Thursday, November 8, 2018

Importance of Materials in Model Aviation

After the end of the outdoor free flight contest season I have been reflecting on what I need to improve on for the future. I have also spent more time reading and watching videos on history and science. One aspect of my model building that I try to improve on is “materials”. In studying history, the most important discoveries that enabled new innovations, were the discovery of new materials. With the discovery of iron and later improving on this to create steel, so many things could be built or improved on as an example.

Witch Hawk 500 - Polyspan & Silkspan Covering

The model airplanes I most enjoy building are built primarily of balsa wood. Balsa is very strong for its weight and great to work with simple hand tools. It does vary in weight and hardness considerably and matching the proper density for the requirements needed in the model construction is critical to build a competitive free flight model. Not paying enough attention to this I have built models that were too heavy or were not strong enough in critical places and broke just from flight loads. Not only is the density of balsa important but also the grain types created by how the balsa was cut from the balsa logs.

1/2 A Streak Wing Does Not Sag with Polyspan Lite

Another part of the model construction I have been trying to refine is the proper covering material for the type of model I am building. In the past I most often used a plastic iron-on covering when possible, for smaller models it was tissue paper. For the past couple of years I have had a fascination with the older free flight models. With the older designs the structure was lacking the rigidity needed without using a covering material that gave the structure the strength needed. Yet another aspect of the covering selection that became apparent with the past flying season was what humidity did to the covering. Esaki tissue which is really the only choice for many smaller models starts to sag terribly in early morning or late evening when I often fly. Besides taking on extra weight now that the covering is no longer tight on the model, the structure becomes less rigid.

Wilbur Center Section Repaired with Tissue over Mylar

Last winter in my building I tried covering with Polyspan and using mylar under the Esaki tissue. Polyspan does not change with water; to shrink it heat is used. It only comes in white and needs to be filled with nitrate dope before applying the color. I had purchased airbrush equipment and this worked well for finishing the Polyspan. There is also a Polyspan Lite which is considerably lighter but harder to fill. I used this on smaller gas models; it did stay tight but will puncture easily. 

New Gollywock Tissue over Mylar

Many people recommend covering models with ¼ mylar and then covering over this with tissue. It sounded like a lot of work but I did try it on a New Gollywock model I was building. On the first model I believe I did not get the tissue attached properly resulting in loose spots and wrinkles. In doing a repair on center section of Wilbur rubber model I did a better job of sealing the tissue to the mylar. The covering does not sag in humid conditions but the tissue will still take on weight from the water.

Eureka e36 Wing Recover with Polyspan Lite - Sagging Tissue on Stab

For me the challenges of getting the choices closer to optimum in building an outdoor free flight model is what makes it so fun. No doubt with more experience I will become more uniform on the choices of wood and covering material. What I appreciate with the rules for the classes of free flight models I fly is that I am forced to do this and cannot just buy a high-tech composite model that was constructed in molds.

Bill Kuhl

Friday, November 2, 2018

Free Flight Fun Projects & Repairs

After the outdoor free flight contest season has finished I have been busy repairing planes and trying some quick new planes. In a previous post I had mentioned working with the STRIX electric timer and flying on a direct drive 7 mm motor. Last weekend I had some good flights using this setup in an AMA Maxi Jr rubber model converted to electric. As I got the turn circle dialed in the flights became higher lasting about 35 seconds on the 10 second power run. The last flight ended when a dog grabbed the plane by the wing and started to carry it back. Only the tissue was ripped. I have to believe with a lighter plane a minute flight is possible, which would be about perfect for a small flying site.

NJAPF Fuselage Repaired 

Only Damage After Carried by Dog

AMA Maxi Jr Electric

At the last outdoor Minnesota contest I flew my NJAPF p30 and the DT line broke, after the contest I decided it was time to recover the fuselage that had numerous patches. What surprised me a little was there were broken glue joints that the covering had hidden. After repairing the structure and twisting it to check for more questionable joints, I did find more. My plan is to replace the viscous stab DT system with a wing pop off using a fuse.

Eureka E36 Test Flight after Repairs

After a crash at the Nats I finished repairs on my Eureka e36 NOS. The wing was recovered with Polyspan Lite because I did not like the sagging tissue in humid conditions. My repair in one spar in the wing was not adequate as it broke again on the first low level DT landing. This has been repaired and more short flights went well. 

FAC Tic Catapult Gliders

G12 Catapult Glider

A couple of simple catapult gliders were finished the first being the FAC Tic from Easybuilt which comes in a 2-pack. The first one I messed up in that I did not get the dihedral blocked up well when using Testors glue, trying to fix this I did not do the best job either. The second one I used CA and got everything aligned. This glider flies really well for only having a 6” wingspan. I also finished up a second G12 Catapult glider with the flapped trailing edge. Richard Bertrand uses this glider in his Free Flight Rescue program for kids. The glider is stable and can handle a full stretch launch with ¼” rubber.

Aleda R Boost Glider
Switchblade S Wing Forward

Wing Back

  For something different I have built two rocket glider kits purchased from J&H Aerospace. One is a boost glider where the motor tube and nose cone separate from a glider. The other is a swing wing glider that has the wing halves move forward when the ejection charge releases the tension on two lines holding the wing halves swept back 90 degrees. This should be fun to see go up.

I have a new fuselage to build for a New Gollywock and then plan to start building some new models.

Bill Kuhl

Related Links  Aleda R Boost Glider  Switchblade S Rocket Glider Easy Built FAC Tic catapult glider