Canard Forum: Show,Discuss, Learn - Page 339 - RC Groups
Thread Tools
Feb 21, 2011, 10:53 AM
Registered User
cwesh, thanks for the info. I am still trying to locate my plans but so far only have the construction manual...And all it says about CG is to consult the plans!........Robert
Sign up now
to remove ads between posts
Feb 21, 2011, 11:18 AM
Registered User
Can anyone show us a picture of the Star Chaser?


No Pics but here is this:
Feb 21, 2011, 12:38 PM
Registered User

Two Star Chasers.

Don Stackhouse
Let me repeat that:

Incidence does NOT control C/G !!!!!!!!

C/G controls stability. As long as the planforms, areas, moment arms, etc., are the same, C/G should be the same. On a canard in particular it also controls how much load is carried by the canard, and therefore whether the canard stalls before the wing (good), or vice versa (bad, probably very bad).
Don, I certainly do respect your views and will edit my post to refer to your post #5067.

The version which Cwesh has with the GC at about 4 inches in front of the main wing's LE must have zero degrees aft and 2 degrees front.

With the Nitro version with 2.5 positive aft and zero up front and a light E motor behind, you would be set up for an outside loop with the CG at 4 inches in front of the wing and neutral elevator. I assume that the model was tested and set up to fly on the main wing with over a pound of 0.46 engine and fuel sitting behind the center of the wing. It would have to fly nose high with a lightly loaded canard.The main wing would certainly stall first as I see it and I picture the CG at 25% to the rear of the wing's MAC. If balanced
with the calculator it may fly with full UP elevator but it would be speed sensitive, I believe. Nick and Trevor: thanks for your views.
repinfl, Your picture certainly helped. Thanks.
Feb 21, 2011, 01:46 PM
Registered User
Don Stackhouse's Avatar
Originally Posted by canard addict
...With the Nitro version with 2.5 positive aft and zero up front and a light E motor behind, you would be set up for an outside loop with the CG at 4 inches in front of the wing and neutral elevator.
Which simply means you would not fly it in level flight with neutral elevator.

The other thing that is being overlooked in this discussion is the reference used for measuring that incidence. What really matters is the incidence relative to the zero-lift-lines of the wing and canard airfoils. If you have a different airfoil for the canard than for the wing, then you need to consider the difference in their zero-lift-lines relative to the chord line or to the tangent to the underside of the airfoil (whichever you're using for your reference of measurement) for your measurement to have any meaning. If you had a more highly cambered airfoil on the canard, based on the chord lines you could have a negative incidence on it in comparison to the wing, and yet its incidence could be positive relative to the wing based on the zero-lift-lines of the two.

Note that you also have to consider things like washout. If you have positive 2.5 at the root and about 5 degrees of washout, the geometric incidence as measured at the wing's MAC, even before considering airfoil effects, would be about zero.

In any case, you still should not use C/G location to set flight trim. Wing incidence sets the angle the fuselage flies at when the plane is at the design airspeed, and the canard incidence PLUS THE ELEVATOR TRIM SETTING is what puts the plane in trimmed flight at that airspeed.

C/G determines how the load is shared between the wing and the canard (which along with the stall characteristics of each flying surface determines which one will stall first), and also the amount of static stability in both pitch and yaw. Those are critical parameters that are fudamental to the design of the plane, not something to be monkeyed with lightly.
Last edited by Don Stackhouse; Feb 21, 2011 at 01:56 PM.
Feb 21, 2011, 09:35 PM
I post, thread dies!
Repinfl, (can you here me through the noise? )

That is the same as the one I have. The box label is dated October 1988.



P.S. If you can't find the plans, I can scan mine in and email you a copy... (will take a bit of time...)
Feb 26, 2011, 10:07 PM
Registered User

Delta Duck 2 Maiden

Our Saturday weather was a deep blue sky with a high of 70 degrees and smooth winds at about 3 to 5 mph straight down the runway. With only one flying session since the new year, my knees were weak with the thought of trying to fly a new model. My friend Richard was at the next table and seemed to have no fear of testing the Duck. At 160 watts per pound it was quickly at high altitude and Richard was happy with the neutral surface settings. It had just the right amount of nose down trim which Richard said
suited him perfectly. He did several inside loops followed by a couple of high speed outside loops which could have removed the wing from the fuselage. The axial rolls were smooth with a bit of added rudder and elevator. When the transmitter was passed to me, I added five elevator UP clicks to feel comfortable with just a touch of back pressure required for level flight. This Duck's canard incidence was reduced by one degree from Duck one's two degrees positive which with the two degrees down thrust, successfully prevented the quick altitude gain at high speed. On landing, I added two more clicks of up elevator on the DX-7 and landed three points at zero throttle with the prop windmilling with no brake. The added area of the wing
has added a gentleness to the handling which I like. I could have flown it many more times in the perfect conditions but did so only once more. The seven clicks of up trim looked like less than two millimeters of change on the main elevons. The next time out, the flat spin will be attempted but my gut feeling tells me it may not happen with the larger wing.
Feb 26, 2011, 10:42 PM
Registered User
John235's Avatar
Hi Charles, Congrats on the success with the Duck 2. The pleasing result is not a surprise to me at all - I'm just glad it went without incident. I don't doubt the duck 2 will be able to do a spin. I think it will be no trouble providing you have enough rudder authority. I'd be more concerned about having enough altitude to get out of it.

I have recently experienced a spin (not a spiral dive!) with my 60" canard slow soarer. I found it will go into a spin if I use too much rudder in very slow turns. I think it occurs due to stalling one side of the main-wing. For future canard soarers I will look at revising the wing planform to avoid this problem. Fortuantely it will come out of the spiral dive very quickly by applying down elevator and opposite rudder. I see it as an interesting discovery because I have learned something I can apply to future canard designs.
Last edited by John235; Feb 27, 2011 at 02:35 AM. Reason: Corrected terminology
Feb 26, 2011, 11:37 PM
Registered User
Don Stackhouse's Avatar
Note, a spiral dive (also called a "graveyard spiral") is NOT the same thing as a spin. In a spin, one wing is stalled. In a spiral dive, neither wing is stalled.

The difference is not just academic. In the case of a spiral dive, pulling more "up" elevator just tightens the turn, and actually makes airspeed and "G" increase, right up to the point that the wings come off. The case of a pilot flying into clouds, either without the necessary instruments and/or the training to use them properly, typically ends with the plane ending up in a spiral dive that accelerates until there is a sudden increase in dihedral and decrease in wing span, followed shortly afterwards by a very deep hole in the ground. It's the classic "VFR into IMC" ("Visual Flight Rules into Instrument Meteorological Conditions") accident scenario.

Sometimes the tail fails first, which causes the plane to pitch nose-down so violently that the wings then break off downwards due to the sudden negative "G".

Conversely, a spin is one of the two maneuvers that is stable by itself and that holds airspeed down at a safe level, without needing corrections from a pilot (the other one is a forward slip). One wing is stalled, the other isn't, they start chasing each other around like two dogs chasing each others' tails, nobody is holding up the airplane, the airplane comes down, but the drag is high enough that it comes down at a safe airspeed.

Back in the barnstorming era, the spin was one of the maneuvers used to safely escape from getting caught on top of a cloud layer. As long as you knew there was enough altitude under the clouds for a safe recovery, you could just stall the airplane and hold full back stick and full rudder, and wait until the plane had spun safely down through the cloud layer, then recover when you could see to fly again. The massive drag of the stall would keep the plane's airspeed down to a safe level. The tricky part of a spin, where you do have a risk of exceeding redline airspeed and/or pulling the wings off, is the recovery, after you stop the rotation and break the stall.

If you had tried to fly blind and wings-level down through that cloud layer, sooner or later your inner ear's inability to distinguish between gravity and centrifugal force would end up working you and the plane into a spiral dive, followed shortly after that by you and the fuselage reaching the ground long before the rest of the airplane.

No, a spiral dive may superficially look similar to a spin, but the two are really quite different maneuvers.
Feb 27, 2011, 12:38 AM
Registered User
John235's Avatar
Don, Thanks for your post. Based on your explanation I think that my model entered a spin rather than than a spiral dive. The first time I experienced the issue was at low altitude and the model decended into some bushes without being damaged. After that I have reproduced it and learned that it isn't very hard to recover from it.
Feb 27, 2011, 02:15 AM
What could possibly go wrong?
nickchud's Avatar
Congratulations Don! No surprises there.

And I've learnt something new about spiral dives, clouds and the barnstorming era. Really interesting! Thanks Don.

Feb 27, 2011, 08:54 AM
Registered User
Don Stackhouse's Avatar
The recoveries from the two maneuvers are similar, but not identical.

To recover from a spiral dive, you ease off the "up" elevator, use coordinated rudder and aileron to roll back to wings-level, and pull out of the dive. Note, a spiral dive is a coordinated maneuver, while a spin is anything but coordinated.

There are a couple methods for spin recovery. The traditional one is to apply opposite rudder to stop the rotation, then down elevator to break the stall (do not apply them simultaneously, because the down elevator could blank the fin and put the plane into a flat spin!) with ailerons neutral, then up elevator to pull out of the dive.

The ailerons-neutral part can be very critical on some airplanes. For example, the Mig 15 was very sensitive to this and would not recover if the ailerons were not precisely centered, to the point that they painted a large white stripe on the bottom of the instrument panel to give the pilot something to aim at with the control stick.

On some airplanes that method does not always work reliably. An alternate that has become more popular recently is to let go of the stick, let the ailerons and elevator trail at whatever angle the airflow puts them, while applying opposite rudder to stop the rotation.

However, this method may not work on models (or at least not the same as in full-scale) because our servos cannot be "back-driven". When we release the stick, our control surfaces will go to neutral, rather than being allowed to trail with the relative wind. However, there's a good chance that will still get you an acceptable recovery. Most models come out of a spin fairly easily, probably because they tend to have much less rotational inertia ("flywheel effect") about the pitch, roll and yaw axes, in comparison to full-scale aircraft. In fact, on many, if not most, models, just releasing all the controls (including the rudder) so that they all go to neutral, will be enough to stop the spin.
Feb 27, 2011, 10:22 AM
Registered User

Congratulations! for your Delta Duck 2 maiden flight.

Feb 28, 2011, 03:45 AM
What could possibly go wrong?
nickchud's Avatar

some advice, please

Below are some pictures of my current project, it's a Polaris converted to have a wide body and twin motors. I'm getting great success with it, lots of fun and very good, tight control. The latest development is to use an elevon mixer to control the motors differentially via the rudder channel. Works a treat if I'm careful with it. BTW, no problems at all with thrust versus pitch.

The outstanding problem I have is wing rocking at low speeds. For high alpha, it seems that I've fixed the problem by introducing a small down deflection of both ailerons controlled by the flaperon switch. However, I have had some mishaps on takeoff if the conditions are not perfect, caused by wobbling and catching a wing tip on the uneven ground.

For the first time in my flying career I'm wondering if expo might be needed. If I am more gentle with the elevator on take-off I can avoid pitching up too much too soon.

Being a canard fan, I wonder if a small, delta shaped wing on each side of the nose, working as elevons might fix the problem. Any ideas?



As a side note, I tried to calculate the C/G, using Geometric Mean for the delta part of the wing and using a weighted average of that and the rectangular central section. But, after about an hour of flying I am convinced empirically that the true neutral point is about 50mm behind that. She flies very well as per the drawing.
Last edited by nickchud; Feb 28, 2011 at 04:44 AM.
Feb 28, 2011, 04:45 AM
Registered User
Trevorh's Avatar
NIck, congratulations on another creative project!

I'll be interested to read what Don has to say but I imagine that your wing rocking is caused by air spilling out of the tunnel between the sponsons. My Miss Hyperion Hydroplane used to do the same thing (and it had no wings at all outboard of the sponsons).

I'm not sure the problem can be eliminated, but maybe a bit of dihedral on the tips would help to avoid snagging them on the ground during takeoff?
Last edited by Trevorh; Apr 26, 2012 at 01:57 PM.
Feb 28, 2011, 06:41 AM
Registered User
Don Stackhouse's Avatar
The way you're calculating C/G doesn't sound right, it sounds like it should be further forward. You should be able to treat the delta panels just like a regular panel that happens to have a lot of taper. Find the MAC, then find the AC, just like you would any other straight-tapered panel:

Of course flight performance is the ultimate judge. If you shove the nose down a few degrees, or pull it up a few degrees, and then let go, does it return to the original attitude smoothly and reliably, with at most two or three oscillations?

Also, if your static pitch stability is adequate, but yaw stability is poor, moving the C/G forwards will help that as well.

Two possibilities:

1. Some airplanes (such as the full-scale VariEze) exhibit wing rocking near stall. However, that doesn't feel right to me in your case.

2. The dihedral effect of sweep increases with angle of attack. Planes with a lot of sweep act like they have too much dihedral at high alpha and Cl, which shows up as a dutch roll problem. At high speed and low alpha, where the dihedral effect goes away, they may also have spiral instability.

Your plane does not have much fin, and the fin effects of those sponsons (which add a lot of side area ahead of the C/G) makes this even worse. Odds are that your static yaw stability is pretty feeble. Combine that with lots of dihedral effect at high alpha from the large amout of sweep, and you have dutch roll.

Assuming it's that second problem, possible solutions are some anhedral in the outer panels, and/or more fin area. The easiest one to try is probably adding more fin area. Just tape in another fin in between the two you have, and see if it helps. If it works, then you can come up with somethign prettier. Staying with just two fins but making them taller and larger should work better than three fins. Leave the horizontal tail and the motors wher they are, but add additional fin area above them.

However, more fin area will cause more spiral instability at high speed. Airplane design is almost always a compromise.

Another quick fix would be active stabilization. Get a helicopter tail rotor gyro, install it oriented to sense yaw, and couple it to the rudder servo. This will add yaw damping, without increasing fin area.

"Spillage problems" from between the sponsons, like we see in hydroplanes, requires a free surface (the water surface under the sponsons) to occur. Unless this is only happening when you're in ground effect, almost touching the surface, then that is not your problem.
Last edited by Don Stackhouse; Feb 28, 2011 at 06:49 AM.