Peter Jenkins

Glad you found the explanation helpful Mark. Don't bother with online Re calculators. A good approximation is to use the air density divided by viscosity figures at 15 C and that gives a figure of 67,680 approx. You need wing chord expressed in metres and speed in metres per second. Then just multiply them together to give a close approximation to the Re you are trying to calculate.

"If it ain't broke don't fix" it is a good maxim unless you are searching for ultimate performance.

Rule of thumb. The first 30 degrees of flap generate mostly lift with a little drag. After that, the lift increase is relatively small but the drag increases all the time as the flaps are lowered. The Spitfire had around 80 deg of flap with nothing between fully up and fully down. Apart from the occasion they had to take off from aircraft carriers when on the mission to relieve Malta. Then, the flaps were lowered, some brave/stupid person held some wooden spaceers and the flaps were realised till they met the spacers! After take off helped by the bit of flap, the flaps were cycled so that the wooden spacers dropped out. On some of today's airliners, the flaps are generally split into 2 or more slotted flaps and they can be be lowered as much as 110 degs for the last flap section. Needless to say, there is a huge amount of lift and drag generated when the flaps are fully lowered.

I'm afraid so JD8. I should bury that one good and deep!

Hi Mark I regret to say that JD8's is (sorry JD8) a complete red herring. The size of air molecules is just so many orders of magnitude smaller than our every day world that having a model even 100 times smaller (that's 2 orders of magnitude or 10 squared) will not matter in the slightest. Your issue seems to be whether Re (Reynolds Number) is the cause of the difference between the 2 different sizes of your Cub. (BTW in my younger days I used to fly full size gliders and power and one of my favourite aircraft was the Piper Cub - this one with the unusual power of 135 hp.) The answer is absolutely. To determine the Re for both then you need to use the Re calculation of: Air density x characteristic velocity x charcteristic length divided by air viscosity. The figures used must be in the same measurement system of course. If we use an airspeed of 40 mph or 17.9 m/s, and wing chord of .28 m we can plug that into the above calculation (using Air density at sea level and 15 C is 1.225 kg/cu mtr and air viscosity at 15C is 1.81 x 10 to -5 kg/(m.s).) We get an Re of 3.39 x 10 to +5 or 339,000. For the larger model at the same speed we get an Re of 4.85 x 10 to +5 or 485,000. Generally, the cutover point is an Re.of 500,000 for the transition from laminar to turbulent flow although some use 800,000. A B737 in the approach will be operating at an Re of 1.5 million and in the cruise at about 5 million. I should point out this is a very simplified discussion. Mass/inertia also changes the way in which our models feel when we fly them. As you have found, Re has a profound effect on how our smaller models fly. Large models fly better and are easier to fly (leaving gyros out of it for this discussion) than small models. The problem we have is how the boundary layer works at different Re. What happens in the boundary layer is the deceleration of free stream air to stationary on the actual wing surface. In laminar flow, the speed gradient is high and the boundary layer thin although it does grow in depth the further along the wing you go. A turbulent boundary layer has mixing between the layers and a much thicker boundary layer results. This increases the drag of not just the wing but all parts of the airframe compared with the laminar flow condition. As I mentioned earlier, a laminar flow will break away once it encounters an adverse pressure gradient i.e. the flow starts to slow down and the dynamic pressure starts to increase. Generally, for a wing this will be at or close to its maximum thickness point often between 1/3 and 1/4 of the chord. Hence my point that only a small percentage of the wing, or tail plane, works in the laminar flow regime. That's why small models have to fly faster to generate the lift required to keep them in the air. Also why their controls feel barely in control! A turbulent boundary layer is not affected by the adverse pressure gradient and remains attached to the wing till the TE. Of course, when the stalling angle of attack is reached then even the turbulent boundary layer breaks away causing the loss of lift. I apologise for reducing a very complex situation to a relatively simple explanation and taking some liberties in the process.

Good point.

Well, it's showing a fair bit of use. As I say, mine was a new in box, unused but don't know how long he'd had it. It certainly looked unused and, like all OS engines it was dead easy to start and set up. Indeed, after the first run of the day, choke, attach plug heater and a back flip gets it going. I suppose for £50 it's worth a gamble. It has some damage to the carb opening. Sorry just seen the bid has closed but you can contact them and make sn offer I expect. He says its got good compressikn so that's encouraging. I'd give it a go.

Problem with most petrols is that they are heavier than the equivalent glow (ignition and extra battery for that) don't have the power output plus they are usually much noisier unless you spend as much as the engine on a decent silencer. The OS91FX is sadly discontinued but examples are still available. Mine was second hand but NIB. It is a bored and stroked OS61FX and has the same crankcase with the same bolt holes. I just took the 61 out and replaced it with the 91 and didn't see any change in CG position. The 91 comes with an extra section for the silencer and evdn with the second smallest prop that is recommemded was pretty quiet. For its size, the Wots Wot is very heavy and not as good as an IC Wot 4 at aerobatics. I have Wot 4 with an Irvine 53 and a throttle pipe and that really is vertically unlimited. It also has better directional stability compared with the bipe. I fly both and they are great fun but I prefer the Wot 4. I wouldn't describe either as aerodynamically pure but by using mixes you can overcome the worst of their habits. Both are knocked into a cocked hat by the Miss Wind referred to in one of the posts above - but then that is a specialist aerobatic aircraft.

I fitted an OS91FX to the tired old example I acquired. I tried an OS61FX but that ran out of puff in the vertical so you could only do small loops. The OS91FX while not quite having the grunt to give the Wots Wot an unlimited performance does quite well. You have a throttle Edgeflyer so don't fall into the trap of flying a low powered model. Aerobatics are much easier if you have the power available to fly smooth round manoeuvres. I fly mine at half throttle on S&L and use the power in the vertical to make nice looking round loops. If you fly it flat out all the time it still won't be a screamer as the WW has so much drag!

Bas, if you are a complete beginner to model aircraft building and flying my short answer would be "Yes". Without knowing more about your capabilities as both a builder and a pilot it's an impossible question to answer. If you are still in the very early stages of learning to fly, then definitely not a good thing to have to deal with a potentially seriously out of trim model.

Always felt that C/L was a great way into aeromodelling for youngsters. It's relatively cheap, and these days uses electric motors so overcomes the problem of the snarling 2 stroke noise. It can be flown in small spaces and gives a huge amount of fun for the money involved. It's also the only type of model flying where you are connected to the controls directly. As you can imagine, I cut my teeth on C/L as an impecunious teenager. I could get to the flying field with up to 3 models on my push bike!

There is a difference between being legal to fly a model aircraft and taking a BMFA Achievement Scheme Test. Andy Symons has stated that if you don't show a pass of the BMFA's RCC then you will be asked the 5 mandatory questions on an Achievement Scheme Test. That is because the CAA competency test does not require knowledge of the Article 16 exemption which is specifically for the BMFA, LMA and the other 2 bodies. For example, Article 16 allows members of the 4 organisations covered to fly a model aircraft above 400 ft provided the take off weight does not exceed 7.5 kg and does not have more than 1 rotor provided you are not infringing any airspace restriction. The CAA test does not cover Article 16 but the BMFA's RCC does and is designed for model aircraft pilots as opposed to other UAS operators.

As I understand it, if you want to take an Achievement Scheme Test you need to pass the BMFA's Competency Test otherwise you will need to answer the 5 mandatory questions. The reason is that the CAA's competency test does not cover Article 16.

If you aren't noise limited you could prop down further to get the revs up. Max power is around 15,000 rpm so running at 10,000 rpm ish on a 10x6/7 gives scope for trying a 9x6/7. Aiming for 12 -13 k rpm may solve your problem.

I think that's too big a prop. I have an Irvine Tutor (also bought second hand) with sn Irvine 40. That's turning a 10x6 prop. Acceleration is leisurely but perfectly acceptable take off performance from the smaller of my two clubs where we have about 200 m but a 20 ft tree line to clear at one end. My irvine 53 used to have 12x6 on it till I put a throttle pipe on it and now turns an 11x9 giving my Wot 4 unlimited vertical performance. Try your 46 on a 10x7 or 11x6. I'm sure you'll get much more power out of it without causing a noise ptoblem.

The other thing to watch out for if your son goes for aerobatics is to avoid using cotton control lines as after a few loops the friction builds up and the controls don't work too smoothly. Metal lines, the used to be called Laystrait I think, don't get this problem.

Thinking back 50 years to my C/L days, I seem to remember the CG needed to be in front ofvthe bellcrank pivot point. Makes sense as that helps to keep the nose of the model pointing outwards thus msintaining line tension. My last C/L model was a combat Wing called Early Bird by Richard Wilkins. My EB had a PAW 249 in it whereas the serious combat pilots had Olivers. I never flew combat but the EB was fantastic for aerobatics. The line pull was so great you could fly it without looking at it! I remember doing mid height figs of 8 behind me while shouting at a mother to get her toddler out of the circle directly opposite to where the EB was. Happy days.

Thanks JD8.

Sorry Simon, you have described the transition point that is when the laminar boundary layer flow separates (separation bubble) and then reattaches as a turbulent boundary layer. Generally speaking if the flow remains laminar that is fine in a pressure gradient where pressure is reducing due to the flow speed increasing. When a laminar boundary layer encounters the opposite, a slow down in flow speed and an increase in pressure gradient then it breaks away completely but even that is not a stall. It merely reflects the fact that at a non-dimensional number called the Reynolds Number that is on the low side flow will be laminar whereas above a certain number the flow transitions from laminar to turbulent boundary layer and the turbulent boundary layer will cope with the adverse pressure gradient that causes the laminar to break away. Small scale models operate in the laminar flow regime and so only the front part of the wing, usually up to the thickest part of the wing, generates lift. The same goes for the tail plane which is why small scale aircraft tend to have larger than scale tail feathers otherwise they don't fly well at all! Larger scale models cross into the turbulent boundary layer territory and all of their wing and tail plane work hence that is why big models fly far better than small ones. A stall occurs when the wing is at an angle of attack when the boundary layer, laminar or turbulent, will not stay attached and the flow breaks away and that is called the stall. It will always occur for a specific aerofoil at the same angle regardless of speed. That's why there are high speed stalls where the aircraft is a very long way from its "stalling speed" and yet it will stall if the pilot increases the AoA to the stalling angle. In full size aviation, a stall speed is arrived at by reducing airspeed at 1 knot per second to give the unaccelerated stalling speed. A high speed stall can occur at any speed provided the elevator is powerful enough to increase the wing's AoA to the stalling AoA. Again, in full size, the pilot is usually warned of this by buffet as the stalling AoA is approached and recovery merely requires a slight reduction the force they are pulling on the stick. Hence the term in air combat of pulling to the buffet which is where you are getting the maximum turning performance of that aircraft. As regards the OP's question then, it depends on the Reynolds number that you are operating at. Reynolds number (Re) is the relationship between inertial and viscous forces - to give it it's formal designation. It is calculated by dividing (air density, air speed, a typical length e.g wing chord) by (air viscosity). It's generally a large number with the divide between laminar and turbulent being around an Re of about 500,000. The wing section and smoothness (or roughness) of the wing surface also affects the answer. As an example of this, a full size glass fibre glider had a stalling speed of 38 knots when the wing was dry. If you flew through a rain shower, the rain droplets on the wing at the leading edge would cause flow breakaway in the laminar flow part of the wing and stalling speed went up to 50 knots! Unfortunately, aerodynamics is very complex so we generally operate to what was done in the early days of aviation and learn what works, or doesn't work, the hard way and build up our approach accordingly.

That is correct Nigel. When you are in level flight at the same speed as before, the fuselage will be flying tail high the same amount you have increase the incidence. The wing will then be at the same angle of attack generating the amount of lift needed to support the aircraft's weight. Raising the tail, or lowering the nose amount to the same thing and the motor thrust line is lowered by the fuselage adopting a more nose down position.

Hi Nigel I would agree that it sounds odd. Incidence is a fixed setting but what matters when an aircraft is flying is the angle of attack i.e. the angle between the wing and the air and that is controlled by the elevator. Regardless of the angle of incidence, for a given airspeed and aircraft weight the angle of attack will always be the same as the lift force must support the aircraft weight and AoA is controlled by the elevator position. With a greater angle of incidence, you will then need to raise the aircraft's tail to regain the AoA that establishes level flight. That is why increasing the angle of incidence is analogous to increasing down thrust as for the same AoA the nose is now pointing down a bit more hence the increased down thrust.

Hi PatMc I did read the text before you deleted it and I hpld my hand up for getting my recommendation the wrong way round. So, for a low wing aircraft,ike the Ruckus, an alternative to applying more down thrust is to increase the wing incidence. This would require the TE to be packed. For the same flying speed, you would need more down elevator trim to reach the same angle of attack as before i.e. the aircraft would be flying more nose down than before and hence more down thrust. Sorry for missing your point first time. Nigel Heather - please note the above.

Well PatMc Motor down/up thrust is set against a fuselage datum. So is incidence. If you reduce incidence it has the same effect as reducing motor down thrust or increasing up thrust and vice versa. If you disagree with that then draw the picture out and you will see the relationship between the two. To be strict about this, I should say for the same airspeed you will need to generate the same lift force for balanced flight. Reducing incidence means that for the same lift force at the same airspeed your wing needs to adopt the same angle of attack as before to generate the lift to balance the weight. So, the fuselage datum line will have to be tipped nose up and we do that by using some elevator up trim. Now if you look at the motor thrust line which is tied to the fuselage datum that has been pitched up .i.e. the up thrust has effectively been increased slightly compared with where it had been.

I completely understand Chris but as an advanced driver your observation should be much better than average. All the examiners are ex Police Class 1 Drivers. I suspect that the drivers to whom you refer would pull out with indicating without seeing you anyway. The point about advanced driving is that you are given a one hour test which covers all sorts of roads by an expert who judges whether you are up to the award or not. I'm sure they fail quite a few but I don't know. Many drivers have the view that by putting their indicator on that gives them the right to move out either to overtake or turn across a road but they haven't seen the traffic that is fast approaching. I'm talking about observation and not just a brief glance or a check after they have moved and often before they have made an indication and usually without looking first.

Having done the IAM test many years ago, the book says that rather than just indicating think about who you are intending to warn of your intentions. If there is no one who will benefit from seeing you indicate, then don't. As an example, if you are turning right and there is no one to warn then don't indicate. If, however, you are going to overtake someone and there is no one else on the road, the person being overtaken deserves to know that is your intention so indicate in this case. I never saw written down what Leccy was told. Perhaps he was doing his own thing in his instruction. I see plenty of people today who seem to think everyone knows where they are going so there is no need to signal - that, of course, is a different issue!