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Member postings for PatMc

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

Thread: Thinking aloud about Spits...
15/09/2019 00:47:09
Posted by Peter Christy on 14/09/2019 08:58:01:

PatMc: I don't think anyone is suggesting raising the ailerons as a substitute for built in wash-out. However, presented with a finished wing that exhibits tip-stalling, the options are either 1) build another wing or 2) find an alternative solution.

One alternative is raising both ailerons slightly (doesn't work with strip ailerons, though!). This is simple, and often surprisingly effective.

Aerodynamics - like my own speciality, electronics - can be something of a black art, especially at model sizes.

Sometimes effective solutions are weirdly counter-intuitive. And at the end of the day, even if it doesn't work on a specific model, you are no worse off for trying it!

wink

--

Pete

Peter, it was once explained to me (by a retired aerodynamicist) that the main purpose of washout (in full size aircraft) was to maintain aileron authority at low airspeeds, tip stalling being a secondary consideration.
Also that raising the ailerons as a fix for lack of washout would help to maintain some aileron effectiveness near the stall. The downgoing aileron would still help raise the wing, the upgoing one would do nothing behind the separation point but because of the overall excess drag being produced by both wings there would be little if any adverse yaw. Overall result would be sluggish & limited but positive aileron control albeit still prone to provoking a spin.

A simple. if agricultural,retro substitute for washout is to progressively blunt the LE for the final 1/3 - 1/4 span to the tips. By blunt I mean still round. Because of the bluntness when the wings AoA is increased the point on the LE that the airflow splits moves down effectively retarding the increase at the tips compared to the root.
I've helped with this mod on models of newcomers to glider comps on a couple of occasions. Used as on field cures to thermal gliders that couldn't be adequately controlled during fast towline launches. Both models were R.E. only, had polyhedral with no washout & had the outer panels LE blunted. A strip of cover film was first removed from the affected LE's. then when blunted by sanding covered as a temp measure with adhesive tape.
I've also seen this method used on aerobatic slope soarers. I see no reason it can't be effective on power models, Rarely an acceptable fix for a scale model but fine for many sports models.

13/09/2019 22:14:23
Posted by Erfolg on 13/09/2019 22:03:14:

The effect of reflexing the ailerons is that the camber is changed, in such a way that looking at a lift drag graph, that at the angle of attack that the section sees, moves the stall to occur at a lower speed than the rest of the wing, assuming that the section in use is pretty standard.

The camber forward of the separation point hasn't changed at the stall & the camber behind the separation point is doing nothing. The effective AoA is the same as if the ailerons were not reflexed.

13/09/2019 21:36:58
Posted by Erfolg on 13/09/2019 21:02:39:

I did come across a paper on the design of the Spitfire, it read somewhat differently to much that has been written.

Is this the paper you are referring to ?

Edited By PatMc on 13/09/2019 21:38:59

13/09/2019 21:33:10
Posted by Erfolg on 13/09/2019 21:02:39:

One or two have mentioned an aspect that a Spitfire must have to fly well at lower speeds, and that is wash out as mentioned by Colin, the full size had it. It had it because it needed it. Reflexing the aileron as mentioned has a similar effect in that the camber is changed,

Reflexing the ailerons doesn't have the same effect as washout, at least not close to the stall when the effect is needed.
As the stall angle is approached the air over the wing uppersurface breaks away at a separation point that moves progressively further forward. This separation point will be forward of the ailerons before the stall is reached, therefore reflexed ailerons will be ineffective or virtually ineffective as washout.

Thread: What replacement for brushed
12/09/2019 23:48:36

I don't think that there are many inrunners of the correct dia that would be a straight drop in fit to replace the 500 brushed motor. However there are some "outrunners in a can" that meet the criteria, MVVS make some apparently very good quality but also very expensive.
OTOH Hobbyking Gliderdrive motors are similar in layout, more reasonably priced & have a good reputation.
This one is 1400 KV & should be good on 2s with something like a 10" or 11" prop, unfortunately it's not currently in stock.
My personal choice for the model would be this 960 KV version of the same motor which would be best on 3s with a 10" or 11" prop and it's in stock at the UK warehouse.
The power requirement can of course be regulated by choice of prop.

IIRC the first version Graupner Match that you have has a plastic (Plura?) fuselage. I've been able to upgrade a couple of same era Protech plastic fuselages from brushed 550/600 can motors to outrunners without too much difficulty.
refurbed unlimited.jpg
This is a re-furbed Protech Unlimited, at 63" span it's a bit smaller than the Match & I normally fly it like a powered slope soarer - 10 - 15 secs of WOT then aeros on the glide repeating the cycle from a fast pass at head height until the battery's low.
A model the size of the Match is more versatile, I'd use it for mixed aeros & thermal hunting depending on the potential lift conditions & my mood but I always only use full power for a fast, up to 30 secs, climb with any of my gliders followed by aeros or thermal hunting.

BTW I notice that HK's warehouse stock seems to be very low (I've been looking for motors & batteries), I presume it's a result of the civil unrest in the area.

 

 

Edited By PatMc on 12/09/2019 23:54:16

Thread: OS 40 four stroke, inverted?...
07/09/2019 15:14:01
Posted by Jon - Laser Engines on 06/09/2019 23:57:07:

i would say that the tank in pat's example is in fact too high. I would set it up as cuban suggests and essentially have the top of the tank level with the spray bar. This prevents a change in head as fuel is used.

Never had a single dead stick, or even poor engine run with this setup.
Virtually the only time the tank was in that position was when the model was flying S&L, which was only a small % of it's quite long life. wink 2

Thread: Covering with Solarfilm
06/09/2019 23:27:24

Geoff, I've never seen the point of using Balsaloc, since like a chain, the adhesive is only as good as it's weakest link.
I've also never had any more adhesion problems with silver Solarfilm than with any other colour. However I have found that silver Solarfilm does not have very good opacity. Joints & different shades of wood can sometimes be quite clearly seen through the film.

Thread: OS 40 four stroke, inverted?...
06/09/2019 23:09:47
Posted by Dave Cunnington on 06/09/2019 21:10:40:

So, without wishing to hijack this thread, am I correct in thinking that the fuel tank pipe to the carb should be at approx. the same level as the carb intake ?

I tried to fit an inverted 2 stroke OS and it kept flooding as I think the tank was too high above the carb level, I gave up and turned it upright, ran fine

Presumably if the pipe is a tad below the carb level, the engine should be able to suck in the fuel, aided of course by the pressure from the exhaust

Edited By Dave Cunnington on 06/09/2019 21:15:15

I always tried to position the fuel tank as per sketch in my previous post but the level isn't as critical as many people seem to imagine, after all when the nose is raised in flight the tank can be much lower than the carb. This is especially true of a 2 stroke where the carb is mounted ahead of the cylinder.

Re flooding - so long as you turn the engine over by hand a couple of turns before connecting the plug you should never flood a 2 stroke engine. In fact if you do this there's virtually no chance of causing a hydraulic lock, unlike an overprimed upright 2 stroke.

06/09/2019 22:53:58

To avoid fuel dribbling out of a model between flights when the tank has fuel above the jet level it's best to route the fuel line in an inverted U as per sketch. With the engine stopped open the throttle then raise the model's nose high enough to allow the fuel in the line to run back into the tank before parking the model.

Fuel tube U bend

The sketch is as used in my old Silhouette showing a 2 stroke engine but the same principle holds good for a 4 stroke.

Thread: Small engine tick-over
03/09/2019 16:57:22

Roger, you may find Peter Chinn's test report on the Enya 15 III of interest.

Thread: Incidence Meter/Gauge Measurement
01/09/2019 21:41:07

Geoff, the incidences of biplane wings & washout did occur to me but I've always managed quite easily with bits of string, a ruler & basic trigonometry, so dismissed them as any reason for the gadget.

Side thrust set using an incidence gadget ? I rarely use any, prefer the full size approach of rudder trim.

Down thrust, I don't believe in the term. I always set the horizontal thrust line relative to the wing incidence at the build stage.

Thread: undercamber wings
01/09/2019 21:12:12
Posted by John Bisset on 01/09/2019 20:59:29:

Thanks for that PatMc.

Is that a Schleicher ASK14 in your picture/avatar? I think there are only two or maybe three of those in the UK - nice machines.

Yes John, it's a 1/5 scale ASK14, photoshoped flying over Northumberland with me in the driving seat. wink 2

It's been an attic queen for a few years but hopefully the 19 glow engine will be replaced with a proper (electric) power source during a refurb this coming winter.

01/09/2019 21:02:52
Posted by Dougie Swan 1 on 01/09/2019 18:26:51:

Well that opened a can of worms, didn't it

My question was in regard to the dh60 I am building to the Mike Sun plans, the undercamber makes things a little more complicated and I was thinking of just making the ribs flat bottomed

Any thoughts on that idea?

Thanks for all the replies, they explained a lot

Dougie

Personally I'd go with the undercamber but I doubt that there would be any noticeable difference in performance or, for that matter, looks.

IMO whichever you're most comfortable with is the best choice.

01/09/2019 20:54:31

Posted by J D 8 on 01/09/2019 18:21:09:

All the aircraft and items you list are things that travel at supersonic speed's or nearly in the case of the Vulcan.

I think we are talking about the effect of undercamber in aircraft with speeds of under 100 mph.

There's no difference because of the speeds.

01/09/2019 20:54:17
Posted by J D 8 on 01/09/2019 16:23:03:

In the early days of full size aviation undercamber wings were seen as the way to as that is generally what birds have.

Some chap also took out a patent covering a number of undercamber sections and many early aircraft builders took it as gospel that this was the best type of section even though the thin section made for structural difficulty's.

By the middle of the Great War undercamber wings were past their sell by date, however most allied aircraft still had them. It was the Germans with Fokker who lead the way with more modern sections.

In the 70's with the return of lightweight more or less one speed aeroplanes [ microlights ] the undercamber wing reappeared and can also be seen on many parawings.

All Fokker WW1 aircraft had undercambered wings. However they a used thick section in order to be able to make them cantilever as Platz (the designer) realised that the external wires & struts created more drag than the thicker wing would. The German authorities & pilots were uncomfortable with the way that the wings flexed independently in flight so insisted on having inter-plane struts. But the rigging wires were the main drag producers anyway so a major reduction in drag was still achieved.

Actually Junkers produced cantilever thick winged aircraft before Fokker. They were all metal, low winged monoplanes with no struts or rigging wires but some of the drag reduction was sacrificed by the use of corrugated metal skinning. Like Fokker they also used an undercambered wing section.

Undercambered wings didn't reappear in the 1970's, the never disappeared at anytime after the Wright brothers.

01/09/2019 19:14:09
Posted by John Bisset on 01/09/2019 18:15:57:

So does that mean that the only benefit of undercamber is drag reduction rather than increase in lift ?

Not quite.
Thinning a section reduces drag, increasing it's camber (not undercamber) increases lift.
Combine the two and a bi-convex or flat bottomed section will remain convex over it's upper surface but could become concave on the lower surface - i.e. undercambered - depending on the new thickness & degree of camber.

A wing section is basically a symmetrical streamline profile bent around a camber, any resulting undercamber is incidental & of no aerodynamic significance.

Thread: Incidence Meter/Gauge Measurement
01/09/2019 17:57:41
Posted by Peter Jenkins on 01/09/2019 02:23:38:

Hi PatMc

Don't know about others, but I fly F3A aerobatics and it is an essential tool to ensure that you have the motor, wing and tail incidences set to the recommended set up. You do need to set a datum to measure from - often the flat fuselage part the canopy rests on. They do not come pre-set even if they are ARTF. Once you fly then there are probably minor tweaks required e.g. re-setting wing incidence to eliminate any trim you have had to dial in to fly wings level. Also essential when trimming the aircraft to go up vertically and down vertically although in the latter case, a mix with elevator to throttle might be the only solution. Motor thrust line can be a very important figure to ensure that motor goes back in the same position after any maintenance e.g. greasing gearboxes used on in-runners or for contra prop gearboxes.

Peter, thanks for the reply but not sure why it's regarded as an essential tool when you have to reset incidences anyway during the trim process.

Actually I was more interested in how & why the gadget is used for more typical types of sport & scale models.

Thread: undercamber wings
01/09/2019 17:44:26

Explanation in the edit.

01/09/2019 17:15:34

I repeat, undercamber has no aerodynamic significance.
Several fast jets have undercambered section over parts of their wings (Mirage III, Vulcan B2 & TSR 2 spring to mind), the BAC Tornado had "inverted undercambered" tailplane, propellers & turbines blades are undercambered.

If any non-symmetrical wing section is thinned sufficiently it will have undercamber but it might then require external bracing to be strong & stiff enough for normal flight. The external bracing would negate the gains in drag reduction made by thinning the section.   

 

 

Edited By PatMc on 01/09/2019 17:41:55

01/09/2019 15:43:05

Undercamber per se has no aerodynamic significance in the lift ability or the relative drag of a wing.

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