Simon Chaddock

Neddy Welcome If your Hawk Aky is old it is probably the original version 1. The Hawk Sky is till available but now as a V2 with 4 channels. It has a more powerful motor and is a bit heavier. The original was sold as a 3channel so did not have a rudder. I would not worry to much about the lack of rudder. A plane like the Hawk Sky with adequate dihedral will be perfectly controllable without one. I fly several powered glider type planes and lots of EDF jets with no rudder. So my suggestion would be to repair it to its original state and fly it. If you find the lack of a rudder is a problem then it is perfectly possible to fit one but be aware even a small servo and linkage will add weight to a plane that is quite modestly powered, I hope this helps

Trevor At the Hawker factory when they were making the Sea Hawk the bifurcated exhaust duct was apparently referred to as "the trousers" by shop floor!

Reprinted the inlet part of the duct meaning I can actually start on the airframe. I origianlly intended to build a half shell over the plan, insert the complete duct exactly half way into the shell and then complete the other side. This was the process I used on my series of Hawker EDFs. However I had second thoughts as the X3 duct is both a complex shape and made of LW-PLA. I felt the duct was just too delicate for this process so plan B is to add the formers to the duct one at a time and very carefully plank the complete fuselage section. This would allow each former to be made a better fit around the duct but would require plenty of "eye ball" to ensure no twist was built in. The process would still be fraught with difficulty but it would be a gentler on the duct. A modest start. The "exhaust" former. The 3mm Depron former is in two halves, cut together to make sure left and right side are identical with separate pieces inserted between the ducts. When the new inlet duct is added there will be 6 more to add.. The tail and nose sections will be built separately and together are actually longer than the centre section. They will only be glued on when the centre section is fully complete. This could all take some time.

Beautiful detailing, very nice indeed. I never can understand why nearly all scale WWI biplanes do not use a scale wing section. They look odd with "thick" section wings when they should use something like this. The usual excuse is a thin section has bad characteristics yet the Pup was known for its ease of handling. Not a criticism just an observation.

The other possibility is the balance plug into the charger. We are taking measuring 1/100 of a volt so just a slight resistance on one balance pin will seriously upset the chargers balance cycle at the very end of the charge. I have had exactly the same happen a few times with a couple of cells "stuck" at 4.19 and the charger showing zero charge tate. Give the balance plug a little jiggle and surprise, surprise after a few seconds the charger switches off indicating "full charge" with all cells showing 4.20. 😊

Erfolg Yes indeed about the X3. Perhaps the moral for aircraft designers is that engine development takes longer than the design & build of experimental airframes. Would the X3 have succeeded with more powerful engines? The example of the similar simple inlet geometry of the Fairey Delta 2 suggests that Mach 2 could have been achieved but with the extra weight of bigger engines its already record breaking take off and landing speeds would have been even higher. The fifth iteration of the duct for the bigger X3. It is 27.5" (700mm) long and in LW-PLA it weighs 25g just 1/3 that of the EDF.. . The 20% bigger airframe also means the scale inlets are now closer to the ideal 1.2 times the FSA. Hopefully any benefit in thrust this gives will compensate for the extra length of the duct. A test run using a substantial 3s. https://www.youtube.com/watch?v=czHn-uf2NS4 This test at least proved that the inlets could withstand the reduced pressure without relying on any support from the fuselage formers. It also passed my simple "blow the door shut" test I used on my other bifurcated duct foam planes. I will however have to reprint the whole inlet duct as I made a small but significant error. it is 10 mm too long! Nearly ready to make a start on actually building the hX3.

A scale model flying plane is most likely only scale its visible features. Nothing hidden inside is ever likely to be scale so why a scale internal linkage?

The origins of the of the BMFA is the SMAE. The Society of Model Aeronautical Engineers. When I fly a plane I am a model plane flyer. If I build a plane from a plan/kit I am an aero modeller. If I build a plane from scratch am I then a model aeronautical engineer? Do the semantics of a name really matter? Considering the amount of time and effort expended I am just a model plane nerd!

I agree Arthur but there is one caveat. The old free flight types were designed to be really stable and were modestly powered Add a modern LiPo/brushless and it is all to easy to over power them. They then become very sensitive to the application of excess throttle which will result in a rapid un commanded climb and possibly a stall. A novice pilot can then enter a form of pilot induced oscillation, PIO, but from rapid throttle movements rather than the elevator. Use just enough throttle to maintain height and the plane will then perform as originally intended. It can then be safely 'nudged' around the sky.

Furura57 My Libelle wing section was a actually a compromise and variable along the span. The scale wing section was remarkably thick particularly at the root. Note the very fine trailing edges! Such thick sections do not work well at small model sizes. however I had to use the scale section at the root as the cockpit canopy fitted around it and was a significant part of the Libelle's "look". My solution was to reduce the section quite quickly from the root to what I felt gave a better thickness chord ratio but retained the general proportions of the original. I actually built a "test" centre section to investigate the unusual thin balsa "surface" top and bottom wing spar flange tnat gave the maximum possible depth to the spar. The wing was built in 3 pieces although was a single piece when all glued together. The root section resting on the fuselage. Over this length the thick root changed to a more reasonable one. This section then remained to the tip but was reduced in proportion to the wing chord. The RH outer wing panel still under construction being tested for bending strength. Note the balsa spar flange tapers to nothing towards the tip. The final few centimetres of the wing simply relied on the foam skins for strength. Sorry there is not much detail but much was done 'by eye' although all the foam and balsa bits were always cut in left and right pairs so the final wing stood a chance of being aerodynamically the same L & R. The fuselage was entirely built using narrow planks as a 'half shell' over the plan. As with the wing the L & R former halves were cut together. Of course each plank had to cut/sanded to the correct shape to match its already in place neighbour before being glued in itself. With 20 odd planks per side a slow and labour intensive process but when carefully sanded it gave a good imitation of a glass fibre moulded structure. Worth all the effort? No way! I simply wanted to convince myself it could be done particularly as I had flown a full size Libelle! Maybe when I have got tired of making foam EDFs I might have another go.

I an sure you know what I am going to suggest as an alternative - thin sheet foam! Not a substitute for all balsa applications as it can range from hard to soft but thin sheet foam is a great deal cheaper. As it is several times lighter than the balsa typically used for wing skins it can be quite a bit thicker yet still bends well. This can have advantages in the building process. Now if you can combine that with "printed" wing ribs there can be no need for any balsa at all. My own rather extreme example of foam "construction". A 2.2m Glasflugel 201 Libelle. Made entirely from 3mm thick Depron sheet foam with a hard balsa/foam/hard balsa 'composite' spar. No carbon used anywhere and no 'skinning' material either just a light coat of acrylic paint Flew well but after several flights I lost sight of it in a thermal flying just that bit too far down wind! Never saw it again.

Peter Perhaps my explanation of how a plane's stability effects its flight is a bit different to yours.

In my many years of full size gliding I only went slope soaring once to gain the last part of my bronze certificate a 5 hour duration. 5 hours using flat field thermals in club gliders is not easy and gets expensive if you try and fail a few times. The other elements of the certificate were rather easier to achieve flat field thermal gliding. Soaring over a Scottish hill with no real safe out landing area except back at the airfield was a very different experience particularly flying so apparently low to the ground! Soaring does require rather a different set of skills and in some respects more discipline too but boring it is not - even after 5 hours going back and forth over the same bit of hill. I expect I am biased but as far as learning about wind and turbulence effects there is nothing quite like flying light powered RC planes especially those that glide passably well just in case there are any thermals about.😉

If a stable plane is in trim, flying at a constant speed and you apply more thrust (power) it will speed up. Its natural stability will cause it to climb to regain its "trim" flying speed. It will continue to climb as long as the higher power setting is maintained. The more power the steeper will be the climb. However like any 'balance' system a rapid application of significant power will cause the system to over shoot before the corrective forces have time to make an effect. Stability forces are very small compared to those from control surfaces. Thus applying too much power too quickly can lead to the plane stalling although there is nothing fundamentally wrong with the setup apart from the throttle application. Models that have free flight heritage will have stronger stability designed in and thus suffer more from throttle effects. The correct down thrust angle can mitigate this effect although technically it only does so at one speed and power setting. Down thrust will not exactly balance the aerodynamic stability effects at all air speeds and power settings. A compromise setting found by trial and error can still be very useful.

Using the inline exhaust duct which give virtually equal thrust on each nozzle this is what it would look like over the elevation of the "bigger" X3 Bigger means it will be 6ft long with a 2ft span! However efficient the 'inline' exhaust duct may be note how close the EDF ends up to the bottom of the fuselage. In addition the inlet duct seriously interferes with the concept of a one piece wing. Some sort of over and under spar(s) design will have to be included with the duct. According to the literature this EDF gives 850g thrust on a 4s. Although not quoted the efficiency of converting Watts into thrust (g/W) rises significantly at slower fan speeds. As the LiPo is going to be the single heaviest item of the whole plane it makes sense to make use of lower power efficiency and use a smaller lighter 3s LiPo. My guess is the EDF will give 550 g thrust on 3s. I have to expect to loose up to 40% with e ducting giving 330g thrust. My guess of the all up weight is 360g. It should fly but with not much to spare. Once I have the full duct completed I can test to see if the thrust it creates is at least in the right ball park. What ever the thrust is the X3 will have to be seriously light.😉

If the D box is solely to provide torsion resistance then a non grain surface material is preferred. Of course a well designed D box also contributes significantly to the bending resistance but the extra forces generated under bending may not simply require a thicker skin but just additional support to resist the skin buckling under compression and drawing in under tension. Typically achieved with closer spaced ribs. This brings on the question of what the ribs and skin are made of. To make matters worse a rib inside a D box is largely a support item whereas a 'free' rib has to resist bending forces as well. This suggests they should have different forms of construction. jd1 is quite right that the strength of the material at the outer surface of the wing has the greatest impact on its rigidity. However applying the same material all over the wing surface is not structurally ideal either. I do not claim to have a magic solution but this sort of analysis does indicate that using the same material throughout such a complex load bearing structure is not likely to give the best result. It quite reasonable to follow "It is proven to work so leave it be" but it is also wise to ask "Can it be done better?" particularly with the availability new materials.

There is an argument that using balsa for an entire airframe is a somewhat an inappropriate use of a single material. Balsa like any wood has a grain. Works well in simple tension and compression as in a spar but it is not so good when used as a thin skin to resist torsion. You need a grain free homogenous material for that to achieve the most efficient use. Once material cost comes into play the need to use the suitable material for the forces involved becomes ever more important. This of course does mean you have to know what the forces are and the properties of the material to be used. As a hobby I am willing to sacrifice my time and follow the principle "Simplicity of fabrication may have to take second place to component functionality" in order to reduce material cost Easier to say then do but then I am retired!

I know I am going a bit over the top but this picture shows what I mean about the EDF exhaust of a typical 50 mm EDF. You can see that the motor has a larger diameter at 26mm than the fan hub at 22mm. Si in theory the exhaust duct should be a bit larger than the fan case to maintain the FSA. Then at the end of the motor the duct suddenly becomes less tan the FSA. In this case to make matters worse there must be considerable turbulence from the bearing support lugs spinning at 30,00+ rpm. This must play havoc trying to achieve any sort of smooth airflow even if the duct is reduced to the FSA in a reason able manner. This is my attempt at a printed duct to go from the fan case to an FSA size duct. The duct fits over the EDF case and initially maintains the case diameter. It expands slightly over the motor area and then smoothly reduces to the true FSA diameter. Of course it would be better if the motor had a cone fitted of the rear end to reduce the turbulence and to maintain the FSA over the length of the duct reduction. This particular fan & motor are so well balanced I rather "chickened out" of attempting one. Does it make any difference? Very hard to tell as the benefit will be small but it does make me feel better for at least trying.

Zflyer Certainly a possibility but my concern would be that although the thrust may be equalised at each nozzle there might well be an overall loss in thrust. With this plane I am already struggling to arrive at a workable solution of thrust to weight to wing area. Even a potential loss in thrust I can do without. I think I may have a potential solution - make the plane 20% bigger! It would keep the same EDF but the increased fuselage dimensions would enable an "inline" duct to avoid the EDF and inlet duct "interfering" with a single piece wing. The extra wing area would be useful too. As all the "heavy" bits would be the same and the airframe still made from the same 3mm Depron the increase in weight should be a bit less than the increase in wing area. It could actually reduce the wing loading. compared to the first design. On paper at least a win/win!

I found an usual result whilst testing the latest duct. The RH nozzle gives less thrust than the LH one and by quite a margin about 12%. 🤕 The two halves of the bifurcated duct are mathematically identical and the EDF is exactly in the middle of the duct feeding the bifurcation. A 12% thrust difference is likely to be significant so I spent some time thinking about why this could happen. The only possibility I came up with is the airflow is spinning as it leaves the fan so the fact the ducts descend downwards at about 30 degrees be a benefit to one duct and a disadvantage to the other. To test this hypothesis I printed the ducts with only a side off set and none downward. Then added to the full duct to test. Holding the duct over a small digital scale showed the thrust from each exhaust was virtually identical to within a gram or two. However this layout would mean the EDF is 'inline' with the wings. It would likely require a substantial "over and under" spar going around the duct. Perfectly possible but it would be relatively heavy. Some more thinking required.

Yes It is a neat way to electrically isolate the ESC from the battery but there is still a risk that a hot hard worked LiPo can decide to "give up" as the heat soaks through. It all comes down to which of many risks you want to guard against. As far as a model is concerned it is only truly LiPo safe when there isn't one in it. 😉

I have a humble single heat Black and Decker gun that has done much paint stripping over the last 15 years and it still works fine for heat shrinking. The heating element of the 2 heat level version we had at the same time failed relatively quickly. I suspect because the two speeds of the motor did not correctly match the two settings of the heating element. More of an observation than a specific recommendation.

Rich There is a slope version of the even bigger 6 engine AN225 but its not by me. Big H? I presume you means the rugby post. I have hit it a couple of times in the past but the last named storm blew it down! It was made of steel and I suspect rust got to it. Apart from the many big trees around it and a clump in the middle there are now no other obstructions apart from it being very low lying, muddy and it can and does flood!

PDB Yes in LE-PLA and in vase mode. I actually print it with the exhaust outlet on the bed and with a 3mm brim to ensure it sticks to the bed and to provide some stiffness to keep the nozzle circular when it is lifted. Erfolg It is generally agreed that the published thrust figures for EDFs relate to it being tested in "free air", with a bell mouth and no ducting at all. There is an argument to suggest that the inlet ducting is usually bigger than the fan swept area so has slower moving air in it thus the losses are lower. It follows that for the most efficient EDF installation it should be placed right at the back with no exhaust duct at all! Difficult to prove as any benefit is likely to be quite small bit it certainly works. The EDF at the back of my Fairey Delta 2. In this case the weight of the EDF is an advantage as it meant the battery had to go ahead of the twin inlets meaning no inlet duct could be of the required 1.2 times the FSA and absolutely straight up to the intakes in fact it was built around a piece of gutter down pipe which was only removed when sufficient fuselage outer skin was in place. It used a lower power 6 blade 55mm EDF so overall efficiency was a concern.

I mad a boo boo! I miscalculated the correct area of the exhausts. I had intended each of the bifurcated exhausts would be 50% of the FSA except for the final "nozzle" which would reduce to 45% FSA but after printing it I judges it looke too small so I redid the calculations only to find each duct was nearly 50% too small. All the exhaust duct had to be reprinted The old and revised. The correct is on the right. A short video of the first test. https://www.youtube.com/watch?v=RqqtZASn8xI So far so good. At least it blew the door shut!