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Oct 09, 2012, 07:48 AM

Back in China . . . sigh . . .

I haven't posted for quite awhile. My first plane flew although it was underpowered as expected - a speed 400 just isn't enough for a 950 gm plane. Good control and stability - which didn't quite make up for a totally inexperienced pilot. Over all a success.

I recently read an paper based on wind tunnel tests of two identical airfoils mounted in tandem. These were 2-D tests so this result cannot be assumed to apply to real wings, however, even with a decalage of -10 degrees (AoA of the the back wing being 10 degrees greater than that of the front), it was not possible to get the flow over the back wing to separate before flow separation occurred over the front wing. The researchers attributed the extreme stall resistance of the back wing to the decrease in the AoA of the back wing caused by the downwash of the front wing. The highest L/D for the two wing system resulted from a decalage of +5 degrees and an AoA of the whole system of +5 degrees.

This might apply to wings of equal span, but I suspect that the up-wash outboard of the canard might precipitate an early stall on the outer part of the main wing in layouts where the canard span is significantly smaller than that of the main wing.

I am thinking of a box wing canard (three wings) for another slow flyer. The box wing has high planform efficiency, combined with some structural advantages for a slow speed design that is not being optimized for low drag. The lateral area of the vertical endplates gives some justification for a canard - this lateral area is further back than in a conventional box wing layout. Whether it is possible to get three wings to work as an efficient system is a question that intrigues me.
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Oct 10, 2012, 12:28 AM
What could possibly go wrong?
nickchud's Avatar

The Dive Test

A picture speaks a thousand words.

Here's a really good diagram produced for glider pilots. The glide test, sorry, "Dive Test".

I thought it might be helpful.

PS Mitchell. I understand all that, but what's a box wing? Are we talking about a bi-plane with solid fins at the wingtips?
such as this one:

Burgess-Dunne 1914
Last edited by nickchud; Oct 10, 2012 at 10:35 AM.
Oct 10, 2012, 01:04 AM
Yup! That's the idea.
Oct 10, 2012, 01:28 AM
I'll find the article about the wind tunnel tests of tandem wings and post the URL here. It is interesting but not really very useful. Because the airfoils are fixed in the wind tunnel any relationship to stability can only be inferred indirectly - not a very reliable method.

From what I remember of the data, positive feedback would likely result from an increase in lift coefficient causing an increase in the difference in lift coefficient between the front and back wings - with the lift coefficient of the front wing increasing more rapidly. Definitely destabilizing! However, there are many canard designs that seem to have at least neutral pitch stability. A bit of a head scratcher.
Oct 10, 2012, 08:34 AM
Registered User
Don Stackhouse's Avatar
Nick, one point to remember on the "dive test" is that the old method (steep initial dive, maybe 30-45 degrees) gives unreliable results, and also can be very abusive to the plane. It can run the plane into corners of its flight envelope where responses can become nonlinear, and where aeroelasticity can distort the results. In extreme cases it can damage the plane.

Use only very shallow dives and climbs, 5 degrees or so. Better results, and much less abusive. Pull the nose up 5 degrees and let go, the plane should nose back over into level flight, with maybe 2-3 steadily decreasing oscillations before damping out the disturbance. Push the nose down 5 degrees and let go, it should nose up, maybe a few oscillations before damping out to the original flight path and airspeed.
Oct 10, 2012, 11:15 AM
What could possibly go wrong?
nickchud's Avatar
Thanks Don, I remember your mentioning that before and I cut my dive angles from about 45 to 30 degrees.

I see what you're saying about extreme corners of the flight envelope producing unreliable results. However, my understanding of the purpose of a dive test was to reduce the moment of the CoG working vertically downwards versus the Center of Lift, working at right angles to the airflow. In this way, the effect of any trimming in pitch becomes more significant, especially as speed increases.

I guess your point might be that the increase in speed is enough to amplify the effect of any trimming, yes?


PS, Any thoughts on my earlier heresy? I have my Starship CoG set somewhere so that the Main Wing stalls first. It's easy enough to get out of trouble by adding power. The landings are now more inclined to slide in gently when I flair, rather than go into a nose dive. Another attempt to guess your answer: "Easy enough? only if there is power available to add."
Last edited by nickchud; Oct 10, 2012 at 11:28 AM.
Oct 10, 2012, 11:49 AM
Registered User
Don Stackhouse's Avatar
30 degrees is still way too much.

If you have a statically stable airplane, putting the nose down, even just a few degrees, then releasing the controls, will make it want to return to the trimmed airspeed and angle of attack. That's all you need, just a few degrees, not a screaming, tuck and/or flutter-inducing dive. Likewise, pulling up 30-45 degrees into a violent stall will not tell you as much about the plane's static stability as you can learn from a gentle pull-up into a shallow climb.
Last edited by Don Stackhouse; Oct 10, 2012 at 11:55 AM.
Oct 10, 2012, 10:58 PM
Here is the link to the article I mentioned:

Note the significant influence of the back airfoil on the front airfoil.
Oct 19, 2012, 06:31 AM
And a video of it flying

Canard fly (1 min 31 sec)
Oct 20, 2012, 01:53 AM
Registered User
John235's Avatar
teopbako, You are too modest using the word "attempt". That looks like a very nice flying model. Especially good considering its flying with only rudder and elevator. Very well done!
Oct 20, 2012, 06:16 AM
AMA 125691
wmcneal's Avatar


Here's a unique canard that showed up at the flying field this week. I think someone got creative with a Balsa USA Enforcer kit to make it look like an XB-70 Valkyrie. Yes, it's nitro, but I just wanted to share it. Not sure they got the cg quite right on it because every time they wanted to bring it by for a low pass it wanted to dive towards the ground, but it made for an exciting video anyway.

Nitro Delta Canard at JaxRC (6 min 22 sec)
Oct 22, 2012, 02:04 AM
What could possibly go wrong?
nickchud's Avatar
I have a theory I would like to run past the people on this thread. CaptArmour and I have been puzzling about the problem of making a plane that can fly both in and out of Ground Effect. I think I have a solution involving a canard and a gyro.

The problem is that, when the plane is set up for GE, the CoG needs to be too far back for safe flight out of GE. For instance:
Flightship more trials (0 min 32 sec)
Oct 22, 2012, 02:22 AM
What could possibly go wrong?
nickchud's Avatar

My theory

This little gyro from Hobbyking costs $29. It works on a single channel between the Rx and any servo.

Lets add an all-moving canard to the WBT plane in my picture. Plug it into channel 6 via the gyro. Out of GE, we make channel 6 into the slave of the elevator. In other words, the canard and the elevator work together for pitch control in normal flight. The gyro will reverse any unwanted changes in pitch.

But, in GE, we flip the switch so that the gyro is in sole control of the canard. It's job is to keep the plane level in pitch. As speed increases, lift increases until we start to leave GE, so there's a nice equilibrium. This equilibrium is normally upset by a dramatic shift forward in the Centre of Lift. Hopefully, the gyro will compensate for that by using the canard to push the nose down.

To escape from GE, we flip the switch again and go flying.

What could go wrong?

Oct 22, 2012, 04:00 AM
Registered User
Trevorh's Avatar
If I understand you right Nick, when in ground effect, the gyro is keeping the model at a fixed pitch attitude using the canard, but you still have control of the elevator.

If so, I can see two issues:
1. What pitch attitude is the gyro going to try to hold? It seems to me that you would need at least to be able to trim this, perhaps with a slider or rotary control on the Tx.

2. What do you expect the elevator to do? If the gyro succeeds in its task of maintaining the pitch attitude then, for a given speed, it seems to me that down elevator would cause the model to ascend and vice versa i.e. down elevator tries to lift the tail, so the gyro responds by applying 'up' to the canard, with the result that both canard and main wing are now generating more lift.

Don't get me wrong, I think that there is mileage in the idea of using a gyro to tame the pitch instability, but I don't think I've fully understood what you are trying to do with this particular control setup.