Wing sweep on low speed aircraft (piper cherokee)

[MHAIRD]

Good afternoon everyone

I am investigating the effects of wing sweep on low speed aircraft (max speed 230 km/h). I am focusing on the effect it has on the drag, lift and stability of these small aircraft.

I am debating whether to focus on forward or aft swept wings, for this low speed application, does anyone have suggestions or criteria I should look at?

Thanks for the help!

 

[WKTaylor]

Look at Mooney, Aerostar, Aero Commander: all have forward sweep wings. Mooney also has forward sweep stabilizers.

I am a structures guy... so my answer is more structures oriented.

Wing structure is complicated by the forward sweep wing: has to be very stiff in bending and torsion to minimize divergent loading, IE: as AOA increases, unstiff forward sweep wings tend to bend and twist the leading edge [outboard] to a higher AOA increasing shear, bending and torsion further... etc. Aft sweep wings tend to do the opposite: relieve load as AOA increases.

Structure has to be tailored to account for these effects so it can tolerate these effects up to ultimate load. Metal structure usually designed to higher strength and stiffness for this purpose. Composite structure can be designed ["tailored"] so that it actually responds to load-induced strain by bending/twisting to a relieving/reducing load state.

Aerodynamically I suspect some benefit... but the structural price can be high. Usually GA bird benefit from the keep it simple and light principle.

Unfortunately, the rounded-tip "hershey bar" simplicity of older Cherokee wings made them simple and rugged but... but with a significant negative effect on performance [L/D]. This is why the wing was redesigned with a longer span and taper ["Cessna look"].

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[rb1957]

i'd look at airpalne design books, like Raymer or Torenbeek ... i think they'll go into the trade-offs more. personally i doubt ther's much to be gained ... you've got to think if there was then they'd've done something about it ... maybe the price of gas is revising the value of some designs. and as composite materials get cheaper you might find other designs becoming more favourable ... fwd sweep has aero-elastic issues (as Will mentions above), but designing the stiffness of the wing carefully (with composite materials rather than Al) you can over-come these issues.

there was a prototype/experimental fighter some years back with significant fwd sweep, i think they were looking for a more manoeuverable plane, guess they didn't find it !?

Quando Omni Flunkus Moritati

 

[Sparweb]

...there was a prototype/experimental fighter some years back with significant fwd sweep, i think they were looking for a more manoeuverable plane, guess they didn't find it !?

X-29: [URL unfurl="true"]http://www.nasa.gov/centers/dryden/news/FactSheets/FS-008-DFRC.html[/url]

The project was more of an experiment to develop advanced control systems, able to handle such an inherently unstable aircraft. The project was considered a success due to the team's ability to marshall control over the aircraft, despite it's radical configuration, but they didn't see performance gains because of it, no. But who really needs to fly at angles of attack greater than 45 degrees, huh?

Sweep: NACA Technical Reports Server

This subject was studied extensively in the 1930's/40's by NACA. If you're new to the subject, try starting the search with the older reports, which are much more readable, then by moving forward in time, you can more easily find what you want and refine the search for terminology that applies to your subject of interest.


STF

 

[MHAIRD]

Thanks guys, really appreciate the pointers.

Wil, I have read about the stresses induced in forward swept wings at higher speeds, but do you think at (relatively) slow speeds such as 230 km/h (max) these stress will be of any significance?

Thanks, SparWeb, for the sweep link!

Mike

 

[rb1957]

i think it's doable with low speed a/c. I'd expect some weight penalty and/or a cost penalty for some questionable drag benefit ?

Quando Omni Flunkus Moritati

 

[SAITAETGrad]

One reason for sweeping wings can be simply to ease the layout. Especially on low wing aircraft, sweeping wings can improve the pilot view. It also allows a slight offset between the wing attachments and cg.

 

[WKTaylor]

MHAIRD... a few thoughts to ponder...

Of all the GA and homebuilt acft with forward swept wings [IE], there are NO competative aerobatic [extreme high-G] acft in this catagory. Inevitiably, they all have "0" sweep at the 1/4-chord for both the wings and stabs. The complexity of, and/or added weight of, forward [or aft] sweep... to ensure stiffness/deflection control and ultimate strength of wing and stab surfaces... makes it infeasible.

I understand that early Aerocommanders had a bad habit of loosing wings above limit load, but not ultimate. "I think" beef-ups were required for wing torsion and bending resistance. The Aerostar was an evolution of the Aerocommander and did not have these failure modes up-to ultimate [much stiffer short wing].

Another parallel cautionary tale is the "V" tail Bonanza. Certain design assumptions led to understrength stabs, on specific "growth" models... which then developed reputations for failing stabs below ultimate for various aero/structural reasons [similar in nature to forward sweep].

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.