BD-4 NEWSLETTER #19, Sept 1991
Roger D. Mellema
17605 SE 288 PI. Kent, Wa 98042
PH 206-631-5324

Oshkosh '91 is over and now it is time to start working on getting more BD-4's there next year. I am ashamed that my airplane has been down so long. Somehow we never plan it that way when we tear them apart for modifications. I have been working hard on the Piper Aztec and now it has been sold. This gives me more time and a little more money. My current problem is that my Prop Speed Reduction Unit (PSRU) has developed a bearing problem. The front and back plates were mis-drilled by 3/16". This caused the rear prop shaft bearing to go out after only 3 hours of taxi tests. I am getting replacement parts now but will still have a chopped up system. The mis-alignment of the top sprocket caused the front of the PSRU to be pulled off center a little, which caused the holes holding the center support of the lower bearing to be pulled off a little also.

I hope you readers with the Lycoming engines will excuse me if I put some automotive engine builder information in the newsletter. I have been trying hard to limit the amount, but a tremendous number of builders have purchased information and PSRUs for their airplanes. Maybe we can develop these systems so that you will feel inclined to use them too.

Back when I initially assembled my PSRU, I had left all bolts loose to make it easier to fit everything. Apparently this masked the problem of the holes being off center. When I tightened up all the bolts it was not obvious that there was a problem. I would recommend the following procedure for insuring proper alignment.

  1. Install back plate to the engine and tighten bolts.
  2. You may or may not have the lower sprocket installed.
  3. Remove all guide blocks or anything that hinders movement of the rear bearing mount.
  4. Install the side and top "C" channels.
  5. Install the front plate.
  6. Tighten the front bearing support block to the front plate and down as far as it will slide. This is about where it will be when the belt is on and tensioned properly.
  7. Grab the prop shaft/flange and move it around a little to see how much slop there is in the front bearing. There will be some but I don't know how to quantify it as my bearing has had some wear in it. Wiggle the shaft left and right and set it in the middle of the motion.
  8. Turn the rear bearing mount so that the top of it is parallel with the top of the rear plate. Check to see if the rear bearing mount is almost touching and parallel to the rear plate. Look behind the rear plate to see if the holes in the rear bearing mount line up with the elongated slide holes in the rear plate (check all of them!).
  9. Put the rear bearing support guides in place and see if they fit without pushing the bearing support to either side.
  10. You might move the front bearing support to the top of its travel and check again to see if the alignment of the rear bearing support is proper there.
  11. If the slide holes in the rear plate are misplaced, they can be corrected by filing if they are not off too far. If they are a long ways off, I would recommend getting a new un-drilled rear bearing mount and drilling it to fit the back plate holes. You will now also have to modify the guide blocks so that they fit against the rear bearing mount properly. I think that it would not hurt if there is some small clearance from the guide blocks to the rear bearing mount so that it can find it's own center.

The V-6 is really running great! I have had no problems in getting it to run right. It has not been dyno'd yet but it turns a 72 inch pitch prop 2500 rpm static, (4000 rpm engine). As you rev it up, it has some spots where there is a little vibration, but at the higher rpm's, it runs really smooth. It is, of course, much smoother overall than the O-360 Lycoming that it replaces.

I have had the airplane up to 75 mph on the runway and the rpm does not increase over static. I guess the prop is still mostly stalled or is doing an "almost constant speed" function.

I am still working on the cooling system and am now investigating the ejector (or augmenter, eductor, blooie-tube, jet pump, aspirator) system of augmenting the system cooling by using the exhaust to "pump it". This involves directing the exhaust into the bellmouth of a larger tube. The exhaust pulls the cooling air along with it and thus enhances the cooling flow. I have a spreadsheet program (Macintosh) that should allow me to determine the correct design. If you want to try the program, just send a floppy and postage. It looks like a lot of flight testing will be necessary to fine tune the system. I built a test rig that pumps a tremendous amount of air at 2000 rpm but totally looses it at 4000 rpm. This system will also be useful in enhancing the cooling of Lycoming engines.

New rumors: I hear there is something called the "Cascade Twin" being dreamed about. If, you feel automotive engines are less reliable than you would like, or if you feel that they cannot be run at high power levels, then this could be the answer. Center line thrust will allow you to safely fly this twin without a multi-engine rating.. All of this and easy BD-4 building technique, simplicity, and materials.

There is also a rumor that there is a new aluminum "build-up" spar designed for the BD4 that weighs 40+ percent less than the tubular spar. It is also stronger so that a 2400 lb gross weight is allowable even with the long wings. The wings will still be removable. Do you have any ideas on how such a design could be released to the public without liability and also compensate the designer some?

Dues: If you look at the address label and have a +, =, or * you are OK. If you are behind (@) $5 will give you a "+", $10 will give you an "=".

Memorial Day Fly-In
We again had a successful fly-in. The weather as usual was not as good as we would have liked. We have decided to have the fly-in next year on Labor day. We had a lot of builders present and 2 BD's. Colin Powers and Dick Gushman.

Monday, September 7, 1992 will be the date of the next BD-4 fly-in at Crest Airpark.

Firewall Modifications
I have had a couple of recent calls from people who have chosen to build their BD's with conventional gear. Their question was: what do most people do about the firewall frame and braces?

Many people have chosen to put all the angles in just as the plans show but most have taken out the nosegear box and the 4130 steel brace that connects to the diagonal box coming down from the spar. Of course, the weird aluminum angles going all over the firewall are not at all necessary in the taildragger and I would recommend getting rid of them (as I have done on my BD). They are extra weight, they help transmit heat through the firewall, and make it difficult to insulate the firewall. The only thing they do for you is to stiffen the firewall so that the instrument panel can be braced to ft.

If you use some bolls in the attachment of the firewall (instead of all rivets), the firewall will act as a shear panel to hold the front of the airplane square. I put 2 diagonal straps of 0.040" aluminum across just to hold everything square while I installed the firewall.

The firewall does need some stiffness. I chose to put a 1.5 x 0.5, 0.063 thick aluminum angle on the front side of the firewall. I put it vertically just to the right of the pilots right foot. This stiffens the firewall and also is where I put most of the holes for the things that must go to and from the engine. I also tied the brace for the center/bottom of the instrument panel to this angle. You could also support the instrument panel by installing aluminum angles from the bottom/center of the panel to the left and right sides of the firewall. If you are starting from scratch, it would also work to make the angle at the bottom of the instrument panel stronger so that it does not need more bracing.

I would recommend insulating the firewall with at least 1/2" foam (sticky-back works best) right after you put it on. You wouldn't believe how difficult, it is to insulate that thing once you have all your controls and instrument lines run through it!

Fuel Pumps
In the last newsletter I reported on an accident where a BD-4 experienced an engine failure shortly after take-off. They had a lot of trouble finding the cause but it turned out to be a failure of the mechanical fuel pump. This is really scary as most of us have flown for years with just a mechanical fuel pump.

Steve Mahoney got very worried as he regularly carries his family over some very ugly terrain and he has always assumed that the system would gravity flow no matter what happened to the fuel pump. He bypassed the fuel pump and had no problem until just about lift-off. The engine would not go over about 1600 rpm. He then thought about all the airplanes that have always flown without a fuel pump (C-150, C-172, Piper Coft, Piper Tripacer, etc.) and wondered why a BD should be different. He then hooked up the fuel pump again and found that the engine had a hang-up at about the same rpm, but by pumping the throttle, he could get it to full power. Steve found that the float in the carburetor had a small shiny place on it where it was rubbing the side of the float bowl. With the float bent so that it didn't rub, he found that the airplane flew fine with or without the fuel pump.

I am not sure quite what lesson we should learn from this. I almost makes you want to try flying without the pump so that you will be sure that your installation will work with gravity only flow.

The Marvel-Scheibier carburetor requires a fuel pressure of about 0.2 Ib/sq. in. to work properly. A cubic inch of gasoline weighs 0.026 lbs. Theoretically, a 7.7 inch head of gasoline should give sufficient pressure to allow proper operation.

Fuel System Tips
Jim McCord pointed out an interesting fuel system pick-up that was featured in Sport Aviation. It is a valve that effectively closes off the pick-up (front or rear) that could suck air. It uses as ball bearing and will switch pick-ups when the aircraft accelerates or pitches.

Care must be taken to make the valve so that the ball can never be sucked tight over the exit hole. This can be done by having a drain valley between the lower ramped rails as shown on the right.
This valve looks like it could really help us solve any problems associated with pitch or acceleration. Remember that it still does not solve the "ball off center, or slip/skid" problems that force the fuel away from the pickups.
Jim also suggested that "flap valves" be used between the ribs so that fuel will drain away from the inboard bay much slower. I suppose these valves could be made so that they are pretty tight. The way you would use them would be to "pump" the fuel into the inboard ' bay by flying with the ball off center to the opposite side of the, tank you wanted to pump (i.e. ball to the left, you would pump the right tank). Of course turbulence and other natural rocking while flying would also "pump" the system. The ribs could have holes through them in the upper 1/3 so that the tanks could be easily filled.
Another suggestion was that the bottoms of the outer fuel cells be filled with increasingly thicker layers of foam and thus force the fuel to the inboard rib. The only thing wrong with this idea is that changing attitudes can "pump" fuel into the foam if anything isn't done just right.
An idea for pressurizing the fuel tanks is to use a NASA vent on the bottom of the wing. Maybe this way they could be pressurized with nothing protruding below the wing.

New Landing Gear Configuration by Mack Weston

I have put on a main landing gear similar to the Tailwind or RV-4. The main landing gear legs are from a 152 Cessna modified slightly to fit. I had to heat and bend the lower bend very slightly to get proper alignment. The legs are mounted onto sockets built into the engine mount. I modified the mount that I had to do the job.

The four corner mount attach box in the fuselage were made of 0.090 4130 steel, the lower boxs are made four inches longer than stock to spread the load into the lower side channels. This gear moves the wheel center line forward about 2.5 inches and raises the nose quite a little (see picture in April '90, Kitplanes); this gives clearance for a long prop and rough field capability. It uses 6.00 x 6 tires.

Another benefit of this gear is that you can no remove the old gear box and fittings, this opens up the floor for good seats (mine are also from a 152) a little lighter than original. I installed 2024 T3 2x2 box (same as door post) between the lower side channels for seat mounting.

The gear set-up in my plane is strong. I have made some landings that would probably take out the original gear. I have flown on and off some pretty rough unimproved strips and the gear handles it well and also gives a good ride over rough ground: The tapered string steel tubes absorb the shocks very well but rebounds With vigor, watch it!
There are no drawings of my landing gear (as long a liability laws remain as they are, there will never be any). All I will do is show pictures of mine and tell how I did it and how I like it. There are no guarantees. I will furnish tube size and a rough sketch to serious inquiries of the gear modification.

This landing gear is a little lighter than the original with all it's fixes.

For Sale

Larry Fattaleh informed me that Performance Propellers, PO Box 486, Patagonia, AZ 85624, PH 602394-2059, is a good company to deal with. The props are made of laminated hard rock flawless maple (16 laminations per inch). The leading edges are made of Uralite and are rain proof. The final finish is multi-layers of polyurethane paint. These props are said to have quasi-constant speed characteristics.

They will help you select a propeller size for your airplane, manufacture it with a little extra thickness, let you try it out, and then modify it to your specifications and do the final finishing. They will also take your order with just a $100 deposit. Their delivery time is usually three to four weeks. The price for a 72 inch diameter prop is $600.

Fuselage Stretch

W.O. Singer has the longest stretch yet in a BD-4. He is up to 26.5 inches and even has a load carrying aft door designed. He also has the rear floor strengthened to hold a light motorcycle. One of his sons commented that "either you are afraid to fly a BD-4 or you are suffering from a delusion that you are going to live forever". William readily admits that "there are other crazier ideas that I dare not breathe a word of

Fuel Gauges

A new occurrence of an old problem has again come to light. Several years ago David Dotson had a problem with one of his fuel gauges that wouldn't work. After a lot of messing about we opened the "instrument cluster" and after a lot more careful work discovered that there was corrosion in the case that was keeping the meters from grounding to the case. We tried to clean up the corrosion and did get everything to work.
Dennis Love recently called me about the same problem. It took me a while to figure out again what we had figured out before. We did find the problem and solved it by soldering a wire to each of the "grounding fingers" and then tied it to ground.

Test Flight

I had the distinct pleasure recently to fly a new BD-4. Dick Gushman and Jim Parker have been building this BD for quite a few years and it was finally ready. Dick very carefully approached the first flight with a lot of thinking, checking, and ground tests. He has many hours in the Grumman American Traveller and I had told him that with that kind of experience the BD-4 should be no problem.

Dick finally went for it and had a successful first flight. He called me shortly after that to see if I thought everything was all right with the airplane as he was "kind of all over the sky". After talking for some time, Dick asked me to come down and see if his airplane was "normal".

Several of us BD builders went down to Kelso-Longview, WA with video cameras etc. to see if we could help. The airplane is really beautiful and we found absolutely no fault with it. Dick said he would like me to fly it and see what I thought. I had to get used to the Murphy nosegear and the "brake steering" again (it works best when you ride both brakes a little). The BD took off with all the gusto that is normal for an O-360 with a constant speed prop. After doing some turns, getting up to 6000 feet, and doing a series of stalls, I flew some formation with the camera plane. Then a touch-and-go and a fullstop landing.

Dick wanted to know "just what I thought was wrong and how could it be fixed?" When I told him that it flew just perfectly and was pure BD-4, he said "Oh ____,it's just me!"

I guess this account points out that the BD-4 is a little different than even "quick" factory builts. The biggest problem I have seen builders have with their BD-4s is the quickness of the controls - especially the rudder. I find that it really helps if the airplane is a tail-dragger. The friction in turning the tailwheel really helps moderate the rudder action. Some builders have even installed a "micarta friction block" on the rudder cables to make the rudder more controllable on nose-dragger models.

Dick flew his BD-4 to my house for the Memorial Day fly-in and even got his BD down between the trees at Crest Airpark (not that difficult, butt spooks some people). He is feeling a lot more confident about his BD and is starting to enjoy it.

Fuel Pumps

George Wittet called me some time ago and suggested using 4 Facet electric fuel pumps to assure good fuel flow for the BD-4. He put two pumps in each wing root, one hooked up to each fuel line. He has 4 switches in the panel so any or all of the pumps can be selected. He says that this system works fine and the airplane will cruise with no pumps 'on'. The pumps each have check valves in them to prevent backflow into the tank when only one pump is turned 'on' on a side.

This pump arrangement is especially good for low wing airplanes as it is always better to 'push' gas rather than 'suck' it (sucking enhances the chance of vapor lock).

This system may add a little weight butt sure allows you to get, gas from the very back of the tanks for take-off and climb! These pumps are a little noisy but only when there is still air in the lines. You couldn't hear them over the engine and wind noise anyway. I have two of these mounted under the instrument panel right now for my V-6.

Jim Huber informed me that these pumps are available with "flare" fitting rather than the hose fittings on the ones I have. I have found that hoses tend to flatten too much when they bend and that they flatten more over time. I may be ordering the flared fitting model soon for my V-6 conversion so that I can use aluminum lines. Another solution suggested by Tim McGinnis is to use polyurethane tubing. It is extremely resistant to chafing and can be bent into a tight radius if it is heated some. This tubing is recommended by Rutan and is available from Spruce and Specialty.

Main Landing Gear
George Wittit also said that he dropped his BD in a bit and buckled the landing gear box. About the same time, I heard that the English fellow that bought Don Hewes airplane (Virginia Patriot) had found fatigue cracks in his landing gear box. The cracks in this gear box were directly caused by Don's modification to make it easier to inspect the main gear upper end. The idea to make removable end panels in the cover of the main gear box is attractive as it is a pain to always jack up the airplane and then remove the many bolts (some of which always seem to strip out the gang channel ruts).

Don's modification allows some of the stresses that should be taken up in the gear box cover to be transferred to the sides of the gear box. George Wittet may have bounced his BD pretty badly but the failure of the box was certainly due to a Hewes type of modification to make it easier to inspect the main gear. This particular modification was made to allow a center console to be permanently mounted and still allow the gear box cover to be removed.

Those of you who have made the Hewes modification should improve the strength of the joint by adding a more substantial "carry-through" patch. One of the builders I talked to suggested the fix shown below.

If you have to replace the landing gear box, I would really recommend using the design shown below. It is really nice to be able to use bolts and nuts rather than the nut plates or gang channel. This allows you to take off the cover without jacking up the airplane.

Personally, I think the 0.063 inch thick aluminum is just fine, but some people have gone to 0.090". Most of us who have destroyed a gearbox have to admit that the original is very strong and there is not much justification for making it heavier.

When you are fitting the landing gear box and lid between the side channels, you do not have to make it fit so tight that you can't move it around. All of the loads are transmitted through the 1/4 inch aluminum plates on the sides of the gear box. My box and lid have 1/4 inch spacing to the side channels.

Oshkosh '91

At the final moment I made the decision to fly to Oshkosh. David Dotson (also a BD owner) loaned 3 of us EAAer types his "V tail" Bonanza so we could make the big event. The Bonanza, of course is a Classic which allowed us to park in the show plane areas. I must say though that it flys like a truck rather than a sportscar-like BD-4! The most interesting things at Oshkosh (for me anyway) were:

Ray Ward's Super BD-4 - Before coming to Oshkosh, Ray had a problem with the prop governor taking too much oil from the main bearings of his big Chevy V-8 and did some bottom end damage. He quickly installed a back-up engine, set the minimum pitch on his constant speed to more of a cruise pitch, disconnected the prop governor, and flew to Oshkoshl The worst part of all of this is that his wife, Ann, had to throttle her BD-4 down a bit so he could keep up. It really is amazing for one family to have two BD-4s at Oshkosh when most of us couldn't even get one there (shame, shame).

Global Positioning System - The Garmin ProNav people were in the exhibition halls and really seem to have a good system. The best price to be had was $1925 and another $600+ for the "aircraft installation" kit. It turns out that the "installation" kit is not worth buying. It does include a standard aircraft avionics rack with a rear section built in that, I was told, made the system compatible with moving map displays. From what we could tell, all that happens in the back of the rack is that the output plug from the ProNav is rewired to a "standard RS-232 bus" plug. In other word - a lot of money for very little. A technician told us that the RS-232 formatting is done in the main ProNav unit.

When I got to Seattle, I found a marine supplier that will sell the basic system for $1300. The antenna that comes with the unit can be installed on the top of the BD fuselage and a small fiberglass fairing placed over it.
Sony also had an impressive GPS unit on display at the fly-in. It is smaller than the Garmin ProNav and it had a couple less features. It has a flat, oval antenna that sticks above the fuselage skin by less than 1/2". It probably doesn't even need a fairing. This GPS will be on sale soon "for less than $1500, retail". Nice, but I liked the ProNav panel unit a little better because it is bigger. These things can now be made so small that the display is larger than the electronics!

I have a feeling that the LORAN companies had better sell all they can now, because everyone will go to GPS very quickly. Just think - no multiple chains to worry about, no gaps, no ASF (additional secondary factor) errors, no center-line extension errors, better accuracy, altitude, fewer problems with interference, much smaller antenna, and fewer environmental effects (lightening, noise, static).

Montana Coyote - This airplane is an Avid Flyer that was made for the comfort of reasonable sized people. It is exactly what I wished for after flying David Dotson's Avid Flyer "Model A".

Bush Master - This rotary powered, ex-TriPacer, bush plane was also at Oshkosh last year. It was interesting because it had an aluminum radiator which had been welded up just like I plan to do with my V-6 conversion. It also had a very narrow HTD belt on its reduction drive. It is certainly smaller than the Blanton belt and this guy claims over 200 hp from his rotary. It looks like he will be doing our bait testing for us!

This plane also had a Czechoslovakian propeller on it that is an automatic constant speed. The spinner has fins on it that cause it to turn opposite to that of the
prop. This runs a hydraulic pump that is used to vary the prop pitch. The higher the engine rpm and the higher the forward speed, the higher the pressure to push the blades into high pitch. The cost was quoted at "around $4000". Something like this is fantastic for an auto engine conversion.

BD-4 Forum - Jim Bede talked about the re-birth of the BD-4. He said that he was committed to supplying kits and parts but that right now the BD-10J has precedence. John Bracher (famous BD-4 owner) is going to be the focal point for the BD-4. The Scotch-Ply landing gear that was used on the Yankee is now the gear of choice for the BD-4. I approve of an energy absorbing gear but find the price intimidating.

BD-10J - The 10J really looks good. The skills required to build it is not out of line with that required for most homebuilts. The kit price of $195,000, of course, is a bit steep for most of us. The total cost (minus labor but with good avionics) is about $350,000. The Fox corporation is ready to build one for you for $105,000 (or $125,000 if you want the avionics installed). The total cost of operation is about $160 per hour. This includes everything but the loss on your money. Not really that bad! Approximately 55, $10,000 deposits have been paid.

The airplane is larger than we first thought. The cockpit area is very roomy and the visibility is outstanding. It really helps to have almost non-existant wings. Even so, a 600 ft takeoff roll and a 30,000 ft/min climb rate ought to be fun.

F-117A - A very impressive airplane - and to think it is already old technology! I think the guards standing around it with machine guns were only trying to make us think that there are some secrets yet to be divulged.

SU-26 - I was ready to see another ho-hum aerobatic act but this Soviet. airplane can do things I have never seen before! Air shows quite often bore me; but I couldn't Wait to see it again (and we thought radial engines were dead!).

BD-4 notes - I just realized that there was a beautiful BD at Oshkosh that had the skins just glued on. Thats right - no rivets to fill and mess with: The skin was bonded on (I use that term lightly) with contact cement. This airplane has a good bit of time on it and some of it at high speed. It is also in a taildragger configuration which loads the aft fuselage harder during ground operations. I really like the idea - but would use Pro-Seal or 3M 2216 instead of the contact cement. It is easier to use and has more strength. As with all ideas in this newsletters, you must decide for yourself what will work for your application.

Wingtip doors
In doing the index, I notice several improvements or "features" that have never been written about. One of these is the wingtip door. This is a necessity if you want to carry around 25 foot flagpoles or somesuch. I like the wingtip door as it lets you store things in the end panel-rip and also in the spar. I always find the heaviest things and put them there right on the Center of Gravity. Engine oil cans lay very nicely in the spar. If you do some very uncoordinated turns or hammer head stalls you might want to restrain the items stored in the tip. In my airplane I have the aileron counterbalances closed off from the rest of the tip so I don't have to worry about things blocking the aileron motion. You want to be a little careful in what types of

things you put in the panel-ribs as you can bend the fiberglass skin enough to crack the paint on the outside.

The thing I like best about the wingtip door is the ease with which you can take
off the wings. I open the wingtip door and push a 14 foot, 2x4 down the spar so that 2 feet are inside the cabin spar and 2 feet stick out of the wing tip door. The person holding the outer end of the 2x4 can rest it on his shoulder as he wiggles and pulls the wing off. The person by the cabin can help wiggle and push without worrying about the wing suddenly falling off. When the wing spar comes off of the cabin spar, the wing will fall or be let down to where the 2x4 will support it. Now he can loosen wires and fuel lines. Then he can grab the 2x4 and help carry the wing away.

The procedure for making a door is as follows:

  1. Mark the location: Mark the outline of the door on the outside of the wingtip. The door should end up being about 6x6 inches, positioned as high as possible, and inline with the spar. It will be just forward of the position light hump.
  2. Reinforce the door area: If the thickness of the fiberglass in the area of the door is thin or if you want a sturdier door, you will want to lay-up a couple layers of fiberglass on the inside of the tip in the area straight out from the end of the spar.
  3. Initial cutting: Cut partially around the door being careful to keep it in place and the shape of the tip unaltered. I would recommend cutting to the corners but leaving about 1/2 inch in the center of each side uncut. You can use a saber saw with a fine blade or a hack saw in a hand holder.
  4. Protect door from fiberglass: Wax the door on the inside being careful to not get wax on the area around the door. You can also use fiberglass resin proof tape or PVA (Poly-VinylAlcohol) to keep the fiberglass from sticking to the door when you lay up the "door flange".
  5. Prepare area for good bond: Rough up the area on the inside of the wingtip and around the door. Lay up 3 layers of 8 oz. fiberglass. Make the strips about 2 inches wide, and lay them 0.75 inch on the door, and 1.25 inch beside the door.
  6. Finish cutting: Finish cutting the door out being careful to not cut through the "door flange".
  7. Trim the flange: Trim the fiberglass so that the flange is about 0.5 inches wide (or less).
  8. Secure door: The door can be secured by just Dzus fasteners or by a piano hinge and a latch. The hinge can be on the bottom or the top. It is important to keep the top edge of the door very near the top of the tip if you want to use the 2x4 technique for removing the wings. I found it best to hinge the door at the bottom.
  9. Reinforce tip: If you think the strength of the tip is weaker than you like, you can add some foam and fiberglass to the top, sides, and bottom to keep it more rigid.
  10. Restriction on use: Do not store materials in the wingtip without closing off the aileron counterbalance area. If anything jams the aileron counterbalance, your poor pink body will surely be damaged.