Cheating The Wind: Question for Eric - DOCTOR - Anyone?

[AAFO_WSagar]

The thought came to me while I was shooting the Hughes Racer last week.... Here's an airplane that was designed by someone who had the resources to build an airplane from the ground up with nothing more than going fast in mind... and, well I guess far, since he had the coast to coast record in mind also (not exactly sure if that was in the original plan or an afterthought though)

But back to the original thought.. Even though this was in 1935, they did the design after much wind tunnel testing. There are some rather unusual features onthe airplane, most notable to my untrained eye is the way he terminated the fuselage... a very interesting piece this tailcone!

Is there anything on this airplane that is of enough interest/value that it would retrofit to any "modern" design?

I know some of you guys can pretty much "see the wind" so I was wondering what thoughts struck you when you looked at that tailcone shot.. or any of the other "stuff" on the airplane.. The wing root fairing is also a very interesting design, maybe not totally visible in the photos but it's pretty extensive.

Wayne

[Eric Ahlstrom]

No free lunch

The tail of the Hughes is a good example of the intuitive reasoning that something aesthetically pleasing (smooth curves) should be aerodynamic. Unfortunately, there is no good connection here. While most efficient aerodynamic shapes are aesthetic, not all aesthetic shapes are aerodynamic!

Tail closure (as it's called in the design world) is a fairly hot topic and most aircraft are very poorly designed in this area. The Hughes design pictured does three things wrong which vitually guarentee separated flow:

1. The pressure recovery areas of the horizontal and vertical stabilizers are coincident. This means that the boundary layers of both roots have to accelerate to higher velocities and recover more pressure at closure than they would if they were separated by some longitudinal distance.

2. The fillet fairings that look so graceful increase the interference acceleration and pressure recovery amounts to the point where it is doubtful that an unseparated closure of the boundary layers can be achieved at all.

3. The pressure recovery area is too short. The combined pressure recovery of the vertical, horizontal, and fairings is too great for the short "stinger" pictured and there is no reflex curve in the pressure recovery area to suck the boundary layer down. Figure that there is about 1 to 1.5 sq. ft. of flat plate drag pictured and that's quite a bit. Tufts would capture this better than oil streak data, though all it would tell you is that you have a problem and not what to do about it.

That's MY job <LOL>

[AAFO_WSagar]

Interesting that they came to this design after extensive wind tunnel testing (with model)...

Knowing that seeing a picture is harder to analyze than in person (that chance will come, hopefully, at Reno this year) that aside, what's your take on the design overall in their "cheats"..

Sure "looks" slick to the untrained eye!

And the replica is absolutely SWEET to see! Fantastic craft at work here!

Wayne

[Unregistered]

Re: No free lunch

"1. The pressure recovery areas of the horizontal and vertical stabilizers are coincident. This means that the boundary layers of both roots have to accelerate to higher velocities and recover more pressure at closure than they would if they were separated by some longitudinal distance."

Presumably this is part of the reason that aircraft like the F/A-18 have the horizontal tail located so far aft of the verticals?

"2. The fillet fairings that look so graceful increase the interference acceleration and pressure recovery amounts to the point where it is doubtful that an unseparated closure of the boundary layers can be achieved at all."

And I presume this has something to do with why the "screwdriver tail" (MD-88 and later DC-9 derivatives, MD-11, 777) is now the preferred closure on high-efficiency airliners. All the activity occurs in a plane, rather than at a point?

[MattB]

Perfect aircraft

So is there a "perfect aircraft" out there? Especially back in those days. Id be interested to know if someone got lucky or figured it out.

[Eric Ahlstrom]

Originally posted by AAFO_WSagar
Interesting that they came to this design after extensive wind tunnel testing (with model)...

Testing and analysis alone do not create a better design. One could test this coincident pressure recovery tail root with fillets all day and never realize that no fillets and staggered pressure recovery regions were better. You'd have to actually build the different tail designs and compare them.

Kelly Johnson was famous for his wind tunnel work because he would try many wildly varied design options (over 20 tail configurations for the F-104...). If all someone does is test what they think already works, all they will get is one data point; no real development occurs.