|Apr 29, 2005, 07:41 AM|
LD-4 Design and Build Thread
Many of the questions posed here enter the 'how high is up' category because they are not associated with a goal or problem. The realities of construction limit design zeal. The two cannot be separated.
So, I thought it might be interesting to some to follow a model from concept to flight verification and see how it works out.
What follows is the fourth in a series of models that I call 'Look Down'. The goal is an inexpensive AP model that is rugged, easy to transport and infinitely repairable. In previous discussions on this board you have suggested SPAD or other construction materials. I have purchased and evaluated some of them. SPAD is about three times the weight and cost of foam board. Also it cannot be assembled with resin glue.
Power..........................OS /15 glo
Propeller.......................Zinger 8x4 pusher
Area............................4 sq ft
Camera/mount weight.....5 ounces
Foam board construction - light, cheap
5:1 aspect ratio - good strength, reasonable drag
Straight line cambered section - has worked well so far
Half inch foam profile fuselage - minimum weight and complexity
Short coupled pusher configuration - easy to transport
Single wheel, stab skids - minimum weight - remove after test
Long nose to allow balance without added weight -
Rigging - wing +3, engine -3, stab 0
I invite your critique -
|Apr 29, 2005, 07:55 AM|
The weight is right at 40 ounces but the nose is not long enough to balance with just the battery. That's OK because I can remove the lead when I add the camera.
This Sunday should be good for a test flight.
|May 01, 2005, 02:02 PM|
Got in a couple of test flights:
+ Take off with the single wheel works well. Once I had to support the left tip until it gained speed but then it raced on down the runway.
+ The straight line wing section works fine. The model is fast and floats in the glide so I do not think the flat leading edge is causing much drag.
+ The OS .15 carries the 4 sq ft wing like a pylon racer. Great little engine.
- I balanced it on the wing spar - bad deal. The CG wants to be an inch or so forward of the spar.
- The foam profile fuselage is weak. It showed damage on both landings. I will patch it with sections of 1/16" ply but think it should have ply on one side if not both.
- (not design issue) The tank is leaking - replace.
I'll work on it and test it again next week.
|May 01, 2005, 02:29 PM|
Tom, pure foam makes good wings and tails, but it's not sturdy enough for the stresses the fuselage encounters.
The front ends on my EPP 3-Ds catch **** with any less than optimum landings.
Some sort of "real" structure backbone with foam filler to smooth out the airflow probably works best.
1/64" ply on both sides...
A pair of wire tip skids would make takeoffs a bit easier.
I'd made up a potential foamie 3-D using 1/4" foam board, but with the same equipment.. motor, battery, servos, it weighed about 7 oz more than the equivalent EPP airplane. Didn't bother to fly it.
|May 01, 2005, 05:16 PM|
I hadn't considered going as low as 1/64th but it makes sense. I'll check the ply shelf and see what's there.
I made the wire skids. just have to glue them on. Once it starts to move it is fine. Looks good on take off!
|May 04, 2005, 01:46 PM|
I put the wire skids on - didn't notice that they raised the tail a bit. Without the + angle it just scooted around and refused to take off. Finally I picked it up and chucked it - took the hand launch great! That was where we started this discussion sometime back. Looks like with enough power the TL doesn't make any difference (or maybe it was the down thrust aft of the wing).
So now it looks fine under power. But, as soon as the engine stops it wants to drop off into a spin. Think I'll put small tip fins on the stab and cut down on the area up front. Also will box the forward fuselage to give it some strength and to make it act less like a forward rudder.
|May 22, 2005, 03:22 PM|
It took more than a week but it all worked (I've done worse). I removed the single gear, boxed the front of the fuselage and added fins to the stab tips. Checked the CG and added about four ounces of lead to the nose. This is about right because I can remove the lead when I add the camera.
The model is very well behaved. It hand launches well, climbs aggressively and has a straight, controllable glide.
The wing section works
The high thrust line does not prevent hand launching
The very short tail moment is not a problem but you have to have a lot of fins.
Next step is the camera.
|May 23, 2005, 07:43 AM|
I still like the foam board but it may be too heavy for electrics.
If you expand the left thumbnail above you can see that the top of the fuse is covered with paper. I asked the art store if they had any paper with the same plastic coating as foam board and sure enough they do. It weighs about the same as 1/16th inch balsa and costs $2.70 a sheet. It is stronger than balsa in tension and isn't as brittle. You just cut it out with scissors and glue it on. Goes around curves real nice.
|May 26, 2005, 01:15 PM|
I ran some numbers for the LD-4 with present mods:
Total fin+rudder area as a percent of wing area = 24%
Total stab+elevator area as a percent of wing area = 42%
The stab is too big. The elevator movement has to be minimal to keep from overcontroling. I think it should fall between 30% and 40% of the wing area but it works fine and I'm not going to mess with it. It trims to almost 0 degrees so there's not much drag and better too much than too little.
The all up weight with the Olympus C-50 is now 57 ounces. Well over the 40 oz goal. That's also 4 ounces over last weekends flying weight. It works out to:
Wing loading = 14.25 oz/sq ft
Power loading ~ 40W/pound
Wing loading is OK but power loading is getting marginal.
Anybody have views on this?
|May 29, 2005, 05:23 PM|
Just flew at the above weight and got pictures. The plane and engine carry the weight but I breath easier when the hand launch is over. Thanks to the excellent skills of pilot CARL MOORE everything went fine. The model even makes 'power on' landings without the wheel. Not bad for a $5 paper airplane.
The photos are:
In the air and taking pics
The little red things in the ground are plastic feathers for runway alignment.
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