Competition Flying In Sigma


E.C.N. Goodhart

The point of this talk is that I believe that competition flying in Sigma will be very different from what we are used to, so I am going to start by showing you what is different about Sigma.

Well, for a start, we can see from Figure 1 and Figure 2, that it certainly looks different. The aspect ratio flap-in is about 36 and flap out it comes down to just under 27. It weighs about 1500 pounds and has a wing loading of 11-1/2 lbs/sq ft flap-in and 8-1/2 lbs/sq ft flap-out, but despite these very high loadings the stall speed is 42 mph due to the very high camber of the wing with the flap out.

Figure 3 shows the flap-in and flap-out wing sections. Both sections are laminar flow Wortmann airfoils. The flap-in section has a good performance over the whole cruising speed range and the flap-out section is designed to give good L/D's in the very high lift range, i.e., solely for circling flight.

That introduces the flap which is the main feature of Sigma. Now, let us look at the other control surfaces shown in Fig. 3. The tail end is conventional with all moving horizontal stabilizer with full span anti-balance tab. But on the wing we have what at first sight seems a grossly excessive number of assorted control surfaces. The ailerons cover over three-quarters of the span but are of very narrow chord; they are attached to the trailing edge of the flap. A similar surface over the rest of the span is used together with the ailerons for camber changing purposes in cruising flight.

The ailerons are not sufficient for max rate of roll at low speed so an additional surface in front of the aileron is used as a spoiler to provide additional rolling moment.

Further surfaces on the inboard wing are used as air brakes in addition to the tail parachute.

When the air brake surfaces are extended, the camber changing flap is deflected to reduce the pitching moments.

One other important point is that the flaps are hydraulically operated thus, provided there is power in the accumulator, they move instantaneously from one position to the other. Keeping the accumulator charged requires a lot of effort and this will be done by pumping with both feet on the rudder pedals. We have arranged this by allowing you to disconnect the rudder adjustment lock and having a wire joining the pedal slide to the hydraulic pump. Maybe as many as 20 pumps per flap in-out operation but there should be about five cycles in the accumulator before you start.

You may wonder why Sigma's fuselage joins the fin in the middle instead of at the bottom as in conventional gliders. The requirement is that the fuselage shall lie along the streamlines in cruising flight and this defines the relationship between the fuselage and the wing chord line. With the flap down, the landing attitude is almost exactly the same as cruising flight so we had to put something under the fuselage to mount the tail wheel on. Part of the fin seemed the obvious answer. Incidentally, in the same context, the rather long, stalky undercarriage is fitted in order to keep the wings Well clear of ground obstructions even when they are in their static deflection position, i.e., drooping.

Now we will look at Signals performance. Figure 5 shows Sigma in comparison with Cirrus. Down at the slow speed end Sigma is very slightly worse: in narrow thermals the rate of climb is likely to be about 35 fpm less an this loss is almost entirely due to the fact that the weight we have selected for Sigma produces a stall speed about 2 mph higher than Cirrus. But at cruising speeds, Sigma has a glide ratio up to 70 percent better. This results first of all in an achieved cross-country speed which is 16 percent better than Cirrus on a day, when Cirrus climbs at, 2.0 knots (200 fpm) and increases to 25 percent better on a day when Cirrus climbs at 5 knots (500 fpm).

Since Sigma's climb is slightly worse than Cirrus, it is clear that all the extra performance comes from high speed cruise at shallow glide angles. Figure 6 shows the cruise speed against achieved rate of climb for both gliders in the normal working range, Sigma cruises 20 knots (23 mph), or more faster than Cirrus and what is equally important, at the best cruising speed Sigma is cruising about six glide ratios better.

To put numbers to this, let us look at Sigma and Cirrus starting side by side at the bottom of a thermal in which Cirrus climbs at 400 fpm. We will assume a climb of 2000 feet is available. Figure 7 shows the distance against time of the two gliders.

Sigma takes half a minute more to climb but as the cruising speed is 97 knots (111 mph) compared to Cirrus' 74 knots (85 mph), Sigma passes Cirrus after 2-1/2 miles and covers 2-3/4 miles more than Cirrus before getting back to start level and needing another thermal.

You will see from the bottom part of the diagram the actual height of Sigma and Cirrus along the track. An important point is that Sigma has to dive off 400 feet to gain cruising speed while Cirrus only dives off 200 feet.

This is an inevitable fact of life and nothing whatever to do with the gliders; it derives simply from the conversion of potential energy into kinetic energy and the higher the cruising speed the greater the height dived off to get up to the required speed. Fortunately it is all recovered (and I mean all) at end of the glide when you pull up to thermaling speed on joining the next thermal, However, it does have a significant effect in that the ground will be that much closer throughout the cruise. Where the working layer is near the ground, this may be discouraging. The other important point brought out by this diagram is that Sigma covers 30 percent more distance in each glide than Cirrus does with the consequent opportunity to choose the next thermal from a greater selection. If, as seems probable, thermal strengths have a standard type of distribution, this increased selection capability should give a significant improvement in the strength of the thermals used on any particular day.

A big advantage stemming from the higher cruising speed is the shorter time to reach the next cloud. This means that one is less likely to suffer the all too familiar problem of the cloud that goes soft just as you get there.

But there are disadvantages in high cruising speeds. For a start the total energy variometer has really got to be good -- unlike nearly all the installations I have flown with. Even with a successful total energy, there is the problem of slowing down where you think there is going to be lift and then speeding up if it isn't there. Contrary to popular ideas, this does not create any great loss -- only a little due to the short time you are not flying at best cruising speed.

In Sigma the flap has one purpose only, it reduces the speed for minimum sink. Nothing else. Minimum sink is the same flap-in or flap-out except that flap-in it occurs at 56 knots (64 mph) and flap-out at 39.5 knots (45 mph). Flap is there only for circling, so if you are slowing down to feel for a thermal in straight flight this will be done at about 55 to 60 knots (65 to 70 mph). This will certainly involve a new technique and may be more difficult but I think we will get used to reacting more quickly; it will certainly lay stress on a really good total energy variometer. If we find it difficult to pick up thermals at these higher speeds there is no reason why the flap should not be put out but this will be a more complicated maneuver and I visualize difficulty in getting a variometer to cope during the flap-out or flap in maneuver. This will undoubtedly be troublesome. However, you must bear in mind that we expect Sigma flap-out to be very similar to any existing high performance glider in the 40 to 60 knot (45 to 70 mph) speed range. So, unlike BJ-4, there is no severe penalty if flap is put out and the thermal turns out be nonexistent.

The actual thermal climb in Sigma should be no different from an ordinary glider although aileron control is unlikely to be as pleasant, since I doubt very much whether we will be able to produce a really smooth relationship between stick movement, stick force, rate of roll, and adverse or proverse yaw. Using, as we are, a combination of ailerons and spoilers the compromise between these four parameters is unlikely to be better than acceptable.

Fore and aft control should be good since we are fitting an all moving tail with anti-balance tab. The tailplane area is also fairly large to take care of the trim changes due to the flap. The rudder should be normal and, in view of the aileron spoiler arrangement for roll control, there should be ample rudder effectiveness for all maneuvers.

How, let's go on a 100 km triangle in Sigma. We take off with hydraulic accumulator fully charged and flap out. If it is an ordinary tug we will be climbing flap-out; at least 70 knots (80 mph) would be needed flap-in. We cannot get the wheels up as the tow hook is on the main undercarriage. At 2000 feet we cut loose and select wheels up (they are hydraulic) leave the flaps down and amble round to join the nearest gaggle. At the earliest opportunity we unlock the rudder pedals and pump up the hydraulic accumulator, say, 15 Pumps to put back what the wheels used.

Let us assume it's a good day with 600 fpm thermals between 3000 feet and 6000 feet. We will want to cross the line at Vne which is 140 knots (160 mph) and we will know that this is achieved by diving from a point one mile short of the start line starting at 70 knots (80 mph) and a height of 4150 feet.

We are now cruising around in the region of the start line at 5000 feet or so and waiting for a few other pilots to get strung out along the line and, even more important, waiting for a clear indication of a good thermal within a couple of miles of the start line. We check the hydraulic accumulator fully charged, everything stowed in its proper place, all instruments running, camera ready, sunglasses clean, hat, sunshade ventilation adjusted, gyro set to compass, right piece of map on knee pad, straps tight.

Nobody else diving for the line. A touch of air brake gets you to 150 feet at 70 knots (80 mph); dive to 140 knots (160 mph); and over the line at 3250 feet. Confirmation of start comes over the radio and you ease up to 110 knots (125 mph). A minute after start and you are two miles out and pulling up under the first cloud, At 60 knots (70 mph) you will be at about 3500 feet and listening to every twitter from the audio. Suddenly there is a heave and you know you have a core. Roll-in, slot the flap lever down, bang go the flaps and a sharp backward heave on the elevator brings you into a tight circle. A couple of circles gets you centered and good steady climb develops. Apart from concentrating on the climb, you are now very busy sorting out the next thermal. Which cloud? Or -- join a gaggle five miles ahead which appears to be doing well?

Four minutes of climb brings you to 6000 feet and the rate of climb begins to fall off. Straighten up towards the gaggle, bang in the flaps and dive 600 feet to get 110 knots (125 mph) and away on the cruise. The first job is to recharge the hydraulics; release the rudder pedals and pump maybe 30 pumps, relock and sort out the navigation. The gaggle is approximately on track to the first turn point. You get to within half a mile of them at 3500 feet and pull up to join them. You gain 500 feet in the climb and pop the flaps at the top. Sigma. has substantially equal climb performance and you are soon away from the top of the thermal with a few other gliders streaming for the turn point in front of you. In flaps, dive, pump up the accumulator and almost at once you go flashing past those who had left the thermal ahead of you. Turn point coming, check the second leg for a good cloud, round the turn point at 5000 feet, photograph, straighten up for the next cloud, you will have taken about 18 minutes so far.

On the second leg there is no gaggle but a good looking cloud is available and you feel your way under it slowing down to 60 knots (70 mph). It is a large cloud and there is a wide area of weak lift. You cruise around and fly through one or two poor cores but the real meat is not there. You still have 4000 feet and a new cloud is showing a couple of miles ahead so you press on and sure enough you find a real boomer. Once again that rather tricky roll-in, flap-out, turn maneuver and you are locked into your last climb, for at 110 knots (125 mph) your final glide with 50 km to go requires just 5400 feet. But as you pass 5000 feet the climb builds up to 800 fpm and your final glide break-off computer tells you that with this rate of climb you should break off at 6300 feet and glide at 120 knots (140 mph). But it dies back again to 600 fpm so you break off at 5400 feet and head for the second turn point.

Now comes that hair raising game of checking height against distance. At times it looks good, at times not so good and it's nerve wracking when you speed up in sink and height seems to evaporate in a flash. The turn point comes and you make the turn at 3600 feet which checks out exactly. Already it is looking good because with 110 knots on the clock you have a good five km of level flight available if you reach ground level short of the finish line. This is why you did not allow any margin in your break-off from the last thermal. Ten km from home you have 1000 feet and know it's made. If the flight works out as I have described, you should cross the finish line just 42 minutes after starting, for an average speed of 143 kph, and if you do I am quite sure of one thing -- you are going to be one helluva tired man! There has been more to cope with and less time to do it in and there is the actual physical effort of keeping the hydraulic pumped up as well. You did remember, didn't you, because you haven't landed yet and there is going to be a nasty incident if you cannot get the flaps and undercarriage down.

The undercarriage drops down under gravity but you'll need the flap or else a very large airfield. So, a quiet circuit to lose speed, gear down, flap down, turn in on final with 200 feet or so, pop the chute and use the air brake for fine adjustment. And that's it.

I'm not at all sure it's going to be fun but if Sigma does work, we ought to be able to rewrite the record book and it will certainly be exciting.

Question And Answer Period

Question: (Schreder) I imagine there are a lot of visitors here who are panting for one of these Sigmas and they'd like to know the purchase price and the earliest delivery date.

Goodhart: The answer to that is we are building at the present time a prototype. We are hoping to learn a great deal from building this prototype. After we build it we'll look at it and see what can be done. It looks like $100,000 to build this program.

Question: (Moffat) I'm curious Nick, what are you going to do when you get this thing built and it works out just exactly the way you said but some chap who never flew anything but a K-6 wins the Nationals under your handicap system and then becomes the Sigma pilot?

Goodhart: We shan't lend it to him. No, I think it's going to take quite a bit of learning to learn to fly this bird. You're going to have to have a lot of high performance time to fly this.

Question: (Seibels) Nick, if this isn't classified, I don't think I heard you say anything about what sort of materials you're building this ship out of.

Goodhart: The ship is entirely built out of aluminum alloys except that the pod on the fuselage -- the part around the pilot is the fiberglass pod. The tail cone is aluminum. Just as a matter of interest, the wing skin thickness at the root is 5/16 inch.

Question: (Roy McMasters) I'm curious about the maximum speed at which you can bang a flap down. It seems to me that if it gets triggered accidentally at 170 mph, that it would disintegrate itself in the process of the flap coming down because of the tremendous forces involved in the center pressure change.

Goodhart: We are making the flap lever a very long and conscious movement. You've got to first move it along a slot and slide it down. Hopefully, nobody would be silly enough to do that at any speed above flap limiting speed which is 75 knots.

Question: (T. I. Weston) I'd like to hear you comment on crew and caravans involved in moving this thing around and taking it apart and putting it together.

Goodhart: Well the overall weight is just under 800 pounds. It's a three-piece wing and it takes four people with lifting bars. We've designed special lifting bars for them to carry it. We don't necessarily visualize putting it together manually, or taking it apart, except for off airport landings. We rather visualize, in fact, building a small crane on the trailer in order to hang up the center section and drive the fuselage under it. This should be no problem. I accept the point entirely. This is not a Sunday afternoon glider at all.

Question: (Seibels) All that fin area down under the fuselage. Isn't that going to be pretty vulnerable to getting banged up on off-field landings?

Goodhart: I think you're right. In the case where the back end drops in a rabbit hole, we may well be in trouble there. If the end comes off we will glue it on again.

Question: (Byars) With the advent of a stiffer laminate such as carbon or boron fibers, is there any thought being given to changing the materials of the wings?

Goodhart: No, we haven't thought about what we'd do yet with the Sigma II. There's no advantage, that we know of, in going to higher aspect ratio if it only permits you to go to a higher span. I am very doubtful of the roll control problems. We stopped at 21 meters because we thought that was about the limit for roll control problems. I'd be interested to see these 30 meter ships George is talking about.

Question: (T. I. Weston) Please comment on the computer program you talked about which took two years. How was it used?

Goodhart; We did a parametric study and developed a program which, given the wind tunnel data on the wing section, does a full analysis of the performance. This applies to a whole series of thermals. We have a fair thermal -model -it may not be right -- but not many people know what a thermal looks like. The program gives cross country speeds for particular types of thermals. We then simply let the computer look for a maximum over a whole range of aspect ratios, spans, and weights. This computer procedure led us to our present Sigma design.

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