually to the end result.

[David Davis]

Please define "camber."

[Dickw]

Try Wikipedia for a decent explanation of camber - save a lot of typing here.

Dick

Edited By Dickw on 01/09/2018 09:29:32