Simon Chaddock
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Posts posted by Simon Chaddock
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Witterings
To answer your question why the amps through a bulb falls away as the voltage of the battery reduces I am afraid you have got to go back to a basic physics formula.
Watts = Volts x Amps
So a 50W bulb at 24V will pass 2.1A
But this only applies at 24V. The bulb is 50W only at that voltage
In basic terms the Wattage of a bulb is proportional to the applied voltage.
Halve the voltage and it will only give half the wattage (approximately)
So at 12V the same bulb will only pass 1A and will glow pretty dimly.
As suggested measurement with a suitable multimeter is the best way to know exactly what the amps are.
I still find is surprising that you cannot set the discharge rate with your charger. How does it know what capacity battery you are connecting to it?
A 5000mAh LiPo can safely handle 10 times the discharge rate of a 1500mAh one. Apply too high a discharge rate to a LiPo it will get hot, be damaged and could burst into flames.
My humble charger is limited internally to a maximum discharge rate of just a 0.5A but you can still adjust that rate to as low as 0.1 A if you are discharging a really tiny LiPo.
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I glue everything in but the battery!
I find a well thought out and efficient servo linkage means the servos last a long time, usually longer than between major crash rebuilds.
An example.
The aileron servos on my sub 250g Depron Super Cub built in 2013 have never been changed and they are driven by super cheap 3.7g servos! It is still flown on clam days.
The servos are buried in the wing with only the tip of the servo horn showing. Mechanical aileron differential is included with the servo horn angled forward about 40 degrees.
Light and simple no adjustment is required as the link rods are made from a paper clip and are bent to be an exact fit. If it is not quite right then make another. It is only the cost of a paper clip! 😉
Even more important there is a very free moving and virtually invisible top tape hinge.
In this installation the servos are never worked hard.
Just saying.
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Erfolg
If the inlet has the same area as the Fan Swept Area I would agree the maximum velocity the fan can generate is ultimately restricted.
However in my XF108 the inlet and all the ducting to the fan is 1.2 times the fan gross area or 1.4 times the true Fan Swept Area. With this configuration the fan can increase the exhaust velocity by at least 1..4 or even more at the 90% FSA nozzle before the inlet is creating any velocity restriction. In this case a cheat hole has very little benefit.
I dd agree that if the inlet & duct is below this sort dimension a cheat hole will reduce the inlet choke effect and improve the EDF performance.
It one reason I use relatively small EDFs and light airframes so that true scale inlets and exhausts are big enough.
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Surely the charger allows you to set the discharge rate just as it does for the charge rate.
You wont go far wrong with a 1c discharge rate.
The really important bit is the voltage cut off level. Too low a voltage will likely damage a LiPo, particularly if the storage is long term.
The object of a storage charge is to reduce the chemical stress on the cell and to limit the total energy stored to reduce the likely hood of a thermal runaway.
It is not uncommon for a charger to stop at 3.85V per cell. It does this with the balance lead connected so it actually "balance" discharges each cell to exactly the same voltage and leaving enough capacity to cover a couple of years self discharge before the critical 3.7V/cell is reached.
I hope this helps.
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Added a bit of the box top skin, the fin and a section of the fuselage spine.
The fin is a just sheet foam skins with some internal webs.
This is about as far as I can go as I have to leave access for the elevon servo wires.
The delta wing are likely to be of a similar form of construction. We shall see.
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Erfolg
I agree a plenum chamber in front of the fan rather than a duct is an interesting concept. It certainly reduces the average inlet velocity, however EDFs operate a very low pressures. A fan is more a process of air velocity and the energy required to accelerate it than pressure.
In flight the air entering the inlet is, relative to the airframe, travelling at the planes speed. If it passes into a plenum chamber it will rapidly slow down in proportion to the inlet and planum chamber cross sections. The fan has now got to expend energy to speed up the plenum chamber air to the planes air speed before it can generate any net thrust. The question is thus would the air be travelling faster down a constant area duct than in the plenum chamber? The closer the air is travelling to flying speed the less additional energy is required from the fan to generate thrust.
Of course at higher speeds dynamic pressure comes into play and as you point out at supersonic shock wave pressure can be very significant but is not applicable to EDFs.
So my hypothesis is to try to maintain the inlet air velocity so as much as possible of the EDF's energy is spent accelerating the air flow to above the planes air speed.
You mention your HE 162.
With mine I had more aerodynamic problems with the airframe (directional stability) than with thrust. It actually it used a drone motor with a small 3 blade ducted prop but being built entirely thin sheet foam is it pretty light.
Above it is in a "bank and yank" configuration, no rudder, but like that it simply did not turn when bank was applied. Even with oversize fins and working rudders it still requires a very careful application of rudder & aileron to execute any sort of turn.
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A bit more progress but it is rather slow. Apart from the cold much of the construction method has to be decided as it goes along!
With both EDFs installed and tested the basic fuselage box is largely complete.
There is still a bit more to do fairing in the nozzles followed by the fin and part of the fuselage dorsal spine.
By covering the box top surface the issue is ensuring there is sufficient access left open to install the elevon servo wires.
Then the wings can be built and added although their actual method of construction is still to be decided.
When the delta wings are attached it will become quite a substantial plane.
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Sorry no flying yet.
Being tissue covered and only lightly doped and I missed the best warm early December weather. It is now so cold it all goes wrinkly.
It aerodynamics are a bit of a stretch anyway so it will have to wait for warmer dryer weather.
Of course in the mean time I am building the XF108. That will be all foam so will withstand the cold and damp but not the wind!
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Toto
When it freezing it may be cold but if as at the present high pressure is in charge it is at least dry and likely to stay that way all day.
Just remember how frustrating it is when "when rain stops play".
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Erfolg
For the XF108 commercial units were the only possibility due to the fact it used true turbo jets and thus they are a relatively small diameter. I only use drone motors and multiblade props as fans on airliner types where the turbo fans give a reasonable scale diameter nacelle.
My own view on cheat holes is they only work if the inlet duct is aerodynamically inefficient meaning the inlet pressure on the face of the fan is significantly below atmospheric.
Duct theory suggests that provided the inlet duct has a constant sufficient area, has a smooth inner surface and changes direction as little as possible then the duct pressure losses per unit lenght are surprisingly low. Like all aerodynamics the principle the object is to keep the airflow as undisturbed as possible. This also applies to the air stream as it approaches the leading edge of the fan blades and is the primary reason nearly all commercial thrust figures quote the fan operating in 'free air' and with a bell mouth to ensure the fan is receiving a smooth airflow normal to the blades. It can make a significant difference.
Surely a cheat hole in the wall of a duct will disturb the airflow proceeding down the duct and the closer it is the the face of the fan the more the disturbance.
You may have noticed that in many cases I place the fan right at the back with a short thrust tube rather than the conventional midway along the duct. My thinking here is the airflow down the inlet is rather different to the exhaust from the fan.
The inlet airflow will be more or less constant across the duct diameter although a bit slower next to the duct wall due to skin friction. For the exhaust the airflow is not constant across the duct diameter. It will be fastest closest from the fan blade tip and much slower from the fan root. This suggest to me that exhaust duct losses per unit length will be higher than the inlet hence my preference where possible for a rear placed EDF.
Of course over riding all duct theory is the simple fact the efficiency (g thrust/Watt) for a given EDF is improved dramatically as the fan rpm is reduced. Light weight airframes with reduced power requirements can result is smaller batteries and a longer flight duration but then I am not a member of the "it must go over 100mph club"! 😉
All a bit contentious. I think I had better keep my head down.
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The fuselage box built in the same way as the first smaller version with the longer 6 section ducts installed.
Note the magnet wire leads from the completely buried EDF run forward towards the nose.
So far so good but the fairing of the box to the twin exhausts is not quite right. Some rather fiddly bits need to be cut away and rebuilt.
Hopefully the wings and nose will be of a simpler and rather more conventional construction.
A view through the exhaust of the 12 blade 40mm fan.
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I spent some time over the new year period printing the ducts for the 50% bigger XF108. Afterall the fuselage "box" is built around them.
Apart from the bigger inlet the other major difference is the exhaust nozzle . It looks massive although its internal dimension are the same.
To achieve the required nozzle area for the 40mm EDF the 18 "petals" have been contracted down to slightly more than would have ben used at mach 3 speed on the full size.
Next to start building the fuselage box.
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Very neat.
350mm span is pretty small!
What is the fuselage to be made from?
Models tend to have a rather higher power to weight than the full size so extra "form" drag tends not to be to much of an issue. It just means it will fly a bit slower and use a bit more power than it otherwise might. A bit like the full size really!
Any "guestimate" of the likely all up weight?
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Just as a trial to see what it looks like a 50% bigger inlet against the original.
It looks rather...er...big!
I have created both plan and elevation CAD images of the full length duct.
To ensure the parts are well within the printer volume and to keep individual part printing times reasonable each duct will be made up of 6 parts rather than 4.
Both ducts represents a total of at least 12 hours single wall LW-PLA printing. 😉
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Michael D
If that LiPo cell really is at 0.9V and is not the result of a bad/broken connection then that cell is already dead. The chemistry inside the cell has been severely damaged and it can never be recovered.
The absolute minimum voltage of a LiPo cell is 3.3V. Any lower and the internal damage begins.
Note
If the damage generates an internal short circuit in the cell it can self ignite and burn fiercely until all the energy in the cell is exhausted. The heat created is such it may also ignite the other cells in the battery.
Any "modification" to a LiPo or its wiring should be avoided.
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The duct and basic fuselage structure is complete/
The underside is fully skinned.
It is remarkably light (40.4g) and rigid considering it is still open topped BUT I have come to a serious conclusion, it is too small. 😵
At that size the XF108 with a narrow delta and a very thin wing will have a span of only 24" (610mm) meaning with two powerful EDFs it will fly FAST. Not ideal for a foam belly lander.
In comparison my 40" straight wing DH Venom uses only one 40 mm EDF and it no slouch.
My intention is to utilise the same sort of construction technique but make the XF108 50% bigger!
At 36" span it will have over twice the wing area and using the same powerplants and battery it should be a more docile flyer better suited to a hand launch/belly land.
As the ductwork will be more or less the same it will be easier to incorporate it all into the bigger fuselage.
Well that's the plan.
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If we assume the balsa structure is rigid enough then it could be some form of aerodynamic disturbance caused by a part of the aileron at small deflection creating a projection that has a disproportionate effect on the aileron function. At larger aileron angles the airflow will be turbulent and thus a small projection have little or no effect.
Where possible for accurate control at small control deflections it pays to fully "shroud" the control surface gap to maintain a smooth as possible airflow over the surface.
Just a thought.
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This is where things start to get tricky as I have no fuselage cross sections at all to work with, just 3 views. It means much of the local detail of the fuselage shapes has to be "interpreted" or put another way my best guess!
With all the duct sections printed and both EDFs available the basic formers between the ducts can be added to hold them in the correct relation.
Plan and side view.
The complex top and bottom profiles will be added later.
One advantage is the entire nose and cockpit can be built separately and just glued on between the intake ducts.
As the pair of EDFs weigh about 4 times the duct they will only be permanently added when sufficient skin has sufficiently stiffened the duct assembly.
I will be amazed if it all works out!
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Its is personal point of view but why joins spars with plywood?
The biggest force on a spar is either tensile on the lower surface or compressive on the upper. The spar material is chosen to best resist these forces so surely they should be joined with a similar material. There is also smaller force trying to squeeze the spar(s) together.
Plywood is a uniform material so for a given weight it is weaker in tension or compression than the spar material and over strength for the smaller squeezing force.
Of course ply being in sheet form makes it easy to cut to the required shape.
It does rather depend on how fanatical your are about structural strength to weight but spars should be joined taking into account the structural requirement particularly at the root where the bending moments and thus the forces are the highest.
How often do you see a wing structurally fail other than at the root? This suggests the "conventional" root join design tends to be weaker than the rest of the wing.
You can, like they do in full size, actually test a wing.
This foam sub 250g Super Cub is centre loaded to 3 times its flying weight and supported just by its wing tips.
The wing does bend but so supported it is generating a root bending load that is about 4 times that from flying so with the extra centre weight the root is withstanding roughly the equivalent of pulling a 12g manoeuvre which the plane has neither the speed or power to ever achieve.
Contact with the ground will break it but not by what I can do with the controls in the air. 😉
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Slow progress but a trial assembly of the complete LH duct showing the offset.
It looks like an lot of duct but all the inlet is sized to 1.2 x the FSA. The nozzle is 90% FSA.
6 sections, all LW-PLA. The complete duct including the nozzle weighs 17.4g (0.6oz). The 40mm XFly EDF is 40g (1.4oz)
At this stage the EDF and duct can only be functionally tested at low power as the inlet would collapse inward. Full power testing will have to wait until the majority of the duct is supported by the appropriate fuselage formers.
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Its all down to the date of manufacture of the DX6i.
After a certain date DSM2 capable transmitters could not be manufactured and sold in the EU. The easiest way for Spektrum to achieve this was to simply remove the DSM2 option in the TX software.
Older DSMX DX6i have DSM2 compatibility. I have both types.
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toto
Wow!
What sort of "MGM film" are you proposing to make in that studio?
For most modellers it is the visual content that matters more than the quality of the acoustics. 😉
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I plan on using a pair of 8 blade 40mm EDFs with the airframe made from 3mm XPS foam (B&Q!).
Following my usual build sequence the first task is to design a "stand alone" duct in LW-PLA.
It obviously translates from a rectangular inlet to the 40mm diameter of the EDF body then to the EDF itself without a bell mouth and finally a tail cone "nozzle" with a final diameter of 90% the FSA.
The first CAD "picture" of the duct. The gap is where the EDF will go.
The inlet and the nozzle are not quite in line so the duct is made up 3 pieces. The middle piece is where the duct shape changes and also has the "offset" between the inlet and exhaust lines. This means it is "handed" to suit a left or right hand duct. The other bit of the duct are common to both.
Printed in single wall LW-PLA it will rely almost entirely on the fuselage formers to prevent it collapsing inward under the inlet reduced pressure.
It makes a long duct but the inlet area is slightly over 1.2 x FSA so should adequately feed the EDF. With the EDF so far back it will ensure the battery will be well forward ideally in the cockpit area with a removable "canopy" for access.
How to actually build up the fuselage around both ducts at the sane time is a problem yet to be solved. 😉
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It works but of coarse the parts are not printed at the same time or in the same way in order to achieve the best strength to weight compromise. It also uses a separate covering material.
You will find the commercial designs also assemble the wing from a number of parts but most incorporate a load bearing printed skin over a lightweight internal structure.
In my experience using built up foam structure with reinforcing were appropriate ends up at about 3/4 the weight of even a commercial print design and has better crash resistance too.
A companion for my XB70, the XF108 Rapier
in Own Design Project Blogs
Posted · Edited by Simon Chaddock
Erfolg
I wouldn't disagree for a minute that a short duct has less losses than a long one which incidentally is why my VTO V-2 rocket has in effect a virtual 360 degree cheat hole and as close as possible to the ducted fan to ensure the maximum possible thrust from the power available.
To me it comes down to what constitutes an acceptable variation from scale appearance and the best way to achieve it. Cheat holes are a last resort.
I would suggest the HK Vampire is an example of a rather visible non scale appearance to incorporate a big EDF but as it is on version 3 it must sell well.
There is some progressing on the XF108
The inner lower wing skin. The completed wing will be an all foam structure that hopefully will get way with no reinforcement as was the case on my the rather bigger XB70. That is strong enough to do a "very non scale" loop and roll. 😮