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Prop Motor

Discussion in 'Airplanes' started by Frederik Edvardsen, Jul 10, 2019 at 11:01 AM.

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  1. Frederik Edvardsen

    Frederik Edvardsen New Member

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    I don't know if you remember my previous tread, where I asked about EDF size for my 2 kg airplane. I have now read more, and from what I have understood, propellers are WAY more effective than EDF's, can anyone confirm this?
    So my question is, will two of these motors (Turnigy D2836/8 1100KV Brushless Outrunner Motor) with the correct propellers be better (and provide enough thrust) than two of these (https://www.banggood.com/FMS-64mm-4....html?rmmds=search&ID=522767&cur_warehouse=CN) EDF's?

    Thank you.
     
  2. rdsok

    rdsok Well-Known Member

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    What airframe are you talking about? What type of flying are you wanting to do with it? What type of wing? What is the wing loading? Prop clearance... how tall are the landing gear?

    As was mentioned in the other thread... there is a lot more to picking the right motor/prop than just the weight of the aircraft and determining the amount of thrust needed.
     
  3. Frederik Edvardsen

    Frederik Edvardsen New Member

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    I want to build a high wing airplane, flying nice and stable. Estimated wing cube loading is about 6 (Trainer). The wings are rectangular, due to the simplicity of building them. I know this might increase drag, but it's a 2 kg airplane so I don't think it is that important. Are these enough parameters to decide whether a propeller or an EDF is the best choice? Do you have any rule of thumb taking into account regarding this topic when building RC airplanes? Thank you.
     
  4. Frederik Edvardsen

    Frederik Edvardsen New Member

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    Btw it says "max current" is 18 amperes on the motor mentioned first, while the EDF apparently draws 40 amperes. Doesn't this mean I won't need as big of an ESC, and have more than twice as long flying time?
     
  5. rdsok

    rdsok Well-Known Member

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    Props are much more efficient when compared to EDF's... and simpler as well. So they are a much better fit for a trainer. You would only use an EDF when they make sense, like with a jet styled airplane where that look fits better. I've never seen a trainer with an EDF setup.

    A trainer will typically have about 25% up to about 50% thrust ratio... So something that provides at a minumum of 500g up to 1kg of thrust will be fine. I would lean towards something closer to the 50% mark. This ( being a trainer ) also suggests what type of landing gear you would want... usually they have trike gear and aren't a tail dragger like you'd see with some WW II war planes. Having a steerable trike gear is much easier to control on the ground than a tail dragger would provide.

    Where I usually start.. is by looking at existing airplanes that are similar to what I'm trying to pick a motor for. Then look at the recommended motor prop combinations they suggest for that model. Then you take that info and see what amp load that motor/prop has so you can pick an ESC that can provide a bit more current than is actually needed. This then also gives you an idea of how tall the landing gear should be.... As an example... say a model recommends a 12" prop ( much too small for a 4kg sized plane but still a valid example )... then add a couple inches ( or so ) to that and that tells you how tall the gear should be so you have enough prop clearance. I also don't look at just one existing plane... I look at several and see what they have in common or differences so I can better pick a motor/prop combination.

    One final detail that probably should be covered... the wings. A wing with an even cord ( width front to back ) is perfect for a trainer and a lot of other style airplanes. One difference you'll find on trainers is the amount of dihedral they have, in other words that they aren't flat. The more dihedral, the more stable they will fly but only up to a point... you still want to be able to change their direction so you don't want too much either. A plane with enough dihedral will "self level" to an extent, a good quality in a trainer but maybe not something you'd want in a sport or 3D flyer.
     
  6. Frederik Edvardsen

    Frederik Edvardsen New Member

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    I see. Then I guess two of the above-mentioned motors will be more than enough for this plane. Sorry for a lot of questions, but do you think I should go with a 2-,3- or 4-bladed propeller? I can see that most RC airplanes have two, but the seller doesn't mention number of blades, only propeller size.
    I think I won't have any dihedral, since it is a high wing airplane. Based on what I've read, the high wing itself should provide enough stability. Please correct me if I am wrong.
     
  7. rdsok

    rdsok Well-Known Member

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    The less number of blades the prop has, the more efficient. So two blade...

    I didn't use the word "stablility" when I said "self leveling" a bit ago. You are wrong in your thinking about the dihedral and that just having the wing on top adds enough stability for it to be a trainer, you also want something that will "self level" to some degree. I'm not sure where you were getting that information, but it's not a credible source OR they weren't talking about a trainer and you misunderstood what they were talking about. A flatter wing ( few are actually flat ) is the sign of a sport flyer... not a trainer.

    The motor you linked to earlier is made for a much smaller plane than the 2kg ( 4.4lbs ) trainer you described. That would result in about a 50" - 60" wingspan airplane ( unless you build way too heavy ) which in turn would probably require something in the neighborhood of a 14-16" prop. The approx kv would be something in the 650 - 900 range. Also think of the amount of air the prop itself must push over the wing during take off in order to provide lift until you've reached a decent take-off speed.

    I really suggest you start first by looking at equivalent sized trainer airplanes and see what motor/prop combinations they use ... and look at the wings and dihedral they have as well... don't just take my word for it. Designing and building an airplane from scratch is an advanced subject but when you are first starting... begin your design specs following known working specs, don't just guess at it.
     
  8. rdsok

    rdsok Well-Known Member

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    Let me add to this some... kind of going off subject if we are really talking about a trainer. If you actually want a different type of airplane and just unknowingly misspoke calling it a trainer... that's ok but then we need to correct it and change the subject to a sport or other type of flyer also. Therefor a discussion on the different types may be more in order for a bit.

    If it's a trainer... most likely ( almost always ) it will be a high wing aircraft and it will have more dihedral than other types such as sport flyers, 3D airplanes or racers. This is a great place to start for beginning pilots of course but I know many skilled pilots that prefer them for the style of flying they like or just because of their looks even. My own father ( when he flew, at 87 he doesn't now ) as well as many other pilots I know of with decades of experience still gravitate to these because that is what they like to fly. Trainers often are made for 3ch flight ( rudder, elevator and throttle only ) but I do encourage going ahead and adding ailerons so when a pilot reaches the next stage in experience, they have that option also and can continue flying the trainer until they get another sport type airplane. The primary difference between a trainer and a sport flyer will be the amount of dihedral used in order to help the airplane self level better. Most trainers wings will also have airfoils that more flat on the bottom as well.

    A sport flyer can be designed just like the trainers in style but will have a slightly flatter wing but they still will usually ( almost always ) have some dihedral still. Of course being a sport flyer also allows different wing placement ( compared to a standard trainer type ) and often different body ( fuselage ) styles. As I briefly mentioned, sport flyers often will have 4 channels and have ailerons included with the rudder, elevator and throttle controls. Just because a plane has a top wing, doesn't mean it's a trainer... but sport flyers also have mid and low wing configurations included. The airfoil type also adds a more symmetrical shape to them. Having a symmetrical airfoil will help the plane fly better when inverted ( if the pilot chooses to ). Most sport flyers will only have a semi-symmetrical airfoil but some also include a fully symmetrical airfoil ( usually for slightly more advanced flyers ).

    A 3D flyer will often have a flat wing that uses an fully symmetrical wing. These aspects allow the plane to fly in whatever orientation the pilot chooses. Most of these are designed with a mid wing configuration so they fly similarly inverted and upright. These will often have motor/prop combinations that provide a one to one ratio ( or more ) on thrust. The control surfaces ( ailerons, elevators and rudder ) are also much larger than you find on trainers or sport flyers... or racers for that matter.

    I'm not going into the aspects of a racer much but... I will mention they often have motor / prop combinations where they spin faster ( has a higher kv ), typically the props diameter is smaller so they can use a larger pitch on them also. I mention this because of the motor / prop you talked about earlier may be about the right size for something a bit more racy which is in part why I mentioned it was the wrong one for what you have said you planned to use it for.


    Having said all of that... if you are a beginning pilot ( and not just a beginning builder ) then I'd strongly recommend staying with a trainer design as I've been talking about. If staying with a trainer, modify some of the concepts you've held previously so they fit the trainer design better so you won't be starting off on the wrong foot ( so to speak ). If you are a more advanced pilot and actually do want some of those aspects in your design, that is fine also but lets call it what it is and say it's more of a sport flyer so you are using the terminology correctly. Oh... one last thing... if it's to be a sport flyer, you still want some dihedral, just not as much as a trainer would have is all.
     
  9. Frederik Edvardsen

    Frederik Edvardsen New Member

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    No, I have been talking about a trainer all along.
    After some reading I've now realised two motors with a kV-rating of 700 probably will fit my plane. I'm thinking about this one: Turnigy Aerodrive DST-700 Brushless Outrunner motor 700kv. Looks like this provides plenty of thrust with a low power consumption. I've looked at a few other similar planes, 700-800 kV seems to be the most used for this size. Therefore, two of these with a 2-bladed 9x5 propeller (Max thrust: 2*700g=1400g) would be way more than enough?
    Regarding the dihedral, 4°-6° would be good enough?
    I was actually thinking about using 5 channels: Throttle, flaps, ailerons, rudder and elevator. I don't know if the flaps are necessary on such a small aircraft though?
    Again, please correct me if my terminology or anything is wrong, I'm not a native English speaker.
     
  10. rdsok

    rdsok Well-Known Member

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    I've never seen a trainer with dual motors.... sounds like you are making things more complicated for no reason IMO. I'll stick with my previous recommendations...

    I think the 4-6° of dihedral sounds about right but I'd have to look at it really. The trainer I built was from a kit and I can't recall the wingspan, but it had me put around 2" - 4" under the wing tip... remember when building a wing, each half is done separately and then joined later so I ended up with a 4"-8" total between both wings when joined.

    I see flaps added to sailplanes... not so much for a beginners trainer airplane.

    No need to correct terminology... I just wanted to make sure we were actually talking about a trainer because you keep mentioning things you are thinking of that aren't typically on a trainer... like the EDF motor etc.
     
  11. Frederik Edvardsen

    Frederik Edvardsen New Member

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    Yeah, I guess I am building each side of the wings, and then a "mid-piece" to join them and get the correct dihedral angle.
    I'm now getting some sense of how to build this, and if it doesn't work, it doesn't work. It's my first scratch-built airplane so some try-and-failure is probably to be expected. Anyways, thank you so much for your time and help.
     
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