Creosote Bruiser - Design and Build Thread

[Tom Harper]

The LD4 design exceeded it's goals. I'll bump the thread in case anyone is interested.

Now, it's time for something bigger and a little better. The LD4s can be iffy on hand launch in dead air and they have a small pay load capacity. I need to loft two pounds of camera, and some electronics gear, into the air with a hand launch. The goal is still an inexpensive AP model that is rugged, easy to transport and infinitely repairable. I'll stick with the foam board construction. It's lighter and cheaper than SPAD and it doesn't require CA glue.

Criteria:

Power..........................ST .90 glo
Propeller.......................Master Airscrew 14D 4P
Span............................80"
Area............................9 sq ft
Aspect Ratio.................5:1
Weight.........................11 lbs
Wing Loading.................20 oz/sq ft
Power loading................150W/Lb.....180W/Sq ft
Camera/mount weight.....3 lbs
Must fit in 40" by 60" rectangle for transport

Design:

Half inch Foam board construction - light, cheap
5:1 aspect ratio - good strength, reasonable drag
Straight line cambered section - has worked well so far
Removable outer wing panels for transport otherwise one piece construction
Bolt on gear for flight testing

Application:

This model is for aerial photography in very rough terrain. Take off is hand launch with landing in creosote bushes. It has to fit into the back of my Land Rover which is 40"x60". Must require minimum assembly at the field.
Easy camera access. Handle on top so I can carry the thing. Sometimes hike a mile or so to the launch point.

Like it's predecessors this one is starting with a sketch. I got the thrust line a couple of inches high.



I invite your critique -

[Tom Harper]

My primary design aid is an Excel spreadsheet. It has three components.

I set up cells to calculate the area of the desired planform based on entering span and base chord. It gives me the Aspect Ratio.

Another line calculates the center of gravity. I enter the weights and position of components and change the position until I get the CG where I want it.

Once the total weight is determined the spreadsheet calculates wing and power loading.

The positions are transferred to an Illustrator version of the original sketch. The result is below.

The wing section is a straight line approximation of a 5% camber line with the max camber at the 40% point. With the LE - TE of the wing in the same plane as the horizontal stab the effective incidence of the wing is ~5 degrees. That will yield a Cl of about .5 . That is the optimum operating point for the wing.

The straight line approximation is easy to build and performs well under power. When the engine quits you have to drop the nose and keep the speed up because the Cd is very high and the glide angle is steep. But, it holds up well when it tangles with the creosote bushes on the ground.

[HELModels]

That is cool looking. So, your program moved the motor way forward to balance? I like the pod. It looks like you could shape one real easy using a printout of some NACA 4 digit for templates. Here is what I think looks close to your pod shape, maybe a little too thick, but close.

Harry

[HELModels]

This is a little closer maybe?

[Tom Harper]

Harry,

Thanks for the sections. The NACA 4 digits are what I used on the LD-4.

I decided to put the camera at the CG. It's safer there but not as easy to service. I may move it back up front. That will shorten the engine pylon. Not sure yet.

Tom

[Tom Harper]

Looked lile a bad layout.

Moved the camera into the front of the pod. That requires that the camera, or a couple of pounds of lead, is carried on every flight. I will make the front of the pod removable. That provides camera access and meets the 40" length requirement.

[HELModels]

Tom,
Actually what I was thinking was make cardboard templates arranged 90 degrees and shape a round pod. Fill the quadrants with blue foam and circular cardboard templates to guide the roundness. Shape, finish with epoxy, wax, then mold it. Your LD-5 might benefit from some parts being more permanent, even the motor mount tube.

I've designed a pod and when I do it again, I will design it from the inside out to more easily fit the gear. By that I mean design the structure which will hold everything and then design the mold around that. The other way around is awkward for me at least.

IT is YOUR Design and I really should just watch with interest.

Harry

[Tom Harper]

Harry,

I am glad to have the benefit of your input.

You are right about the inside out construction. I tend to build the thing and then see if I can get the camera in it. Not a smart approach.

One thing I have learned about foam board is that everything needs to have a similar rigidity. The foam and paper will 'work' under stress. If anything is stronger than the paper and foam it just pulls away. So, I use poster board for planking instead of balsa and packing tape for reinforcement and hinges. Packing tape also works to protect surfaces from oil and fuel spills. It's faster and cheaper than epoxy.

These are not show models. When I give seminars on the photographs, I display one of the airplanes. It is always patched and dirty. Kind of the airborne equivalent of the archaeologists shovel.

Good advice. Think I'll spend some time on the camera mount.

Tom