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I have had a few e-mails about this feature where the correspondent is under the impression that I set out to design a Wing-in-Ground effect model. Not so. I set out to explore the flying characteristics of a swept forward wing. It's subsequent reluctance to leave terra firma led me to consider WIG as a possible cause. However to those who have taken the trouble to write; thanks for the interest.

 

 

This article first appeared in RCM&E in November 1998.  The original plans for these model are available for down load from this site in DXF format.

 

 

 

Please read this article before building and flying; its not difficult just different!.

 

Problems with WIG

This is not an article about the perils of  wearing a toupee while operating a model (although I dare say there are one or two funny stories out there somewhere)! No, the WIG referred to in this story is "Wing-in-ground-effect".

 

Ground effect is a natural phenomenon, as an airfoil approaches the ground its lifting ability increases and the drag reduces. The lift to drag ratio increases as the pattern of circulation around the airfoil changes.

 

dragratiothumb.jpg (3333 bytes)

The increase in lift is simply described by an air cushion which is created by high pressure that builds up under the wing when the ground is approached. When the ground distance becomes very small the air can even stagnate under the wing, this gives the highest possible pressure, so called ram pressure. The increase in lift is not all, ground effect also decreases drag.

This decrease can be explained by considering a full 3d wing.  See fig 1 left.

 

When an aircraft is flying high, without ground effect, the lift of the wings generates a downwash field behind it. This downwash field is bounded by two tip vortices, these are sometimes visible at an airshow : when a fighter flies at a high angle of attack, the water in the air condenses in the low pressure vortex and you see two curled lines extending backwards from the wingtips.

 

A lot of energy is stored in the tip vortices and this energy is wasted. This type of drag is called "lift induced drag". When an aircraft is flying close to the ground the vortices generated by the wing cannot be the same as in free air, because the ground is in the way. The vortices are smaller, so there is less energy stored in them. Conclusion: an aircraft experiences less lift induced drag when flying in ground effect. (Note for the purist:- downwash is also effected and reduced; thus reducing the induced drag from this source.)

 

All aircraft experience this, and some sea birds exploit it by flying close to the water surface, which requires less effort precisely due to WIG effects. This all sounds like something for nothing; so where is the problem?

 

Well, the exact point at which WIG effects occur, and the ways they effect the aircraft are a function of the aspect ratio and size of the wing. A conventional layout model with conventional aspect ratio will not see very significant WIG effects. Sometimes you may notice that the glide suddenly starts to flatten as you are about to touch down (remember lift increases and drag decreases), and that is usually the end of it. However, if you experiment with unusual planforms and configurations, then other WIG effects can be seen; sometimes with less desirable results.

 

Swept forward flying wing.

Some years ago I started toying with the idea of building a flying wing with a swept forward wing plan. An article in RCM&E in June 1992 by Alan Ham piqued my curiosity, but seemed to suggest that this type of design was fairly difficult to get right. This idea remained on my extensive list of ‘things to do when I win the lottery’ but was reactivated four years later when I saw Andy McNally’s Predator in RCM&E Sept. 1996. This was presented as a slope soarer, with small canards and exactly the type of wing plan I was thinking about.

 

Not being a slope soaring type, I determined to build it as a powered model, and decided that the ubiquitous .40 size engine would be ideal, as I had a couple of likely candidates languishing in my cupboard. For this size engine I ‘guesstimated’ that a 54" span model would be a good bet, and proceeded to eyeball scale Predator from the free Pro-plans provided with the magazine.

 

At the same time I decided to include ‘all the bells and whistles’ and so gave the model a rudder and all moving canards. A late decision was to include a tri-cycle under carriage. This type of u/c is not ideal for our patch; which can be very rough at times (i.e. whenever you want to fly from it!). However the design lent itself readily to a tric. u/c rather than a tail dragger. I did consider hand launching; but the thought that if the model wouldn’t fly it would have further to fall persuaded me towards an u/c.

 

The drawings where duly completed, and the model constructed, pretty much as drawn, with the result as shown on the photo. As I said this had pitch control via the all moving canards; roll by ailerons and yaw by rudder. The engine was a rather tired OS40. As per convention the u/c was set up to give a slightly nose down pitch. The wing incidence, c of g, and all other geometry were, as best I could tell, as per Predator. The first photograph shows the model just after completion in August 1997.

 

Flight trials (and tribulations)

The first flight attempts all shared one frustrating common characteristic; the aeroplane showed no tendency towards flight what so ever. It would move very smartly over the ground, but stuck to it like you know what. The only time it left the ground was when it hit one of the many bumps and lumps on the patch.

 

A series of modifications took place as follows:

  • Move the c of g back until the model was ‘teetering’ on the rear wheels:- no noticeable effect
  • Increase the throw of the canards to ±30° or so:- no noticeable effect
  • Increase the angle of attack of the wing: no noticeable effect
  • Change the u/c geometry to pitch the nose up:- no noticeable effect
  • Fit a hot .45:- moved faster and run out of patch quicker!

After many days (dragging into weeks with breaks for poor weather, etc. and many high speed runs into the long grass at the edge of the patch, and one spectacular cartwheel when one wheel came off during a run the model was looking decidedly worse for wear, but no more likely to fly!

 

Reflex reactions

As a last resort I started to angle the ailerons up, using them more as ‘flaps’ for extra lift. This also created an effective ‘reflex’ section to the wing; which some authorities suggested may be necessary in a swept forward flying wing. At long last the model showed a slight tendency to get airborne; although it was difficult to be certain, and my fellow club members where convinced I was just getting better at hitting strategic bumps!

 

Finally I re-programmed the radio to give flaperons, so that I could adjust the angle of the rear wing from the flap knob, and continued to try even more extreme angles for the rear control surfaces. This proved to be the key. With the ‘flaperons’ set to circa 20° positive (and very little up servo movement left) the model just left the ground with the assistance of a suitably large bump right on the edge of the patch. Whoopee!!!

 

The model immediately climbed away very rapidly indeed. First attempts at feeding in control movements showed very positive aileron responses smooth turns, and quite a docile, albeit fast, model. This was just as well, as I quickly found that the canards, which where coupled to my ‘elevator’ stick, had no effect on the model pitch what so ever! So now I had a model rapidly climbing towards the clouds with no pitch control. Super!!!!

 

"Don’t panic Mr Mainwaring!"

(This is an 'in joke' for UK modlellers of a certain age that remember a popular sitcom)

 

As I said ailerons were very effective and positive, so I could at least control its direction. I attempted to ‘trim’ the model using the flap knob; and found that this gave the desired pitch control. In fact I found that I could effectively fly the model on the flap knob! I actually did a couple of circuits or so like this, but decided not to push my luck, and chopped the engine and brought the model in for a fairly respectable landing. A different meaning to flying with positive reflex.

 

So I now had the answer. Couple the rear controls as elevons and fly with a suitable ‘reflex’ section by setting the elevons to a fixed positive angle. I retired to the work shop to carry out the necessary modifications. With this done the model proved to be very easy to fly, with no nasty vices, and a novel look in the air. The stall is none existent, it just flys at a higher and higher angle of attack, until it starts to drop in a sort of mushy glide.

 

With sufficient elevon movement rolls are spectacularly fast, and it does a very interesting tail slide with hammer head exit. That said the model is not going to win any prizes for its aerobatic capability.

The canards; which where still mixed to the elevons for pitch control, had no noticeable effect on the flight. So I took them off altogether, to create a true flying wing, which was what I had wanted in the first place.

 

But there is still something not quite right!

I flew the model on a number of occasions, but take offs where still its weak point. It seemed very reluctant to get airborne, and invariably only did so with the aid of one of the many bumps on our patch. This puzzled me. The model flew very well once airborne, with more than adequate power. It had a relatively low wing loading and a very low ‘stall’ speed; which it easily exceeded within a few feet of starting the take off run. So why was it such a pain to get off?

 

The model had been put to one side for several months, while other projects and activities took precedence; and I hadn’t thought any more about it. Then one evening I watched Channel 4’s Equinox programme about the various ‘Ground effect’ craft that are being developed, and the history of the Russian work during the cold war in the sixties and seventies. A very interesting programme and well worth catching if it comes round again.

 

During the programme they gave a fairly basic, but accurate, description of what happens to an aerofoil when it gets close to the ground. One remark in particular about the centre of pressure (CP) moving rearwards started me thinking..."is that what is effecting my design during take-off?"

 

Immediately after the programme I started to look for references to ‘ground effect’ in the few books I have about aerodynamics and drew a blank, other than general statements about increased lift and reduced drag, which I was already aware of. I then started to ‘surf the net’ and found a wealth of information on the various ground effect craft, and a few, albeit sketchy, bits of real technical data on the phenomena.

 

For those interested the best starting place is an excellent web site is THE WIG PAGE created by Edwin van Opstal. This has lots of good stuff, including most if not all the material shown on the Equinox programme, and links to probably every other site about WIG and WIG aircraft. Very intersting site, lots of pics and well worth a visit.

 

Under pressure.

Based on the a couple of graphs and statements about lift, drag and CP that a gleaned from this exercise, and with a few back of the envelope calculations I came to the following conclusion. The movement in the CP is rearward; at ‘zero’ ground clearance its is at 50% mean aerodynamic chord (MAC); while in free air it is typically at 30% MAC.

 

hieghtratiothumb.jpg (2271 bytes)

From the data I found, for a wing with aspect ratio (AR) of 3.0, the ratio of L/D starts to change when the ground clearance is down to 0.3MAC, and is virtually complete when it reaches 0.15MAC. See figure 2. To date I have not found a similar graph for the movement of the CP; but it is reasonable to assume that it moves over the same interval of 0.3-.015MAC above ground.

 

My model has an AR of circa 3.3, with a MAC of 16ins (40 cm). Hence 0.3MAC is 4.8 ins (12 cm).The u/c gives a ground clearance of 4.5-3.5ins (11.4-9.0 cm), with the wing angled up and the rear closer to the ground than the front. Hence the wing is flying into the ground effect region, and so the CP is starting to move back.

 

This being the case then during the take of run the CP is more rearward, so the nose is held down and aircraft is reluctant to lift. However eventually the model has sufficient momentum to it hit a bump in the patch hard enough to throw it into the air. This immediately ‘breaks’ the ground effect and the model flys away rapidly.

 

Once into the air everything is OK; but this is why it seems to stick like glue to the ground and then suddenly leap skyward.

 

It is possible that with a smooth runway the model may never get airborne, as it will not be assisted skyward, and will never move out of ground effect conditions. Alternatively enough airspeed will be achieved for the elevons to be effective enough to overcome the forward pitching moment caused by the CP shift. If anybody has a suitable site I would be very happy to bring it along and test the theory!

 

So there you have it; my theory on WIG. If anybody has any other views then I would very pleased to hear them. I am not an expert and my analysis is far from rigorous, but the theory seems to explain my observations. I have tried to obtain a more detailed description of ground effect theory and practice; and have spend several hours browsing through NASA’s technical library (again through the internet) but have found nothing more definite than the ‘simple’ description cited above. I would love to find a source of information that describes this effect, and in particular at the low Reynolds numbers that model aircraft operate at. If any body has any information I can be contacted by email .

 

Post Script

 

Since this article was written I have flown the model again off the patch of my current club Aldershot Model Club. Now our strip isn't a smooth strip of tarmac but it is a lot smoother and longer than the patch I was originally flying from. In place of the 'hot .40' I fitted my good old standard Laser 60 (plus a significant lump of lead at the rear to counter-balance!)

The model took off with no trouble although it did require a lot of runway, and was certainly going a lot faster at lift off than at landing. I didn't do much flying though before I took the engine out again. So I'm still not sure if this model sees excessive ground effect or not.

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