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Member postings for Bob Cotsford

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

Thread: Prop size and efficiency
20/08/2019 10:32:42

To take an extreme example, using a big enough prop you could achieve the model's mass in thrust by moving a lot of air at say 10mph. Great acceleration from a standstill up to 10mph (ish) but unfortunately that also limts the model's flying speed to a similar figure. Moving a smaller mass of air at 60mph for the same static thrust (M*V*V and all that lark) may give lower acceleration but a practical achievable airspeed. Stalled props also have a big effect on the static figure as Martin pointed out.

Static thrust is a guide, but as has been said, pitch speed counts for a lot. It's all a compromise to suit the airframe and how you want to fly it.

Thread: Hospitalised, my own fault - but ?
19/08/2019 17:37:02

Once again Colin - OUCH!! Certainly not offended, I'm just thankfull I got away with past mishaps. I could have in your position many times but for the grace of your chosen deity.

Thread: Prop size and efficiency
19/08/2019 10:50:50

One feature missing is the air velocity as this would reflect on the sort of speed your airframe can achieve. I'd agree that this table gives a fair representation for lightly loaded airframes where speed (hence drag) isn't critical but when you want to go fast you need air to be displaced even faster (to account for that airframe drag). That takes low pitch and very high revs or higher pitch at lower revs. Which means a smaller prop/fan diameter to keep power within acheivable limits.

Thread: Cap1763
17/08/2019 12:13:43
Posted by john stones 1 on 16/08/2019 15:58:19:

Yep I recently had to renew my driving license, car goes for yearly MOT, there's billions spent on road marking, signs, policing, advertising, my insurance is way more than my toy plane insurance, and I've never made a claim in my life, road tax is £200 + a year, there are cameras, speed bumps and speed limits all over the place these days, and everyone on the roads is a terrible driver apart from me. wink

Always been the rebellious sort myself, but a simple online test n £16.50 a year (less than half the price of your BMFA) and how many throw that one into discussions ? Keep the noise down when you riot lads, and don't forget to wrap up warm. smiley

we really do need a 'like' button! Also, all of the above costing is relevant for each vehicle owned, that single £16.50 operators registration allows you to have as many aerial toys as you like, or even as many as your family/friends etc like as long as you take responsibility for their safe use.

Thread: Balsa skin on wing
16/08/2019 19:24:56

Same here, pva on the ribs and outline then paperback text books, magazines, sandbags - just make sure you have the wing jigged securely straight or with any washout so that it can't move under the weights.

Thread: Single servo ailerons
16/08/2019 14:56:31

I still say the biggest benefits of two servos for the average club modeller are that the force acts nearer the centre of the control surface, and that this can be accomplished with less slop than using either pushrods and belcranks or curved snakes, as we used to do.

Also incidentally, I've only ever had two cheapy servos fail in flight, one on a flying wing elevon (early Corona) and the other on floaty glider's rudder (DYS). Both proved interesting.

16/08/2019 14:45:16

RCPF - one servo or two has no effect on how differential acts, dif is dif. Only the modeller cares as two servos makes it easier to implement differential, either by offsetting the horns or by using two channels and implementing it at the transmitter.

Now how about twin servos through a Y lead (minimum 3 connectors) or twin channels (minimum 2 connectors)?

Incidentally, my WotsWot failure was with a Futaba 148, and while I controlled the roll I failed when it dropped into a spin/flick while trying to turn it on rudder. Ultimately pilot error - ambition exceed ability to quote Casey Stoner.

It turned out that the soldered joint between lead and pcb failed (fatigue due to insufficient support?)..

15/08/2019 18:35:53
Posted by Martin Harris on 15/08/2019 18:13:28:
Posted by Jon - Laser Engines on 15/08/2019 17:01:14:

Unless the servo fails at anything other than zero deflection, then its game over :D

...which is a pretty good reason for having a second servo!

Didn't help my WotsWot when it failed at less than 50' after take-off, maybe a better flyer could have saved it but I couldn't!

15/08/2019 18:04:16

On the point of aileron differential - not really needed on a moderately short winged sports model regardless of the wing section in my opinion (RIP BEB). On longer glider style wings or WW1 and similar bipes, yes. Sport monoplane, no!

Where it does come into it's own is precision aerobatics, if a club sports model has a bit of corkscrew to the roll no-one will really notice but for competition style flying the differential will probably be needed to get a truly axial roll.

Twin servos, imho the primary benefit is to move the point of actuation away from the end of the surface which helps reduce any twisting and can help fighting flutter.

If an aileron servo fails by running to the end of it's travel on a Wots Wot you need a new airframe.

Back to the OP - sorry!

14/08/2019 19:37:44

I find that I'm using expo more often in recent years, arthritic thumbs cant sense small pressures as well as they used to so I find small control movements harder to judge than I remember. Slow rolls ain't as smooth as they used to be! One day I'll try fitting some stronger springs on aileron and elevator.

On the question of mushy neutrals, IIRC sharp trailing edges and even better squared off ones are said to give a better response for small movements and rounded off give a reduced effect. I somehow doubt whetherthe average club flyer would really notice much difference, just like the ever increasing resolutions being advertised on modern radios.

Edited By Bob Cotsford on 14/08/2019 19:38:53

Thread: ASP prop driver stuck!
13/08/2019 17:55:15

Way back at the end of the last century I had a 65 four stroke which used the same collet system. On mine the prop driver split, also resulting in locking up the motor.

Edited By Bob Cotsford on 13/08/2019 17:56:08

Thread: Single servo ailerons
13/08/2019 15:25:24
Posted by Simon Chaddock on 13/08/2019 13:55:39:

RC Plane Flyer

Definitely more up than down!

A down going aileron for the same deflection creates more drag than an up going one so although the plane banks it tends to yaw outwards away from the turn or in other words a good start for a spin if you are flying too slowly!

With modern light weight but powerful servos it is much simpler to use a servo for each aileron and then use the transmitter functions to control the aileron travel, end stops and differential action. Most Tx will have a differential function already available.

Simon, for a new build I'd maybe agree about twin servos if the wings had been cut for twin servos but this is a completed model, please don't confuse the lad now! Having owned a kit built Acrowot I'll just say that in my opinion this model doesn't need a complicated aileron setup and mine certainly didn't need differential in order to handle nicely. Thousands flew perfectly well with just a humble 148 sitting in the middle of the wing driving both ailerons. I would suggest that 1/4-3/8" (7-10mm) each way is plent of aileron movement while getting used to the model.

13/08/2019 13:55:36

If you find it twitchy I don't think differential will be your answer. For reference you want more up than down if you do go with dif. Personally I'd try reducing the rates with say 70% as low rate on a switch or add maybe 30% expo, try one at a time and see which suits. Also make sure you have the expo going the right way, some systems use +ve for less sensitivity around neutral, some -ve.

Thread: Hangar 9 Ultra Stik 10cc
12/08/2019 23:43:22

I replaced the 1/4-20 threaded plastic inserts with good, British standard (wink ) 6mm captive nuts on my H9 US 10cc as one of the inserts stripped. It was tight to start with, the bolts were hard to turn even after easing the holes in the wing.

I may have pushed the boat out a bit far on the power front, a Power 52 motor that I had lying about, an FrSky Neuron 80A esc and 6S 4000mAhr Lipo giving me 1400W to play with.

Apart from the wing bolt issue I've found it to be an excellent piece of kit both in terms of fit/finish and flying.

Mix the flaps in with ailerons for roll rate at low speed, mix the ailerons with coupled flap/elevator for silly loops and full span flapperons, crow brakes to stop it dead - it's a brilliant model for safely playing with mixing. The only thing that lets it down in the air is the coupling in knife edge, it's just plain hard work. On the other hand it's extremely tolerant on balance points, mine came out in the middle of the acceptable range after adding an oz or two of lead in the nose and it's so stable I intend to take some of the lead out.

Ali suggests .46 power and up so 10cc petrol should be fine. If one that has similar power characteristics to the Evolution that I had than a 13*6 prop should do the job.


Edited By Bob Cotsford on 12/08/2019 23:45:59

Thread: Bangood Extra 330 power setups
09/08/2019 17:07:54

The advertised flying weight is 800gm and your setup is giving around 230W so if you have achieved the optimistic sound 2lb rtf weight your 110-120w/lb should give a reasonable performance.

Thread: Three bladed props (again).
09/08/2019 13:23:13

I can't say that I've noticed any great loss in speed using three blade props, primarily on larger four strokes turning 15*7 up to 16*8 props on biplanes such as a Super Skybolt and Christen Eagle. Did you follow the old rule of 1" less diameter or 1" less pitch?  That turbine like whine might be the tips overspeeding so maybe you need more pitch?

Edited By Bob Cotsford on 09/08/2019 13:24:21

Thread: Bangood Extra 330 power setups
09/08/2019 13:08:03

My similar sized Sukhoi from Hobbyking has a 3548 900kv iirc motor and 60A esc, I can't remember what size prop but the log tells me it pulled around 500W on a 3S 3000 pack.

Edited By Bob Cotsford on 09/08/2019 13:08:59

Thread: Best Dremel Clone
08/08/2019 10:03:20

I've got two Dremel 395 mains powered tools, both now need a strip down for maintenance of the front bearings. After only 20 and 25 years of use. Don't make them like they used to.

Who remembers the 12v brushed motor drills that we used to get palmed off with as 'modellers tools'? Anything currently available will be a 100 times better than one of those!

Thread: Solartex, any tips?
08/08/2019 09:58:15

Generally Solartex will need a higher setting on the iron than Solarfilm. If going over the edges with a hotter iron doesn't do it then it will need a wipe of Balsaloc or thin PVA around the edges of the airframe. I've got some red Solartex the kept coming loose, I have cured that with a judicious wipe of Balsaloc here and there. I'm using scraps to refurb a Baronette and am finding a smear of thin PVA just as good.

Thread: Hospitalised, my own fault - but ?
07/08/2019 21:44:22

A lucky escape Colin, as you say that could have been so much worse. With the power level that you report and two 'lips' with which to hold the model (in)securely you didn't stand much of a chance really.

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