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Old Jan 23, 2015, 11:13 PM
who has rabbit ears down
Captain Canardly's Avatar
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looks nifty sir Addict! now it looks like a folding prop is in order!- oh yeah- yours spinnss alll the tyme!- I ferget!
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Old Yesterday, 09:11 AM
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Thank you, Captain. A folding prop probably would not work with the large nacelle which could have been smaller. You mentioned the word spin which raised my blood pressure. What is the probability that the BD will spin? The static margin is about 7% unless weight is added rearward. I hope it will thermal.

Charles
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Old Yesterday, 03:04 PM
who has rabbit ears down
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I'm talkin' of the propellor- ya see, myne only spin for a period of tyme,switch off @ 400 ft, or 30 seconds, then automatically switch on in ground effect (i'm working the details)
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Last edited by Captain Canardly; Yesterday at 03:06 PM. Reason: sequenced language
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Old Yesterday, 11:07 PM
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Don Stackhouse's Avatar
United States, OH, Bradford
Joined Jun 2005
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Quote:
Originally Posted by canard addict View Post
...My Boy Duck was designed with no previous model to base it's performance on. Could you tell by looking at it what I should expect regarding turning ability and dutch roll tendencies?
No, but I can make some wild but somewhat educated guesses.

Quote:
The CG is at the center of the motor and the fuselage. The dihedral of the canard is 3 degrees
Bad for yaw stability, requires additional fin to make up for it. The bulge at the forward end of the fuselage is also destabilizing in yaw. However, all of that is still better than all the lateral area ahead of the C/G on the pusher version in your earlier post.

Quote:
and the main wing tips are 10 degrees.
Looks like overall an equivalent dihedral angle ("EDA") of about 8 degrees, which should be enough for adequate roll response. More than what it needs for basic roll stability, but if you want to use rudder for roll, you have to do that. Doing so does open the door for possible dutch roll unless you have enough fin.

Quote:
The wings are carrying a balanced load because it lifted off momentarily with a horizontal attitude. The wing loading is 8.7 oz./sq. ft. It has a 30% canard and the small 1300 battery sits only 7 inches forward of the CG.

Charles
Prop is slightly ahead of the C/G, but close enough that it's effect on stability should be minimal either way. I expect you will have some nose-down trim change with power, and there will probably be very little you can do about it. Normally with a conventional aft-tail we would use an upward thrust line, blowing downward on the stab, to hold the nose up despite the high thrust line. In this case that is not an option.

If it had tricycle gear you would also see an increase in takeoff run because of the lack of propwash on the elevators and the nose-down effect of the high thrust line, but with a tailwheel you should be OK, assuming it doesn't nose over when you first open the throttle (you might have to be a little careful with that at the beginning of the takeoff run, and open the throttle gradually).

You have what looks like a decent fin area, but the moment arm is short, and the nose shape and the dihedral in the canard don't help. When it comes to dynamic stability, moment arm counts for more than tail surface area. The motor is on the C/G (good for inertia in pitch and yaw), but the battery is well away from the C/G (bad for pitch and yaw inertia). You might be OK, but between dutch roll and spiral instability, I would expect dutch roll.
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Old Yesterday, 11:55 PM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
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Thank you for that input Don. This is one of my reasons for following this thread.

Charles showed me his BD last week. I think I remember that the battery was very close to the CG. Low down however.

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Old Today, 11:10 AM
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Don, Like Nickchud, I want to thank you for your thoughtful inputs. With our combined thinking I now feel much better about the maiden flight. I measured the lever arm from the rudder hinge line to the CG and got 18.5 inches. You are right about the forward torque of nose down with sudden throttle especially on grass. I was worried about the almost 2 inch square firewall behind the motor but at speed it may offer enough drag to be beneficial to hold the nose up. I can always add a taper to it forward to the motor front. Let's just hope that the dutch roll is absent because I have passed on two models with it. I am not clear about the bad effects of the nose hump. The high thrust line worked well on the Equaduck because at full throttle the nose stays down an avoids the fast climb which was present on the Delta Duck 1 with a one degree canard as I recall. Once more it was nice of you to offer you thoughts.

Charles
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Old Today, 11:52 AM
Lookin' up at the centerline
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Old Today, 11:57 AM
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Nick, While you are sitting at home eating popcorn I am taking my new Duck to the field. I ate popcorn Friday and only got elevated BP.

Charles
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Old Today, 04:44 PM
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Don Stackhouse's Avatar
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Charles, the drag of the firewall is NOT an advantage. The plane needs a certain amount of thrust to sustain flight. The drag of the firewall is in effect negative thrust. You need more power from the battery making more thrust from the prop (which also reduces the prop's efficiency in the process) to make up for the firewall drag. The total force of the thrust minus the firewall drag has to be the same either way, so there is no net effect from firewall drag other than to alter the rate at which you drain your battery. If you can streamline the firewall, by all means do that.

The hump on the nose adds area in the side view, which acts like fin, only on the front end of the plane, like putting the feathers on the wrong end of an arrow. That reduces your yaw stability. The square corners on the fuselage up front accentuate this effect. If you can more generously round those edges, that would help reduce the effect. The dihedral in the canard makes it act a bit like a V-tail in terms of yaw stability, except that once again this "tail" is on the wrong end of the plane, reducing the total stability in yaw. You need extra fin on the back end of the plane to counteract all of this.

Whether you have enough now to take care of this could be calculated, but it's pretty tedious work that requires a full set of drawings and a bunch of time. Or you can proceed with what you have and hope for the best, or you could make a small FF model of what you have and do some glide tests and spin recovery tests before flying your bigger and more expensive RC model.
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Last edited by Don Stackhouse; Today at 04:53 PM.
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Old Today, 05:47 PM
What could possibly go wrong?
nickchud's Avatar
Market Harborough
Joined Apr 2006
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Good luck Charles! I'm sure you'll have no need of the quick reactions and piloting skills you displayed when I was there.

The small FF model that we were using for glide tests looked very steady.
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