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Scale Curtiss XP-55 Ascender

Discussion in 'Giant / Scale Scratch and Kit Build Threads' started by Larry B, May 29, 2016.

   
  1. Doc

    Doc 50cc

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    Makes my brain hurt to think about that, A stand holding the model on the CG would be "balancing" the model hence the weight on the canard would be zero just as the weight at the tail. One could find out the weight on the nose gear by placing a scale under the nose wheel while the aircraft mains are raised so the model is level. That wont tell you how much the canard is lifting in flight though, just how much it has to lift to rotate the model. To find out the lift provided by the canard would take more smarts than are permitted at my pay grade ie... canard area, airfoil lift co efficients at various speeds, etc... way out of my wheelhouse. I understand what you are using it for Ace but it doesn't involve a canard or trying to figure the lift from one.
    Doc
     
  2. Doc

    Doc 50cc

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    Larry, some body out there somewhere knows just how much of the area (percent wise) of the canard to add to the area of the main wing for computing true wing loading. Regardless of that, with the loading being at 57 without the canard if you added the entire area of the canard and recomputed the loading how much does it come down? Is it significant or is it still over 50 say? I doubt it lowers it enough to provide a warm fuzzy feeling SO! it flies well at whatever the true wing loading figure is and that seems to be what counts. On larger models the cube of the increase in size is what influences how well an aircraft flies at whatever the weight is not just the square. I'm willing to say you've found a nice mix for this particular craft.
    Doc
     
  3. acerc

    acerc 100cc

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    If the plane is perfectly balanced at the cg then the wing loading is restricted to the wings, and the canard is no different than a normal tail. But did you not have to change the balance to get it to fly right, change the in flight cg versus on the ground. None of my planes are balanced per on the ground, I only do a cg for the first flight I then move things to obtain the proper flight characteristics of that aircraft. I thought that was what you did. For example my big Pitts, the Red Eagle, does carry some of the tail lift by way of 1-1/2° stab plus the weight shift for flight, by the scales it carries approximately 4%. I could move the cg back to specs and lighten the tail but then it would fly to high for the model.
    I do understand what you are looking for but in my opinion the canard is no different than a tail, if the cg does not account for it. Remember when I said it looked nose heavy by the way it acted.
     
  4. acerc

    acerc 100cc

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    And I also have to agree with DOC on the weight issue. Most planes, other than 3D'ers, are to light in my opinion. Both of my current giant scales are seriously over weight but I feel they fly more scale with it. The Eagle is nearly 20lbs overweight but flies like a dream. And I have almost always added weight to correct flying tendencies of a to light airframe such as the Seagul models B-25, added five pounds to get it flyable.
     
  5. Larry B

    Larry B 50cc

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    Hi Ace,
    I have to disagree with you about the similarities between a canard and a traditional airplane. With the standard airplane design the tail always applies a down force to counter the nose trying to dive. The reason the nose tries to dive all the time is because the center of lift is behind the CG, this has to be the case if the airplane is to be a stable design that recovers when upset in flight.
    A canard is different in that the canard surface is applying a up force which actually contributes to the overall lift of the surfaces. The canard surface must have a higher angle of incidence so when the airplane stalls the nose will drop first. In an ideal situation the canard will stall way before the wing, the nose will drop and the wing will never stall and you will have aileron control thru the entire process.
    If you want to prove this just move the CG of a model behind the center of lift and go fly it, it will be an exciting flight!
    Now there are certain exceptions to this rule, take Sal Calvagna's big Sikorsky IIya Russian bomber, he is flying it with the CG way behind the center of lift and in that case the tail is actually generating an up force, but this is not a terribly stable configuration for an airplane. His Sikorsky is more like a canard due to the fact that both the tail and the wing are lifting the weight of the model.
    images.jpg

    Larry B
    Larry B center_of_pressure_gravity1.jpg
     
    Last edited: Dec 21, 2016
  6. acerc

    acerc 100cc

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    The stab does not produce down force on all aircraft, if it does why do so many have a positive incidence?, to create the lift needed for level flight maybe.
    Also what I speak of as to weight to cg I am referring to the center of lift not the center of gravity. When the plane is in straight and level flight does the canard and tail both not generate some lift and the degree of this lift can be affected by the displacement of on board weight. This weight is what I refer to when I say I set up my aircraft to fly right not balance on the c of g. C of g is a good starting point for the maiden but after becoming air born we realize the weight need's to be shifted to create a certain flying characteristic. Same as you did in the beginning by shifting weight to change the characteristics of how the nose was relating to the airflow, remember it would not lift and was over sensitive when forced. Could this also be why on any given model the c of g is usually a range instead of a hard line.

    Think of an Extra flown for 3D, the tail is heavy and will be made slightly more so for extreme 3D or lightened back to normal for normal flight. I am preparing to maiden an Extra tomorrow, it is slightly nose heavy to aid in a stable first flight. Once I trim it out and assure myself everything is right I will then shift the weight rearward to accommodate more of a 3D style of flight. If I get good enough to try extreme3D I will shift the weight ever so slightly more rearward.
     
  7. Doc

    Doc 50cc

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    I read some things Sal wrote about the Ilya Muromets and I recall him saying that it was balanced near the trailing edge. Had to be a real scary first flight.
    Doc
     
  8. Larry B

    Larry B 50cc

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    Hi Ace,
    The reason your stabs have positive incidence is because the wing has to much incidence so you are having to compensate for that. Do you set up your IMAC airplanes to a zero/zero incidence? The newer aerobatic models are flying the CG and the center of lift very close, and like you said it takes shifting the CG around to get the flight characteristics that you are after.
    I set up all my big scale stuff to be on the nose heavy side which makes for a very well behaved model, where as the 3D guys and IMAC pilots are after a different kind of handling model.
    Another big factor is the pitching moments that different airfoils have, and this changes with airspeed which is partially why you have to change trim on some airplanes as you change power settings. Another factor is you engine thrust line, up thrust, down thrust, left or right thrust all come into play.
    Have you ever flown a J-3 Cub? As you burn fuel you will find yourself cranking in more positive incidence to the stab, this is because the nose is getting lighter and thus the tai is required to apply less of a down force.
    Larry B P1010030.JPG P1010023.JPG
     
  9. Doc

    Doc 50cc

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    In my experience with models positive incidence in the stab is required to bring the stab and wing into somewhat equal angles of attack so the airplane doesn't suffer from the climb dive tendencies with throttle changes. I fly a lot of biplanes and also have decalage (SP) ie difference in angle of upper and lower wing. Most times biplanes are set up with the upper wing having more positive incidence, say a degree or two. I'm a rebel and set mine up with the upper a bit more negative so as the lower wing begins to stall the upper is still flying and thus the center of lift moves upward increasing stability. Seems to work for me. Doesn't answer any of the canard questions but all part of setting up different models
    Doc
     
  10. Doc

    Doc 50cc

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    My bad , sometimes I don't pay attention when I read.
    Doc
     
    Last edited: Dec 21, 2016

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