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Another speculative thread - adding forward sweep to solve CG problems

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karmarepair

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AERODYNAMICALLY, what would the impact be of adding some slight forward sweep to an existing Light Sport sized single place parasol, originally fitted with a Rotax 2-stroke?

The idea is to re-do the wing attach points and the struts and get the center of lift a wee bit further forward to accomodate a heavier 4 - stroke engine installation of comparable thrust.

My recollection is that the forward sweep will tend to make the inner surfaces of the wing stall earlier than the tips. Moving the CL forward will also increase the tail volume a little, not necessarily a bad thing either. What other effects?

Structurally it needs examination of course.

Lead in the tail would be easier; granted.
 

Dana

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Structure is one issue, yaw stability is another. Also if it's a fabric covered wing, the ribs won't be parallel to the airflow any more which could make for some funny (i.e. undesirable) effects.
 

rbarnes

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My reading on the subject talked about wing twist and needed structures to offset said twist being substantial.
 

karmarepair

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Structure is one issue, yaw stability is another. Also if it's a fabric covered wing, the ribs won't be parallel to the airflow any more which could make for some funny (i.e. undesirable) effects.
Yaw stability would DECREASE, yes?

Any thoughts on how the spanwise flow component would affect the the CsubL? It seems to me like it would make the airfoil look thinner than it would otherwise.

Good catch on the Ruffles Have Ridges for a fabric covered wing, especially one WITHOUT a forward D-cell.
 

karmarepair

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My reading on the subject talked about wing twist and needed structures to offset said twist being substantial.
The one relatively common example of a light E-AB airplane with forward sweep is the Cygnet, and it has geodetic wings, relatively torsionally stiff. I think some of the French LSAs also employee it, Bleu Citron perhaps?
 

Vigilant1

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My reading on the subject talked about wing twist and needed structures to offset said twist being substantial.
Right, the wing twist will tend to increase AoA at the tips, which leads to ungood stall behavior.
With a parasol, I suppose you'd also need to move the lift strut attach fitting forward to match the new location of its wing attachment.
I'd definitely consider the more conventional approaches first -- shorten the engine mount, move the wing forward, if there's a battery, move it aftward, etc.
 
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Will Aldridge

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Kitfox models 5-7 sometimes sweep the wing forward a little if using a heavier engine, a la lyc-240(or bigger). So definitely some precedent.
 

cluttonfred

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The one relatively common example of a light E-AB airplane with forward sweep is the Cygnet, and it has geodetic wings, relatively torsionally stiff. I think some of the French LSAs also employee it, Bleu Citron perhaps?
Yes, also some older factory types like the Nord NC.850 series. That said, I think the effects of *slight* forward sweep are not as extreme as have been suggested.


 
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pictsidhe

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If you stall the roots first as they will tend to with forward sweep, there will be some pitch up. Others have already mentioned torsional effects washing the tips in under high g. The structural problem can be designed around with a suitable shear centre of the wing.
 

Doran Jaffas

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AERODYNAMICALLY, what would the impact be of adding some slight forward sweep to an existing Light Sport sized single place parasol, originally fitted with a Rotax 2-stroke?

The idea is to re-do the wing attach points and the struts and get the center of lift a wee bit further forward to accomodate a heavier 4 - stroke engine installation of comparable thrust.

My recollection is that the forward sweep will tend to make the inner surfaces of the wing stall earlier than the tips. Moving the CL forward will also increase the tail volume a little, not necessarily a bad thing either. What other effects?

Structurally it needs examination of course.

Lead in the tail would be easier; granted.
Take a look at the Cygnet.
 

jedi

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Rather than reworking structure for sweep wouldn't it be just as easy to move wing forward or aft on a single place parasol.

Re-do the wing attach points and the struts to move the wing a wee bit further forward to accommodate a heavier 4 - stroke engine installation of comparable thrust.

Most parasol wings have the wing panels join in a narrow center section about 6 inches wide. I would incorporate a wider center section similar to the Stearman biplane that has about three feet of wing section (with a fuel tank I think). This would also add some wing area and wing span to help with the increased weight.
 
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cluttonfred

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As much as I love design challenges and alternative approaches, I think this may be a solution in search of a problem. A few points:
  • Keep in mind that something like an industrial V-twin conversion is going to come out shorter overall than, say, a Rotax 447 or 503, naturally moving the CG of the engine itself closer to the firewall, so the impact on the overall CG may be minimal.
  • If it is still too nose heavy, a little weight in the tail might solve that.
  • Another option might be to angle or cut back all or part of the firewall and engine mount to allow the heavier four-stroke engine to sit a little more aft.
  • A neat trick I have seen is to cut a hole in the firewall for an appropriate-size flanged stainless steel mixing bowl to make space for a magneto or what have you to allow the whole engine to be mounted closer to the firewall.
Any of these solutions would solve your problem without messing with wing structure or aerodynamics at all.

Cheers,

Matthew
 

Riggerrob

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If you keep wing (forward) sweep to less than 10 degrees, it has little effect on structure or stall characteristics. See Cygnet, Bolkow Junior or the next dozen sailplanes.

However, I prefer Cluttonfred's ... er ... Jedi's suggestion of building a new set of cabane struts that move the stock wings forward by a few inches. That is one of the beautiful things about parasols and biplanes ... allowing you to fix wing position very late in the build process.
 
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TFF

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Do calculations, how much sweep vs moving the struts. If you move the wing, you probably get to reuse everything. Sweep the wing will require new wing struts. To me it sounds cheaper and easier to move the wing but the guy who is redoing the tail on his LSA did the opposite looking for speed, removed the sweep. I think it would matter what building materials are being used too.
 

blane.c

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Or leave the wing alone in relation to fuselage. Move occupants or move engine. ... ... Use two or more engines mounted on CRI CRI type mountings as needed to solve CG problem and gain multi-engine redundancy. Actually some engines could be engines but some could actually be "motors" and the batteries could be placed as need for CG, a hybrid.
 

Tiger Tim

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Also if it's a fabric covered wing, the ribs won't be parallel to the airflow any more which could make for some funny (i.e. undesirable) effects.
I think it would be a stroke of luck if any rib on a fabric covered wing was truly parallel with the local airflow once you consider inflow over the wing, outflow beneath it, interference around struts and the fuselage, propeller slipstream, etc. In any case, the Tiger Moth has all its ribs at 90° to the (swept) spars and it doesn’t seem to be an issue.
 
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