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Member postings for Dave Hess

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

Thread: Radio Link
11/03/2019 23:45:07

I think if you want a cheap radio, you should have a look at the Jumper T12. I've been using one for quite a while now and I prefer it to my Taranis. Hobbyking now have the new version with hall gimbals for $110 or my version for £68

Thread: Fresh petrol in a lawnmower every season?
14/02/2019 10:43:11

I recently recommissioned a motorbike that I hadn't used for 14 years. There was about a gallon of petrol still in the tank. All I did was charge the battery and press the starter. It worked perfectly, though it stank of old petrol. Even the MOT tester commented on the smell. After it passed the MOT, I topped up the tank with fresh petrol and went on a 300 mile tour of Yorkshire. In summary, I'd say that petrol would have to be substantially evaporated before you get any detrimental affect. It can hardly evaporate if it's in an enclosed container like a fuel tank. I don’t think there's even a breather hole in my lawnmower, so even less chance of evaporation.

Thread: Are all watts created equal?
14/02/2019 10:30:58

You need to make sure you're dealing with output power and not input power. When you say that the motor produces 1300w, where does that come from? Efficiency ranges from about 50% to 80% depending on the motor's speed, Kv, voltage, propeller and things like that, so if 1300w is consumed power (amps x volts), output power could be anything from 650w to 1040w. That's motor output power. Actual motive power will be less due to propeeler efficiency, which depends on things like type, quality, size, pitch, and rotation speed.

I would say that the best way to evaluate an electric motor is to measure its static thrust while measuring the voltage and current with a wattmeter. All you need is cheap luggage scales that cost about £2 and a wattmeter for £8. Those measurements will tell you the thrust per watt, which is a good measure of efficiency and power, which will tell you more about what your motor can do than any spec sheet or size chart.

Thread: I Charger + blue smoke = bin
02/02/2019 00:26:17

Smoke ftoma charger normally means that you conne ted your battery the wrong way round, so make sure you check your battery before connecting it to anything else.

Thread: Another scam? Sinister this time
01/02/2019 12:51:29

Anti-scammers, like Jim Browning, do more to disrupt the them than the police. It might be worth seeing if you can get the details to him. He can infiltrate their computers and systems to see what they're up to. Interestingly, he normally gives everything about them to the police, but they never seem to be interested. In one case, he gave their names, adresses, bank details, photos and everything, butheard nothing back. Here's an example of what he does:

Thread: Advice please on balancing LiPos
30/01/2019 09:20:02

The common iMax type chargers balance the cells by charging the pack to a fixed voltage then the charge is switched off and bleed resistors are opened to drain down the higher voltage ones, then the charging is switched back on to bring the pack voltage back up again. That process is repeated until all cell voltages are close enough to each other. There is a bleed resistor on each cell channel. The voltage difference on each channel is read by a microprocessor. The microprocessor can open each bleed resistor separately via transistors on each channel.

There are other chargers that manage and charge each channel separately. In effect, you have X number of single cell chargers in one device. They charge through the balance leads and dont use or need the main power leads.They have the advantage of quicker and more reliable balancing, but have the downside that you can't see if you have a weak or damaged cell unless you buy one of the expensive ones that show how much charge has gone into each cell.

Thread: Fried battery
22/01/2019 19:18:50

If you can solder, repair isn't too difficult. As I said above, the strips that join the cells act as fuses abd vaporise when you short the battery. Any sensible DIY person should be able to repair one as long as there's enough of the tab left to solder to. I've repaired several batteries like that. It's no more difficult than soldering a connector to a lipo, which some of us do to nearly every pack.

I used to use lipos for electric bikes up to 16S, and when connecting the harnesses, it was easy to get an accidental short, which normally resulted in the problem you have.There's very little chance of fire. If you do short anything, the contact area gets vaporised so immediately disconnect it.

Obviously, ham-fisted persons or dunderheads should leave well alone.

22/01/2019 12:13:12

If it still works, it'll be OK. If it doesn't, it probably means that one of the inter-cell conne tions has vaporised. You can sometimes repair them by pealing off all the outside layers to expose them, then solder some thick copper wire across what's left.

You can sometimes do creative things, like convert a 4S into a 3S and 1s or two 2Ss.

Thread: Which Watt Meter
21/01/2019 12:15:58

It's true that current is the biggest cause of heat and measuring the current gives a good idea of whether you'll have a problem with over-heating as long as you know the motor's current rating, but that's not the whole story. Many of the rules and formulae given above are over-simplified. They're correct for a resistor, but not for a motor because the coils are inductors. It's the impedence that resists the applied voltage and the impedence is the sum of the resistance and the reactance. The reactance changes with the motor's speed, which is why the current changes with speed, and it's also why the efficiency is dependent on speed.

It's very important that people understand the relationship between speed and efficiency because speed not only affects the way the motor heats up but also the rate at which you waste your battery during flight.

If you watch this guy's videos, you can get a much better idea of what's going on. In some of his earlier ones, he explains how the graphs work because there's a lot on them and it'll probably take a while to figure them out. The coloured zones are the efficiency zones with blue as good and red as bad. The black line that ramps down from top left to bottom right represents full throttle for each propeller, which is where we need to look for over-heating. If you get stuck, I can probably explain it. Once you have them figured, they're dead simple, and you should be enlightened:

Edited By Dave Hess on 21/01/2019 12:20:37

20/01/2019 12:23:43
Posted by Attilio Rausse on 20/01/2019 10:31:29:

Dave don't think your statement re efficiency makes any sense, its just too many amps that will kill the motor.🤣🤣

It's basic physics - the law of conservation of energy. The energy you put in has to go somewhere. If you have an efficiency of 60%, it means that 60% of the energy going in is converted to motive power. The rest will be converted to heat and a very smal amount to sound. The amount of power is the rate of that conversion, so at 60% efficiency and 100W on the wattmeter, roughly 60w motive power and 40w heat. The ESC is not 100% efficient either, so a small proportion, something like 5% to 10% won't reach the motor an will be converted to heat in the ESC, which is why they can burn too if you put to much power through them.

The current that you measure on an ameter or wattmeter is not the current going through the motor, so it's beter to consider the power going through it. The current is split into three phases and pulsed at a high frequency by the ESC, so it becomes much more complicated to try and explain what happens in the motor. That's why I prefer to think about power and efficiency, though for some strange reason, the motor manufacturers and testers very rarely give us the full throttle speed vs efficiency tables or graphs, which are really useful to see whether the motor is likely to burn.

Sorry for a not simple answer. Unfortunately, the closer you look the harder it gets, though if you look at it from the principle of conservation of energy, it is simple.

20/01/2019 01:06:38

I've used most of the cheap Chinese wattmeters that you can get from Ebay from £8 to £13. They all work perfectly. The thee button type are a bit more expensive, but have more functions. For aeroplane motors watts aren't as important as amps. Wh/Ah are useful for checking battery capacity..

Don't forget that any watts or amps you measure are power in, not out, and the efficiency depends on the speed the motor spins. It can be anything from about 30% to 80%, and low current with low efficiency can damage the motor much quicker than high amps with high efficiency, e.g. 50 amps at 80% efficiency gives 10 amps burning power and 40 amps at 60% gives 16 amps burning power.

Edited By Dave Hess on 20/01/2019 01:15:06

Thread: Trouble with squirrels
14/01/2019 21:40:17

In case you haven't seen it:

Thread: Motorcycles and model flying
14/01/2019 20:43:06
Posted by Bob Cotsford on 14/01/2019 19:45:43:

Strangely the supposed bullet-proof Honda CBR600 (FX model) that I had was prone to occasionally cutting out for no apparent reason, I never got to the bottom of that one

 

Many Hondas of that era had trouble with the contacts on the fuel pump burning, so much so that there's an after-market teplacement kit for them. Symptoms are random cutting out. The same pump was used on many different models:

**LINK**

Edited By Dave Hess on 14/01/2019 20:44:08

14/01/2019 20:33:10
Posted by Cuban8 on 14/01/2019 13:49:43:

Thanks gents, food for thought. Clearly, an exposed airframe is a non starter, so a simple, practical container that the model (after dismantling) can fit into, mounted on the pillion/rack will have to be made up . I'm only talking about a small foamy type model, perhaps a flying wing, for a couple of hours fun, rather than my big stuff.

Or......this could be my big chance to get into drones - they come in nice strong boxes from what I've seen, and will easily strap to the pillion...........................I jest wink

Haven't got the bike yet, rather fancy the new T-100 Bonneville. Will let you know if anything comes of it.

Edited By Cuban8 on 14/01/2019 13:50:59

Nice bike, but it's going to be expensive when you drop it when trying to park it on the flying field mud or gravel unless you have a hard carpark.

Also, you'll find that the pillion is surprisingly small when you want to strap anything to it. You'll need an old-school type rack. The modern ones are really crappy, mainly because they're tiny.

Having said all that, you'll most likely give it up as a bad idea like I did. I can't drive at the moment, so I'd love to be able to use my motorbike, but it's just not practical, even though I have a lot of very small planes., though if I was really serious about it, I'm sure I could find or make something that would work.

14/01/2019 11:25:07

I tried it. I took one of these, figuring that it was small enough to fit on the seat behind me on my Deauville, which has a fairing that should reduce the wind on it:

**LINK**

It took me ages to get it held down tight enough to hold it, but not too tight to damage it. I drove sedately to the flying field, but when I got there, the wing was broken off by the wind force.

I would conclude that it's a complete non-starter to use a motorbike for transporting your plane to the field. Sure there are lots of theoretical solutions, but it'ss simply too inconvenient and the risk of damage is too high. The nearest equivalent solution that I can think of, if you can't use a car, would be an elctric bicycle and trailer. An electric bicycle is my preferred means of transport for journeys up to around 20 miles (round trip). The lower speed is unlikely to do wind damage and a dog trailer will allow you to more or less throw your plane in as long as it has detachable wings.

If anybody needs any advice about electric bikes, I have a lot of knowledge and experience in that area, which I'm happy to share.

Thread: My last foam board build before the balsa journey
11/01/2019 10:15:28

Here's how I stffened up the tail and control surfaces on my FT Edge: 3/16" square balsa strips are exactly the same thickness as that foam-board, so I removed 5mm from all the edges that I wanted stiffened and glued on strips of the balsa. It makes a big difference, looks very neat, glues nicely with hot-melt and costs hardly anything.

Thread: Nutball
06/01/2019 16:38:28
Posted by Nigel R on 06/01/2019 14:55:23:
Nice work on the Hots Dave. Foam board is ideal for angular shapes. Is it a proper airfoil?

Yes, much easier than balsa. The wing section is symmetric. I made a test piece that you can see in front of my toolbox back left.

06/01/2019 11:13:36

You can buy Flitetest waterproof paper covered foamboard in the UK for £2.50 per sheet. It's an excellent material for building planes like the Nutball - and anything else for that matter. Here's my Hots 40 coming on nicely. A plane like this took me only a few hours to make.pre finish.jpg

05/01/2019 16:57:48

There are instructions and free plans here if anybody else wants to build one:

**LINK**

loads of other free plans for nice-flying foamboard planes here:

https://forum.flitetest.com/index.php?threads/sp0nz-plans-index.17136/

Edited By Dave Hess on 05/01/2019 17:02:50

Thread: Ooooh Errrr! Don't know that happened here
04/01/2019 00:24:08

Here's my thoughts for what they're worth. It's always possible to get a loose pin, but two at a time would be pretty unusual. I can't see that being the cause. I would say that too much current has been through those wires.

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