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Old Feb 28, 2011, 07:51 AM
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Don Stackhouse's Avatar
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Further thoughts:

Your wing apparently uses a flat plate airfoil with a squared-off leading edge. That airfoil is likely to have a laminar separation bubble just behind the leading edge, and that bubble may be unstable, especially at high alphas, which could result in aerodynamic hysteresis.

A proper airfoil, or at least a proper leading edge shape, could help. Note, a simple full-radius leading edge will probably not be sufficient.

Another fix might be a row of vortex generators just behind the leading edge, or alternatively a sawtooth leading edge. The sawtooth edge (maybe 0.5-1.0 cm 60 deg. included angle teeth) might be the easiest to add, but could become a manitenance headache, very prone to having the teeth broken or worn off.
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Old Feb 28, 2011, 08:05 AM
What could possibly go wrong?
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Mac

Don
Quote:
The way you're calculating C/G doesn't sound right, it sounds like it should be further forward.
That's what I think too. I used the geometry below to find the MAC via GMC of the delta wings and the standard 25% for the rectangular section. I then assumed that lift would be proportional to area, took the area of each section to make a weighted average. I flew with that at first and then steadily moved back until I got what I liked. The flying test I use is, after trimming for straight and level at about 50-60% throttle, is to get some altitude, then cut the engine and dive, hands off for as long as I dare. If she pulls out of the dive, then the up-elevator has been compensating for a too-forward C/G, and vice versa.

I wasn't surprised to find out that I ended up almost 50mm behind the calculated position, but I have no mathematical explanation. The outer section is flat panels, the centre section has an airfoil, roughly Clark Y. Maybe in taking its area I should have ignored the area taken up by the floats. That would work out closer. You would expect the tail and the idling props to have a pitching up effect in a dive, it seems to me, but that would result in me wanting to move the C/G even further back. Anyway, I'm happy as it is.

Don, I shall certainly try the extra fin you suggest, next time I fly. Thanks for that.

Nick
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Old Feb 28, 2011, 08:25 AM
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Note the statement in that diagram that "the MAC is only slightly bigger than the GMC, EXCEPT for highly tapered wings." You don't get much more sharply tapered than a delta, which suggests that your MAC is further inboard, and the C/G should be further forward.

Your stability test is the infamous old "dive test". It's unnecessarily abusive to the airplane, and does not give reliable results. It puts the plane in such an extreme position in its flight envelope that things like aerodynamic nonlinearities and aeroelasticity can cloud the results.

Just push the nose down a few degrees or pull it up a few degrees and let go. It isn't necessary to hold it at that position and let it stabilize there, you're looking for the transient responses. Just change the attitude, let go, and watch. VERY closely.

You can also input a transient disturbance in yaw, kick the rudder enough for 5-10 degrees yaw (enough to see the response clearly, but no more than that) and let go. Note the dynamic yaw stability in particular (how the oscillations damp out, how many oscillations, what change in amplitude from one cycle to the next, etc.), since that will tell you whether or not your fins are big enough.
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Old Feb 28, 2011, 08:30 AM
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Are these vortex generators? The 4 little plates spaced out under the leading edge:

Nick
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Old Feb 28, 2011, 08:39 AM
What could possibly go wrong?
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Don
Quote:
Just push the nose down a few degrees or pull it up a few degrees and let go. It isn't necessary to hold it at that position and let it stabilize there, you're looking for the transient responses. Just change the attitude, let go, and watch. VERY closely.

You can also input a transient disturbance in yaw, kick the rudder enough for 5-10 degrees yaw (enough to see the response clearly, but no more than that) and let go. Note the dynamic yaw stability in particular (how the oscillations damp out, how many oscillations, what change in amplitude from one cycle to the next, etc.), since that will tell you whether or not your fins are big enough.
I'll try those, thanks. That yaw test will necessitate the disabling of my link between motors and rudders - it's very fierce!

Thanks

Nick
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Old Feb 28, 2011, 09:18 AM
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Quote:
Originally Posted by nickchud View Post
Are these vortex generators? The 4 little plates spaced out under the leading edge:

Nick
Those are a form of vortex generator called "vortilons". They act more like wing fences to prevent stalled airflow from spreading out along the wing span, rather than energizing the flow over an entire region of the wing's surface.

Traditional vortex generators take the form of rows of small pairs of angled vanes:

http://en.wikipedia.org/wiki/Vortex_generator

Newer types that are a little more durable and easy to make are rows of triangles or rectangles:

http://www.it.cas.cz/files/fluid-dyn...ouckova_dl.pdf

In your case you could just cut a series of 0.5-1.0 cm equilateral triangles from Depron and glue them in a row on the wing just behind the leading edge, with a gap between them of maybe about 2-3 times the width of the triangles, with the point of the triangles aimed into the incoming airflow. The edges of the triangles should be sharp and square with their faces. Each of the two forward faces of each triangle will act like a vane, shedding a vortex from its aft tip that stirs up the boundary layer behind it, and helps keep the flow attached. See figures 4 and 6 in that second link.
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Old Feb 28, 2011, 02:02 PM
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Thanks John 235 and Perdu for the kind words.

John 235
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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.
I am envious of those who spin conventional canards. It seems that they will not spin without a rearward CG. The Delta Duck designs almost freeze from a spin with neutral controls. The spin is entered in a slow climb with right aileron, thanks Don, then full left rudder full up elevator and 3/4 throttle.
The amount of right aileron and throttle will dress up the spin for flatness and a slow descent rate. Then there will be plenty of time to admire the show and learn how it might be altered.

Nickchud, Your never ending ideas are great together with the comments which follow. Right now, my ideas are stuck in a rut. The Deltas can do much more than I can. It is fun to watch a great pilot wring them out.
Charles
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Old Feb 28, 2011, 02:35 PM
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Nick, Regarding your rocking problem, it seems that you have a flying center section with not enough area in the triangular wings for good lateral stability.
I would add a triangular section to the front of the wing to add area and to make a wider wing tip. It could be helpful to create a bit of washout on the wing tips to prevent stalling. Longer wings would be good if you have room to handle them. You will get less dihedral effect also. I am glad that it flies and you are enjoying it.
Charles
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Old Mar 03, 2011, 08:48 AM
What could possibly go wrong?
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Here's my WBT flying. Not a very good video, it was a dull morning, but it does prove she handles well over wide range of speeds and manoeuvres. I have the vortex generators in mind, so consider this the Before version.

Also, theres at least a couple of oz of lead in the nose, to counter the motors at the tail. Perhaps this has an inertia effect for pitching. I'm still hankering for small, delta shaped canards to speed up the pitch response. I flew my mini Polaris yesterday. Apart from the wide body and the extra weight, it's just the same but the handling is way more responsive.
WBT Flying (5 min 41 sec)
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Old Mar 03, 2011, 10:01 AM
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Building, flying, filming and voiceover - you're a veritable one man band, Nick!

The model seems to fly very well but the main thing the video shows is how much better the human eye is compared with the FlyCamOne!

I look forward to the next video made with a real camcorder on your head - just as soon as you've worked out how to operate the zoom with your eyebrows.
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Old Mar 05, 2011, 06:13 AM
What could possibly go wrong?
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Vortex generators

Hello Don

Is this what you had in mind, or have I got the wrong idea? It seems to help. I'm getting on really well with slow flying and rudder control. Apart from the usual pilot errors.

She flies off water, though the forward CoG and the flat, delta wing means that I don't have a lot of space to rotate for take off, The step is nearer the front than the back and the TE is inevitably lower than most conventional seaplanes. I screwed up a landing on the lake, flying a little too fast and not truly straight into the wind. Watch ..
Polaris WBT flying from water (4 min 53 sec)
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Old Mar 05, 2011, 08:24 AM
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Originally Posted by nickchud View Post
Hello Don

Is this what you had in mind, or have I got the wrong idea? ...
That's what I was thinking of. You could probably get away with making them not quite as high, maybe 1/2 to 2/3 that height, which might save some drag. It might be effective to have more of them, maybe twice as many, half as far apart.

Imagine a 15 degree included angle zone of air trailing aft behind each one, and note how far aft of the leading edge the zones intersect each other. That's the portion of the wing that's in their influence. I'd try to space them so their zones intersect maybe an inch or two (2.5-5 cm) aft of the leading edge, so there is little or no chance that you could get a region starting to separate before it's within the coverage zone.

It's going to be a bit like using a turbulator, you may have to experiment a bit to find the setup that has the desired effects but does not cause unnecessary drag.
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Old Mar 05, 2011, 01:28 PM
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Delta Duck 2 Flat Spin

John 235
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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.
Yes, John, The Duck spins better with the larger wing. At first it did a nose down tail spin but after moving the battery rearward by 3/4 inch, the flat spin worked really well. It is different from Duck 1 because it required no right aileron for a full left rudder spin. So far I have tried full left rudder and full up elevator with moderate throttle. The spin is rather floaty with very low descent rate. The landings are gentle but with positive nose up stability for three points. With less canard influence, incidence and motor down thrust, it flies fast without climb, flies inverted with less forward stick and with better visibility it meets all demands that I was hoping for. Maybe a video later if my man is for it.

This morning I took the Millennium Slow Stick Canard to the Perry, Ga. model show for sale. It sold in fifteen minutes with several wanting to buy it. There is definitely a more positive attitude now about canards. The new owner was getting lots of attention with it. The owner of Millennium was there and seemed happy with the enthusiasm.
Charles
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Old Mar 05, 2011, 01:36 PM
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Hey Charles!
Quote:
It sold in fifteen minutes with several wanting to buy it.
Wasn't that my plane?

Congratulations with the Duck 2. Bigger is often better! Good to hear of your results.

Cheers

Nick
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Old Mar 05, 2011, 01:59 PM
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Nickchud
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Wasn't that my plane?
Yes it was Nick , but there are more where that one came from to fly on your next visit. I also sold a Four Star 20 made from reduced plans and a Playboy Senior. All three were sold in 30 minutes and I left at 8:45 AM because I was afraid that I would buy something if I shopped.
Charles
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