LD-4 Design and Build Thread - RC Groups
Thread Tools
Apr 29, 2005, 07:41 AM
Lift is cheap - Drag sucks
Tom Harper's Avatar

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
Weight.........................40 ounces
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 -
Last edited by Tom Harper; May 01, 2005 at 01:50 PM.
Sign up now
to remove ads between posts
Apr 29, 2005, 07:51 AM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Look Down Design and Build Thread
Apr 29, 2005, 07:55 AM
Lift is cheap - Drag sucks
Tom Harper's Avatar
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
Lift is cheap - Drag sucks
Tom Harper's Avatar
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.
Last edited by Tom Harper; May 02, 2005 at 07:00 AM.
May 01, 2005, 02:29 PM
Ascended Master
Sparky Paul's Avatar
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
Lift is cheap - Drag sucks
Tom Harper's Avatar

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
Lift is cheap - Drag sucks
Tom Harper's Avatar

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
Lift is cheap - Drag sucks
Tom Harper's Avatar
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 22, 2005, 09:02 PM
Ascended Master
Sparky Paul's Avatar
Tom, good stuff!!
I'm looking around for a more expendable/expanable airframe that slaps together..
May 23, 2005, 07:43 AM
Lift is cheap - Drag sucks
Tom Harper's Avatar

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
Lift is cheap - Drag sucks
Tom Harper's Avatar
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?
Last edited by Tom Harper; May 26, 2005 at 01:22 PM.
May 29, 2005, 05:23 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
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


Almost down

The little red things in the ground are plastic feathers for runway alignment.
Jun 04, 2005, 03:10 AM
Registered User
Looks very neat !

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Ho XVIII (Horten 18 Amerika Bomber) - Design/Build Thread Cyberman Electric Ducted Fan Jet Talk 59 Aug 01, 2011 08:42 PM
Northrop X-4 Bantam Build thread Robert May Electric Ducted Fan Jet Talk 232 Dec 14, 2009 03:45 AM
Build Log Profile SU-37 Design/Build Thread Kai_Shiden Pusher Prop Jet Models 377 Nov 14, 2007 03:26 AM
RBC A-4 Skyhawk build thread X.X Willmsy Electric Ducted Fan Jet Talk 6 Nov 22, 2005 11:01 PM
LD-4 design and build thread under Modelling Science Arthur P. Aerial Photography 1 Jun 27, 2005 04:01 PM