Fat strake mod

[Monty Roberts]

Rather than hijack existing threads with similar themes running through them I thought it best to start a new one.

 

I am not advocating or suggesting anybody do this, but I am trying to hash through the issues associated with increasing strake volume by making the top and bottom of the strake more curved and less flat. I’m not suggesting anything as extreme as the King Cozy but more along the lines of what Robert and Valerie are doing.

 

The issues I think are most significant are Aero, CG location, and construction difficulty.

 

 

Aero:

 

I really don't think that making a slightly curved top or bottom will change much. My reasoning is that the strakes are essentially a low aspect ratio delta wing with a rounded leading edge. The lift curve slope for this type of wing is shallower than a high aspect ratio wing (main and canard). The high aspect ratio main wings should stall long before the strakes do. It is possible that the center of lift of the strake relative to AOA could move some. My only concern is that it could affect the CG range adversely. I don't think it would, but I don't know for sure.

 

Deep stall: I have often wondered if putting a sharp leading edge on the strakes would help this problem. It would allow the strakes to move into the vortex region of lift generation at a lower angle of attack thus shifting the CL aft and helping to push the nose down......Don't have a wind tunnel in my back yard to try it out. Since the leading edge is already rounded, I don't think rounding the top and bottom a little would make much difference in this area.

 

Drag: probably a slight increase to no different. More wetted area, possibly a more efficient shape who knows.

 

 

CG:

 

Anybody contemplating this would need to make sure that the center of volume of the tank is somewhere near the CG of the airplane. A WB analysis would need to be made to make sure you were not causing problems here.

 

There is also the more difficult fabrication issue, but I am going to make that go away with a lot of arm waving.

 

Any other thoughts?

 

Thanks,

 

Monty

[steve]

C:\Users\Steve Parkins\Desktop

Aero:

The high aspect ratio main wings should stall long before the strakes do.

 

Any other thoughts?

 

Thanks,

 

Monty

at osh..06? we heard nat say that the strake has bad air from the canard and there for has little lift (he said no lift).

just more gas for the fire.

[Marc Zeitlin]

... I am trying to hash through the issues associated with increasing strake volume by making the top and bottom of the strake more curved and less flat....

The AeroCad prefab strakes are basically this - the top and bottom are very slightly rounded. I have them, and my strakes hold about 59 gallons total, rather than the stock 52. Other than that, there's no aerodynamic or structural effect that I found in all of my stall/stability/flutter/etc. testing during Phase I.

[TMann]

There also exists the opportunity to fatten the strakes by adding thickness to the c/s spar by adding foam to the top and bottom and bevel from inboard to outboard along the straight section of the spar. ( After completion of the spar per plans. )

[Steve Innova]

C:\Users\Steve Parkins\Desktop

at osh..06? we heard nat say that the strake has bad air from the canard and there for has little lift (he said no lift).

just more gas for the fire.

 

What calculations did he perform to arrive at this conclusion?

[steve]

What calculations did he perform to arrive at this conclusion?

i dont know, but you can call him:confused:

[TMann]

I wasn't there and I don't have the background to claim expert status BUT what I have read on the subject may explain the statement.

 

The strakes sit in the wake of the canard. A lot of discussion has been made about improving the profile of the strake to create a laminar airflow in this area. It was something I looked at seriously as well. The problem is you can't smooth out turbulent airflow (in the wake of the canard ) and the effort in this area is futile.

 

For the same reason, I'm thinking that a curved top vs. the flat profile top skin isn't coing to cost you anything as far as the aircraft performance.

[Marc Zeitlin]

There also exists the opportunity to fatten the strakes by adding thickness to the c/s spar by adding foam to the top and bottom and bevel from inboard to outboard along the straight section of the spar. ( After completion of the spar per plans. )

If I understand you correctly, you would be putting a layer of foam between the top spar surface and the strake upper surface inner skin, and between the bottom spar surface and the strake lower surface inner skin.

 

This would be a VERY bad idea. You would be a piece of foam in what is a major shear joint for the structural torsional ridigity of the aircraft. You would lose a large part of the torsional rigidity of the wing, and make yourself far more susceptible to wing bending, twisting, and flutter.

 

Not to mention that the joints between the strake inner skins and the spar are sealing joints for the fuel tanks.

 

This is not something anyone should even think about.

 

And with respect to the claim that the strakes produce no lift, Nat notwithstanding (at OSH 2005, not 2006), this is a very misleading statement, and we've discussed this here (and in numerous other venues as well) many times. At cruise, when the AOA is low, the strakes produce less lift per unit area than the wings, but they do produce a little. At low speeds, at high AOA, there is no question that the strakes are producing lift - it would be physically impossible for them not to be contributing to the overall lift of the aircraft, given their shape and orientation to the airstream.

[TMann]

This would be a VERY bad idea.

Thanks for that, Marc. Good to know ahead of time.

[Monty Roberts]

Thanks for the replies everybody.

 

I agree with Mark on the foam on top of the spars. The proper way to do this would be to taper the spars so that the spar caps are farther apart. Then make sure the strake skins are bonded to the spar caps. This would not be that difficult to do and would result in spar that is stronger in bending and torsion than the original due to the increased separation of the caps and strake skins.

 

The other option is to use a 30 deg or less pan - down to the standard spar and put the foam filler on TOP of the skin with a cosmetic lay-up over that. This is of course the heavier, weaker, and less elegant solution.

 

Aerodynamically speaking if you do this you will then have to deal with a more abrupt contraction aft of the strake where it joins the cowl. This could create problems with separation which is right in front of the prop.......already a problem area now made worse. Robert and Valerie obviously don't have this problem to deal with

 

 

 

Monty