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    #31
    HI guys...this is not part 2 but interim reply...

    Firstly, Single big unit fan or engine that produces twice the amount of thrust has been proven everytime better than to have duplicate units except for safety redundancy. The weight savings alone is enough reason. A mere arbituary 25% weight increase would double power ratings as oppose to 100% for duplicate units.

    However having said that I can understand why it is common to come to that conclusion of two, I understand may be your set up (motor batt and fan) combination is already at optimum or near optimum, as such if you double increase the voltage of batt or change the motor or just change a new fan, you have realise the increase in thrust is not apparent, as such it is better to have two duplicate units running at optimum to double the thrust. I have personnally came to that conclusion. What I had realise later was that this is where the problem lies with EDF. When we find we do not get what we want, we change something, fan, motor, batt,...the pemutation is almost non exhausting...hence the common conclusion.

    However later I have learnt, to increase thrust, the whole set up needs to be changed to produce the required result. Eg, 55mm fan on 3S on 3900KV 250w is a reasonably efficient set up, However to increase the thrust any further, 55mm fan needs to be on 4S 2500-2900KV >250W. If you want a bigger fan say 64mm fan, 3s 3200kv.>300w would yield good results. However if you only change one item from the basic configuration like a higher kv motor, or higher wattage motor or just run on 4S or just change the fan, the set up is set up for disaster not only for powerplant also for your plane.


    Cheers.

    Part 2 on the way...

    Comment


      #32
      okay, now you guys got me confused... hahahahaha
      DUCT DUCT DUCT GO!!!!

      Comment


        #33
        Hi Joe Yap , Electric Ducted Fan is diff from real jet.
        The key to efficiencies are :-

        1) The resistance from each EDF is theorectically half of a single ESc setup if we compare apple to apple, same wires, same ESC therefore reducing the total electrical losses.
        For the twin, the total resistance=1/Rt=1/R+1/R where Rt is the total parasitic resistance of the twin and R is the parasitic resistance of the single unit.

        If the sinle EDF resistance is 0.1ohms, the twin EDF electrical equivalent resistance=0.05ohms ....you have less electrical losses.



        2) The key is not to drive each EDF of a twin too hard.

        3) The disadvantage of twin or multi fan is the cost weight and more complexity in the installation in its own right.

        At some point, larger single EDF is also more efficient than smaller EDF from the aerodynamics but not on its electrical point of view unless you can find other means to reduce the parasitic resistances for the bigger and higher power setup associated with larger EDF unit.


        Cheers

        Comment


          #34
          OK, found sometime to complete my posting.

          Part 2. my take on the questions and a little more.


          1)Does all the electrical power you put into the plane turns into meaningful thrust or your systems is just using up all the batteries energy to warm up the coffee or your favourite tea?
          -This mostly due to aerodynamic efficiency otherwise the electrical power is just used to peddle and disturb the air. Proper combination of fan, motor and batt is required.

          2) Does one use high KV motor or lower KV motor?
          -As thrust is produced my accelerating smaller amount of air to a higher speed, higher KV motor compared to propeller is req'. However exactly what range, again depends on the set up.


          3) More or lesser blade Paddle count is better?
          This question has been asked too many times for the wrong reasons. Everyfan is design for a optimum rpm for a optimum aircraft speed with a optimum set up. Using the fan on different set up e.g 2s to 4s, 3200 kv to 5200 kv. will yeild different result. Basically reducing no of blades off load the motor as such it will be able to turn faster. This may reduce amp drawn and optimise operation if the blades were loading the motor preventing it from reaching the optimum rpm, however if the motor was spinning the impeller at the optimum rpm, reducing the no of blades will increase the rpm a little but may stall the blades as the angle of attack of the blades have increase. This will cause the set up to become less efficient. Again down to combination.


          4) Does one use light weight motor or heavy weight?
          -Assuming light and heavy means wattage of motor. Well more powerful motor is generally req' as the size of fan grows and rpm increses. The different in rated rpm (motor KV X volts) and actual rpm dictates amps. eg 250w motor at 3000kv on 10v installed on Fan 'X' say draws 25amps at full throttle. If 150w motor at 3000kv on 10v on the same fan 'X' would probably draw more than 25 amps at full throttle as the motor could not reach as high a rpm as 250W, as such the esc will increase the amps in the hope of narrow the rpm different...in some case this will either burnt your esc or you motor or both.


          5)More or less number of cells (Lithium or NICAD) added to the power sytems?
          -In my eperience, anything below 60MM fan 3S works fine but 4S runs better-lower amps).60MM fan and above 4S up to 6S has perform quite well but the motor kv drops back to 1800 kv to 2200kv 450 to 600w for the higher battery voltage ratings.



          6) What else can be done to get more static thrust for lesser electrical power systems?
          -Statis thrust or thrust at low speed has always been a challenge for EDF, GDF and even the jet engines. So far thru my research, the best chances of success at a reasonable price are:
          a) A diverging duct for intake and a converging duct for exhaust. Area increases toward the fan. This is to increase air pressure before the fan thus giving more meat to bite. The exhaust duct suitably converging, not only to keep velocity but actually increasing it further. Arbituary figure, 30 % diverging intake and 50% converging exhaust. This set up will load the motor quite significantly hence giving good thrust with low or no fwd speed.

          b) A inlet guide vane just before the fan. This means mounting a set of vanes that would swirl the air flow just before the fan making the fan blade angle of attack near optimum. This set up is a compromise, it would no doubt improve low speed efficiency but high speed efficiency will drop. Unless the vane angles can be alter accordingly with the use of another servo. Generally another impeller mounted in reverse epoxy permanently to the duct in front of the rotating impeller is the easiest set up to acheive this. This method has been documented in some aerodynamic for model a/c books. Unfortunately no specific performance figures were published.

          7) How critical is the air intake ?
          - It is everything. Without it, it is just a shrouded propeller, and with Coarse pitch of fan impeller it would just be a shrouded inefficient propeller.


          Well there you go. My take on the subject. As mentioned it is a little dry but if you are reading this to this point means you are really serious about edf. Congrats. Lets share out the result of our experiments and reinterate the fact that propellers are for Boats!!!

          cheers.

          Comment


            #35
            I would propose feedback contribution on successfull edf combinations just so we would have a sound data base to start shopping for a powerplant for our new airplanes...also to safe time and money...


            fan motor kv batt plane esc remarks

            1)GWS 55mm himax 2025 5300 3s Lear 45 40A DS runs hot at full Pwr
            2)EPF 70mm epf 16/15/ 3900 3s F-14 40A DS good
            3)
            4)
            5)
            6)
            7)

            Comment


              #36
              more thrust out of your EDF modification

              Hi, check out this link ....accordingly this modification improve the static thrust by 25% of the 50mm EDF



              Note: Thrust tube don't improve your static thrust but a correct geometry cone shape and fitted at the exhaust end does improve performances.

              Cheers

              Comment


                #37
                Originally posted by Babylon5
                Hi, check out this link ....accordingly this modification improve the static thrust by 25% of the 50mm EDF



                Note: Thrust tube don't improve your static thrust but a correct geometry cone shape and fitted at the exhaust end does improve performances.

                Cheers
                Since you made this statement, I don't think you have any 'correct geometry cone shape' fitted at the exhaust of your self designed Twin Ducted EDF plane. Aren't you contradicting yourself on many of your posts here? Perhaps your self designed Twin Ducted EDF plance should have a proper canopy too. This would reduce the drag on the airframe and improve the performance? Perhaps you should have an intake lip in your Twin Ducted EDF as well.

                Comment


                  #38
                  Originally posted by $h@d0w
                  Since you made this statement, I don't think you have any 'correct geometry cone shape' fitted at the exhaust of your self designed Twin Ducted EDF plane. Aren't you contradicting yourself on many of your posts here? Perhaps your self designed Twin Ducted EDF plance should have a proper canopy too. This would reduce the drag on the airframe and improve the performance? Perhaps you should have an intake lip in your Twin Ducted EDF as well.
                  Sorry I disagree, Although the taper thrust tube help to force the air at higher velocity allowing the plane to fly the plane at higher speed due to increase in dynamic thrust at "level flight." It won't help in the speed if you are flying in a tight circle .


                  However from the physics point , the tapered cone constriction will cause a back pressure to built up inside the EDF chamber, causing the blade to stall. This will reduce the static thrust.




                  Take a look at the link of the twin GWS50mmEDF setup, there is no thrust tube at the end of the 50mm EDF just the cone attached to the end of the motor and the intake ring


                  See my other thread link thread #30


                  Here is the extract from the thread 30

                  "Hi Jeff, coming back to this topic. The cut down 3 blade offer some advantage and some disadvantage. According to my observation, the speed of the Jet on EDF do not always rely on thrust although most of the time it does.


                  1.)more thrust=higher flying speed--->generally true

                  2.)More thrust does not necessary translated to higher flying speed.
                  Some multiblade EDF are design at lower rpm like the Super Flying Model is spec at 25,000rpm max. To improve the jet level flying speed in some practice, a hollow taper cone is delibrately place at the exhause to reduce or act to constrict and force the air out at a higher speed thereby increasing the jet flying speed. (There is a limit you can reduce the exhause opening and improve the level flying speed before flowback air pressure reduces efficiencies . )
                  The tail cone /tube do not increase the static thrust but actually decreases static thrust..
                  In this case you have the following:
                  When constricting the exhause air opening....
                  Lower Thrust=higher exit air speed=faster flying speed

                  This is a situation where not all the thrust develop by the EDF is translated to higher level flying speed."

                  I have mentioned this phenomenon many times in my other thread.
                  I don't think I contradict myself.

                  Cheers

                  Comment


                    #39
                    Originally posted by $h@d0w
                    Since you made this statement, I don't think you have any 'correct geometry cone shape' fitted at the exhaust of your self designed Twin Ducted EDF plane. Aren't you contradicting yourself on many of your posts here? Perhaps your self designed Twin Ducted EDF plance should have a proper canopy too. This would reduce the drag on the airframe and improve the performance? Perhaps you should have an intake lip in your Twin Ducted EDF as well.

                    Hi $h@dow,
                    You are right, Sorry to say that I had read through "babylon5" previous threads in this forum on his testing and findings on small electric ducted fan. For a starter like me in EDF, I also find his statements sometimes very contracting and misleading, I am not sure wheather does he know what he is trying to tell us because It sounds like quoting on others finding sometimes. At the end, I am like ???
                    The more I read the more I become confuse..
                    Sorry babylon, no hard feeling.

                    Comment


                      #40
                      You can write all you want about your findings. I think it's open to all to see. Like I said, it just looks funny when your Twin Ducted plane doesn't even have a canopy to reduce the drag on the plane...

                      Originally posted by Babylon5
                      Sorry I disagree, Although the taper thrust tube help to force the air at higher velocity allowing the plane to fly the plane at higher speed due to increase in dynamic thrust at "level flight." It won't help in the speed if you are flying in a tight circle .


                      However from the physics point , the tapered cone constriction will cause a back pressure to built up inside the EDF chamber, causing the blade to stall. This will reduce the static thrust.




                      Take a look at the link of the twin GWS50mmEDF setup, there is no thrust tube at the end of the 50mm EDF just the cone attached to the end of the motor and the intake ring


                      See my other thread link thread #30


                      Here is the extract from the thread 30

                      "Hi Jeff, coming back to this topic. The cut down 3 blade offer some advantage and some disadvantage. According to my observation, the speed of the Jet on EDF do not always rely on thrust although most of the time it does.


                      1.)more thrust=higher flying speed--->generally true

                      2.)More thrust does not necessary translated to higher flying speed.
                      Some multiblade EDF are design at lower rpm like the Super Flying Model is spec at 25,000rpm max. To improve the jet level flying speed in some practice, a hollow taper cone is delibrately place at the exhause to reduce or act to constrict and force the air out at a higher speed thereby increasing the jet flying speed. (There is a limit you can reduce the exhause opening and improve the level flying speed before flowback air pressure reduces efficiencies . )
                      The tail cone /tube do not increase the static thrust but actually decreases static thrust..
                      In this case you have the following:
                      When constricting the exhause air opening....
                      Lower Thrust=higher exit air speed=faster flying speed

                      This is a situation where not all the thrust develop by the EDF is translated to higher level flying speed."

                      I have mentioned this phenomenon many times in my other thread.
                      I don't think I contradict myself.

                      Cheers

                      Comment


                        #41
                        Originally posted by $h@d0w
                        You can write all you want about your findings. I think it's open to all to see. Like I said, it just looks funny when your Twin Ducted plane doesn't even have a canopy to reduce the drag on the plane...
                        Hi Shadow, just because I forgot to bring the canopy on that one day of the twin jet, you jump to to this conclusion that I am flying without canopy.

                        I find it strange and funny that you can write and make this comment of my twin EDF just because I forgot to bring the canopy along just this one time off.


                        If this is how you can jump to the conclusion with regard to the twin EDF that I have nothing else to say. It is for other to see as well.

                        Comment


                          #42
                          Thrust tube explanation

                          Since there is some bone of contention with regard to Thrust tube here is the extract from the expert forum see link


                          Extract
                          "
                          Intake and Exhaust

                          ..... Exhaust ducts are a bit different. Unlike their intake duct counterpart, they do not require strengthening as the air output flowing through the duct will keep it expanded. Also, they can vary in size from those equal to the output diameter of the fan shroud to ducts that are smaller than the fan shroud output diameter. The larger the diameter, the more static thrust, hence making them easier to hand launch, however you will loose some top speed. The smaller the diameter, the higher the velocity of thrust, hence your top speed will increase. However smaller diameter thrust tubes produce less static thrust or “push”, so they are more difficult to hand launch and may require a bungee launcher or landing gear for take-offs. There is an optimum setting for each exhaust duct and the model manufacturer will usually tell you what works best for their jet; for the Minifan 480, an exit diameter of 53-55mm is normal."


                          Did I say different from the expert forum with regards to the application of the thrust tube or exhaust openings.

                          I rest my case.


                          Cheers

                          Comment


                            #43
                            Originally posted by Babylon5
                            Hi Shadow, just because I forgot to bring the canopy on that one day of the twin jet, you jump to to this conclusion that I am flying without canopy.

                            I find it strange and funny that you can write and make this comment of my twin EDF just because I forgot to bring the canopy along just this one time off.


                            If this is how you can jump to the conclusion with regard to the twin EDF that I have nothing else to say. It is for other to see as well.
                            Errrr............. you DID fly without the canopy, right? I rest my case too. Don't worry, I won't comment on your expert findings anymore.

                            Comment


                              #44
                              whoa guys, relax.... let;s not turn up the after burners kkkk?
                              DUCT DUCT DUCT GO!!!!

                              Comment


                                #45
                                Hi all,

                                My take is simple. When you say efficiency, which efficiency are you referring too? We are trying to fly a model and it is the total efficiency we should be looking at, not just electrical efficiency alone. You can have a twin setup that is more electrically efficient in the power system, but if I can have a better aerodynamically efficient single setup which happens to be less electrically efficient, I may still beat you hands down in total efficiency. You can't simply look at electrical system alone.

                                You mentioned that gas turbine engines are different from EDF, I agree to a certain extend. But the main difference is the source of motive forces. They still need to breath air and the more you allow them to breath, they can produce more useful work, either by increasing the fan diameter, or using a bigger powerplant,etc.

                                If a twin setup has the same total fan areas as a single ducted fan unit, aerodynamically, the single unit has been proven to have better aerodynamic efficiency, which also can be translated to same input power with more thrust/ higher velocity or same thrust/ velocity with less power input. The twin setup may seen to have draw less current, but due to the smaller ducts and more interference within caused by the centrebodies and struts, there will be more aerodynamic losses.

                                At this point, I can't say which is better, since individual fan unit and setup are very different, you'll need to compare them very fairly by calculating all the figures before you can jump in to a conclusion of which setup is better. Anyway, I didn't even consider the weight penalties of twin fan setup in the picture yet.

                                Exhaust ducts for EDFs are designed to restore efflux velocity in a proper design, not meant to increase it and causing back pressure to the fan. The whole idea is to accelerate air by spinning the fan, not through squeezing the air behind it. We only have a single stage fan, not axial flow compressor.

                                Comment

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