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    #16
    Originally posted by edmond22
    I thought that as well! But it worked well for edf mounted on top like a wing etc. But for those scale ones with proper intake at the bottom below the canopy kind keep eating grass! Always a headache cleaning those up esp during wet weather.
    It's a compromise. A top mounted ducted fan unit or gas turbine engine suffers badly on intake air starvation or turbulent during high alpha manouevers, as the airframe and wings blanket the smooth airflow into the inlet. Having the intake at the belly solves this problem but prone to FODs. However, it's a small price to pay since the airplane will spent more time in air with the engine running than on ground.

    Having side intake strikes a compromise but differential intake air velocity during manouvers can also cause problem, not to mention the complexity of the duct design. Twin engine side intake seems to benefit from this design though.

    Comment


      #17
      Originally posted by edmond22
      Hey Babylon,

      the Jet is very different, it compresses air, consists of stages and actually puts in a combustion process later to achieve the high thrust velocity output. See http://travel.howstuffworks.com/turbine3.htm

      The EDF to me (IMHO) is mostly a asthetic factor for RC modelling, which works well to power planes but it is in no way close to a Jet engine. For all out speed, I think propeller is still the way to go, so using EDF is more for scale reason.

      Thanks Edmond22 for your your contribution.
      Agree, turbine jet engine develop its thrust from the combustion process
      while a EDF develop its thrust by sheer compression of air.

      Since we are exploring ways to improve efficiencies , one of the ways to get more thrust out of the electrical systems is to move to larger diameter EDF provided the larger size EDF make sense when use on a given size model plane.

      Beside size, I think twin or multi EDF setup also improve efficient. The trick is not to drive each fan too hard as compare to a single Fan setup.
      For a fix diameter fan ,what I found out that the thrust verses electrical power of a EDF is not linear.

      Infact there is a diminishing return in terms of thrust versus the electrical power when the power is setup on the high side.

      generally
      High power setup= lower efficiencies
      Lower power setup= higher efficiencies.

      2 EDF of a size X unit set up at lower power is more efficient 1 EDF of the same size X at high power setting.
      The disadvantage of a twin EDF is mainly cost and the complexity in the setup.

      Cheers

      Comment


        #18
        I disagree with the last assumption....

        generally
        High power setup= lower efficiencies
        Lower power setup= higher efficiencies.

        You cant generalise like that.

        Your whole powertrain has to be properly tuned to the the whole fluid mechanics of the duct,fan,incoming airspeed etc etc.....and efficiencies change at various speeds...

        Why do bicycles have gears....??? Does is a big strong cyclist less efficient than a small weaker cyclist??? Maybe yes, maybe no.....it all depends....
        Do you feel the RUSH....!!!!

        Comment


          #19
          Two smaller EDF would have higher efficiency as compare with a single large EDF.

          Hmm.. a, twin EDF may have large area in total as compare with single. Result, more air flow, more mass, more thrust. Not sure if this will give high air velocity, that is higher flying speed.

          May be this sound stupid, I am thinking two EDF in series. Smaller in front follow by larger fan? will this have higher air speed at exhaust? Set full power during take-off, and cut front motor during cruising. Open by-pass duct to reduce friction from front EDF.

          Just think.,....

          Comment


            #20
            Originally posted by Hawkeye
            I disagree with the last assumption....

            generally
            High power setup= lower efficiencies
            Lower power setup= higher efficiencies.

            You cant generalise like that.

            Your whole powertrain has to be properly tuned to the the whole fluid mechanics of the duct,fan,incoming airspeed etc etc.....and efficiencies change at various speeds...

            Why do bicycles have gears....??? Does is a big strong cyclist less efficient than a small weaker cyclist??? Maybe yes, maybe no.....it all depends....
            Hi Hawkeye, thank you for your feedback let me response to your statement.


            The EDF that we know and setup have wires, ESC, motor wires. They sum to what we call finite resistance. When electrical passes through this resistance it gets wasted power as heat.


            Lets put in some electrical figure so that it is easy to understand why higher power setup result in lower efficiencies and wasted power

            Case 1 Assuming you have a EDF setup with the following

            1. 3 cells LIPO so that
            2) Average voltage is 10V
            3) Average current is 20 amps
            total power = Current x voltage=10x20=200watts



            Case 2 assuming you double the power to 400 watts(10V ,40amps )setup and you expected the thrust to double.... . I can tell you it won't happened. Why because there are electrical resistance built up in the setup.
            There are resistance in the dean connectors, the wires, the bullet connector , ESC, motor winding. Sometimes this resistance are call "parasitic resistance."


            Lets calculate the losses by introducing the parasitic resistance into the case1 and case 2


            Assuming the parasitic resistance R=0.1ohms

            case1 at 10V,10 amp setup ,
            Power lost= IXIXR=10x10x0.1=10 watts
            Useful power=200Watts-10Watts=190Watts

            Use ful power as a percentage=190/200X100%=95% efficiencies

            case2 10V , 10 amp setup
            Power lost=40x40x0.1=160watts
            Useful power=400-160=240watts
            Useful power as a percentage=240/400x100%=60% efficiencies

            For every doubling of current the electrical lost is square factor which make the electrical lost more acute.
            To make matter worst, the power lost in the cable will cause heating effect.
            All copper wire has positive heat co-efficient meaning their resistance increases with temperature and aggravate such lost.

            From this example you can see that the electrical efficiencies drop drastically when you double the current setup.


            One technique to mitigate this lost is to use higher battery cells count with lower KV motor. However , the result still show lower efficiently when you compare lower power setup with more cells count.

            Another very effective method is to go for multi EDF with seperate ESC and pararallel wiring. The parasitic resistance can be significantly reduced.

            This is only considering the electrical factors but from this example is a major factor. Don't you think so?

            Cheers

            Comment


              #21
              Sorry case 2 should read 10V ,40 amps setup....
              too fast finger...

              Cheers

              Comment


                #22
                Power = current ^2 x R


                Babylon..in your example you assume you are pushing the higher current thru the same set of wires and resistance. Which can only happen if you increase the size of the fan or increase blades. There is a point of diminishing returns in doing it that way.....The other way of pushing more current thru the same set of wires is to increase the voltage.

                If the fan does not change....Current does not increase in a simlar electrical setup unless voltage increases or resistance drops.....

                I agree that the way to go to higher power is to go to lower kv motors and and higher voltage....not increasing the current flow on your exisitng setup.......but than that comparison with above is like apples and oranges.

                Jus a note....regarding power setups in the running electric pylon racing...2 schools of thought have emerged....

                1. 7 cells on Hi Kv motors (5000 odd kv)....about 800-1000W
                or
                2. 18 cells on Low Kv motors like a mega 16/25/2(2600 kv) also 800-1000W

                Both these setups have shown to be very very fast.......winning the 1 and 2 position in the world championship....
                Do you feel the RUSH....!!!!

                Comment


                  #23
                  I did not explore the merits of having 2 EDF vs 1 yet. But I did explore reducing weight extensively. I think for EDF to be effective and more fun so that you can also fly patterns at lower speed, it is probably good to look at ways to reduce weight.

                  At one stage I was even able to reduce my Nicad of RC2000 to CR1300 (if me memory serves me correctly). I achieve the same flighttime simply because I lose a lot of weight, less power is needed to fly the model and the 1300 mah actually gave me better flight time If I do some throttle management. This has led me to believe that if you are able to shave some weights, you will probably gain most from there and achieve pretty amazing results in performance.

                  Joe mentioned about the disadvantage of top mount EDF, which is exactly what I experienced on some of my speed wings. If I attempt to slow them down too much in a high alpha, the wing will start to do no warning stall. But if I kept it light enough, the low wing loading factor comes in and I was able to achieve pretty fun high alpha flights with EDF which is pretty cool too.
                  Last edited by edmond22; 30-06-2007, 02:17 AM.
                  Stop looking for a gyro in my plane, they are all in the head.

                  Comment


                    #24
                    I have noticed certain setups on pusher planes that make hell-lot-of-noise, but do not necessary make the plane a lot faster than when it is making less noise.... is this similar(in certain sense) to what you are explaining?

                    (this is an interesting lesson)

                    Originally posted by joe yap
                    .....
                    A lot of modellors do not understand this and they try to recover the efflux velocity by spinning at even higher RPM. It will work if you pump in enough power, but doesn't it sound stupid that you are burning thousands of watt just to produce some useful power and fan noise? .....

                    Comment


                      #25
                      Originally posted by mave
                      I have noticed certain setups on pusher planes that make hell-lot-of-noise, but do not necessary make the plane a lot faster than when it is making less noise.... is this similar(in certain sense) to what you are explaining?

                      (this is an interesting lesson)
                      Actually, the prop noise in a pusher plane setup is another story. But the difference is that once the air has been accelerated by a prop, the thrust is being produced right away. A ducted fan unit produces thrust only after the air leaves the exhaust nozzle, and so it is very important not to let the air decelerates beforehand or some efflux velocity will be lost.

                      However, one point which is common to both type of powerplant setups is that both a prop and ducted fan needs to produce efflux velocity way beyond the aircraft stall speed in order to overcome the total aerodynamic drag. If not, the plane could hardly fly even though the motor can produce tonnes of static thrust.

                      So in order to fly an airplane, producing enough velocity should have more priority over thrust. To increase it, you can spin at higher RPM, which often results in adding more cells or using a higher kv motor, which can cost quite a bit. A cheaper way is simply choose a prop with higher pitch bit make sure you check the amp draw.
                      Last edited by joe yap; 30-06-2007, 08:02 AM.

                      Comment


                        #26
                        Thanks !

                        Originally posted by joe yap
                        Actually, the prop noise in a pusher plane setup is another story. But the difference is that once the air has accelerated by a prop, the thrust is being produced directly. A ducted fan unit produces thrust only after the air leaves the exhaust nozzle, and so it is very important not to let the air decelerates beforehand or some efflux velocity will be lost.

                        However, one point which is common to both type of powerplant setups is that both a prop and ducted fan needs to produce efflux velocity way beyond the aircraft stall speed in order to overcome the total aerodynamic drag. If not, the plane could hardly fly even though the motor can produce tonnes of static thrust.

                        So in order to produce enough velocity should have more priority over thrust. To increase it, you can spin at higher RPM, which often results in adding more cells or using a higher kv motor, which can cost quite a bit. A cheaper way is simply choose a prop with higher pitch bit make sure you check the amp draw.

                        Comment


                          #27
                          Sorry, I typed the last post before bed and did lot of grammar mistakes. I've edited since.

                          Comment


                            #28
                            HI.

                            Firstly, I am glad there are so many EDF fan out there. With this amount of resources, the black art will be resolve soon enough. I do know a few EDF flyers in singapore who have already went thru quite a bit of research over the years and have successfully flew huge EDFs. So to all who are trying, pls keep up the enthuism, almost there.....Followings are my take on the subject, however it is a little technical and dry, will try my best to use everyday terms for easier understanding...will try..however ongle not vely gud.

                            1) EDF produces thrust by accelerating small amount of air to high velocity, F=mA, therefore exhaust air speed very critical (Efflux velocity..as Joe mentioned). Propellers produces thrust by accelerating huge amount of air to a slower speed, F=Ma, therefore angle at which blade cut the air (blade angle of attack) is critical.

                            2) Efficiency, contrarary to practical results, edf,theoratically can be more efficient than propellers. This is due to massive tip looses at the tip and thick inefficient root airfoil for propeller,also engine cowl proximity at root. All this problems are almost all but eliminated with edfs.

                            3) The problem with EDF, in academic sense, is difficult to get optimum aerodynamic performance. The electrical performance can be easily overcome. Measuring watts thru amp and voltage is a measure of the efficiency of the electrical system not aerodynamic. Eg. if the impeller are all 90 degrees blades, it will definately not produce any thrust but the electrical reading may seem apparently valid. Aerodynamic optimization then is the holy grail.

                            4) Next is propulsive efficiency...which basically mean, how much of the power produced is used to propell the aircraft fwd and how much is lost to producing the thrust. In this respect EDF looses out at lower speeds and recovers at higher speed due ram recovery. Propellers are in the reverse. Propellers has a relative narrow efficient speed margin ie a fix speed eg high speed for pylon racers, medium for aerobatics and slow for 3D. As such when it is flown not at that optimum speed the efficiency drops right off. eg. Pylon racers static thrust is not always apparent and amp drawn can be high because the blades are too coarse, however in the air it unloads and amp drops but thrust increases. Similarly, for 3d flyers, once it goes beyond a certain fwd speed, the plane just doesnt want to accelerate anymore.

                            5) As for EDF, a match must be made between the Motor KV, Watt with a fan and suitable intake and exhaust duct, all of which changes air speed as such of paramount important. It is basically due to so many variables that sometime causes frustration to achieve efficiency. There are basically 2 critical factors, angle at which impeller blade meets the air and velocity of exhaust air leaving the exhaust duct. As joe explained, intake must feed enough air to the impeller, exhaust duct must hold exhaust air velocity if not increase it further.

                            ......to be continued, wifey in the house.

                            Comment


                              #29
                              Hi Vortices, I fully agree with you that there is so much enthusiasm to know more about the EDF.I hope more of you will contributed your finding in the mystical power of "ducted force".
                              I am now currently exploring multi EDF setup.
                              The good news about twin EDF is the plane can take quite a degree of unbalance thrust between the two EDf if both EDF set up is in close proximity to each other and are more efficient as compare to a single EDF setup.

                              The bad news is it cost twice $$$$ and additional weight.

                              Cheers

                              Comment


                                #30
                                Actually, I am not sure about whether multi-ducted fan design really improves efficiencies. But I know that full size jets do that for redundancies. I would rather use a big fan unit than to use 2 smaller fan units, as pipe losses would actually increase in smaller ID ducts, not to mention the parasitic drag created by the centre bodies of the units. Just ask any ducted fan designers and they'll tell you to pack a fan as big as possible in a plane. The amount of mass flow increased will be much more by simply increasing the fan diameter, than to use another unit.

                                As you can see the trend on modern jetliners like the B777, twin large turbofans have much better efficiency than 4 smaller turbofan B747. The only main limitation are ground clearance and sonic transition at the fan tip. There are still newer 4 engine designs, but the main benefit is that they do not have ETOPS limitations.

                                If what you said is true that doubling the current draw will square up the electrical losses in a single fan unit, for a twin setup, although is seperately connected, also sees double in current draw, isn't it? Let assume that you use a single battery pack to feed 2 units, the pack will still supply twice as much current draw, and thus the same amount of losses, right? If you use 2 seperate packs to feed, then it'll make a difference electrically, but the aerodynamic losses will also be increased. However, without any figures, it's quite difficult to tell which factor has higher effect and I think it's only through some testing with individual fan unit designs can the answer be surfaced.

                                So in short, I personally think that we can't simply assume what kind of setup is more efficient without considering all other factors. Afterall, the so call efficiency should be based on how much useful power is being extracted from the available power, not just simply on any single factor like electrical, mechanical and aerodynamics. Even if the figure shows how efficient a setup is, if the plane doesn't fly, it'll be simply meaningless.

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