Moving the rear spar in?

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ToddK

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Hey all, I am working on a little pusher project and want to tuck the engine into the rear of the fuselage 12-16" rather then put it up on top, or hanging way out the back. Engine options should be anything from a 912 to a Yamaha, to a Suzuki/Honda. The obvious problem with that is that it does not leave a good structural place to attach the rear spar.

The plan is to use tube spars of some sort. Either a big one like a Kolb or a pair of 2.5" or 3" front and rear spars. My first thought was to move the rear spar in, and put a false spar on the back to mount the ailerons to. Has that been done before? I really don't want to go off the reservation here. I am trying to stick to proven construction techniques.
 

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wsimpso1

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You can stick the aft spar further forward than the usual position, or skip it entirely. Having the aft spar as far aft as you can reasonably get it reduces forces on everything having to do with the wings and thus allows the lightest assembly that way. To make things work well with the spar forward means substantially higher forces on the drag spar, maybe on the main spar, on the ribs aft of the drag spar, and on the struts too. The min weight for all these pieces usually goes up some when that happens. It may be that you are up against minimum gage everywhere, and so no weight gain would occur, but in spars and struts and such, that is pretty unlikely.

Are you working in fabric covering over metal tubes and ribs? That kind of constrains you to available tubes and flat stock.

You have some analysis ahead of you.

Billski
 

ToddK

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You can stick the aft spar further forward than the usual position, or skip it entirely. Having the aft spar as far aft as you can reasonably get it reduces forces on everything having to do with the wings and thus allows the lightest assembly that way. To make things work well with the spar forward means substantially higher forces on the drag spar, maybe on the main spar, on the ribs aft of the drag spar, and on the struts too. The min weight for all these pieces usually goes up some when that happens. It may be that you are up against minimum gage everywhere, and so no weight gain would occur, but in spars and struts and such, that is pretty unlikely.

Are you working in fabric covering over metal tubes and ribs? That kind of constrains you to available tubes and flat stock.

You have some analysis ahead of you.

Billski
Fabric covering for the most part. Not opposed to making some bits or covering some bits out of sheet aluminum. For example, the two inboard and outboard ribs will probably be converted with aluminum like the challengers do.

Candidly, there will be zero structural analysis by me. Don’t have the background or the patience to learn how to do it, hence my desire to use proven structures and sizes. I have no shame in ripping off stuff that works. Many successful designs have been built this way. A guy with solid works and bit of an education can get in real trouble by going off the reservation (raptor). Once I think the design is sufficiently complete, I will beg or more likely pay someone to look over the critical bits. Far cheaper and faster to pay a professional to make sure everything is correct.
 

Hot Wings

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3 robust ribs to mount the the control surface pivots and anti-drag wires? The 'false spar' could then be little more than filler.

3 rib.jpg
 

Riggerrob

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A major issue is torsion ... the wing tip trying to twist relative to the wing root. Aileron loads further complicate aero-elasticity.
This can be solved with cantilever wing structure that is stiffer, or by struts.
How many wing struts were you planning to install. A V-strut will solve most torsion problems. It will also reduce loads on wing roots. At that size and speed, struts will contribute only a little more drag.

The other extreme involves building a thick and stiff D-spar that incorporates the main spar web and leading edge. Pitch loads can be resolved through a pin joint (attached to the fuselage) near the leading edge or root trailing edge. The shorter the distance between the main spar and the pin joint, the more precise and stronger the pin joint needs to be. A disadvantage is the strong and heavy ribs needed to transmit aileron loads to the main spar.

Finally, you might want to consider some of the reflexed airfoils used by helicopters and flying wings. They produce almost as much lift, but have docile (bordering on zero) pitching problems.
 

Riggerrob

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Munk M-6 comes to mind.
Munk M-6 airfoil has a bit of reflex near the trailing edge, but is that enough reflex to make it neutrally stable in pitch ... with a horizontal stabilizer?
Yes, we know that Munk M-6 was used on GeeBee racers.

More modern, laminar flow reflexed airfoils have been developed for helicopters.
 

ToddK

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Since its a steel tube cabin section . Perhaps a "U" shaped truss under the engine and also make it part of the bed engine mount.
The more I look at the the various engine options that I have considered the more sense this makes. On all of them there is path that would allow both sides of the fuselage to be joined, especially the Suzuki and Yamaha engines.

This greatly simplifies the wing allowing me to use one of many current designs.
 

mcrae0104

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Given the design considerations, I might strongly consider a zig-zag rib arrangement (similar to an Ercoupe). The ribs form a space frame, transferring the torsion generated by the aft portion of the wing to the main spar. The false (rear) spar wouldn't need to extend all the way to the centerline of the aircraft if designed properly, and only a single strut would be required.

1635041133077.png
 

TFF

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Moving the rear spar forward and a false spar added is not common on rectangular wings and aluminum. Wood wings that are elliptical or tapered or widened at the aileron, very common. Most of this is time in history. Wood wings and fancy shapes were common in the 1930s for fancier airplanes. Aluminum tubes from the 1970s on usually had a different simpler mission.
 

challenger_II

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The OP is looking to build an ultralight: a laminar flow wing section would be counter-productive.
On the subject of reflexed airfoils, keep in mind that, if One is using full-span ailerons, the reflex will act as a trim tab, and induce a downward force on the control surface. Food for thought...


Munk M-6 airfoil has a bit of reflex near the trailing edge, but is that enough reflex to make it neutrally stable in pitch ... with a horizontal stabilizer?
Yes, we know that Munk M-6 was used on GeeBee racers.

More modern, laminar flow reflexed airfoils have been developed for helicopters.
 

Pops

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Another observation: do not use such an extremely swept back vertical tail leading edge, it makes the tail rather ineffective. No more than 20 deg recommended. Ask Eugene with his Skyboy, which looks exactly like this, if he likes it!
And another thing the swept back vertical tail does is a pitch change coupled with the rudder. I can demonstrate this with a C-172.
 

Riggerrob

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Given the design considerations, I might strongly consider a zig-zag rib arrangement (similar to an Ercoupe). The ribs form a space frame, transferring the torsion generated by the aft portion of the wing to the main spar. The false (rear) spar wouldn't need to extend all the way to the centerline of the aircraft if designed properly, and only a single strut would be required.

View attachment 117259
Ercoupe's zig-zag ribs also carry drag loads. They replace the diagonal cables seen inside many fabric-covered wings.
Overall, a clever innovation.
 

Tiger Tim

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Another thought would be to sweep the wing to keep the trailing edge from interfering with the engine. The fringe benefits would be the directional and roll stability from the sweep coupled with any washout giving a bit of pitch stability would allow you to size the tail feathers on the smaller side and save weight, drag, etc.
 

ToddK

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I kitfox type wing would also fit perfectly. Looks like I need to sort though some options.
 
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