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Thread: Frontal Area and Drag

  1. #1
    Mike Switzer's Avatar
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    Frontal Area and Drag

    I found Barnaby Wainfan's article in the new issue of Kitplanes interesting.

    It is causing me to consider a major design change. If I switch from tandem seating with rear baggage to a 2+2, using the +2 as a baggage area when nobody is sitting there (sort of like the "rear package shelf" in my 944) I can significantly reduce the wetted area, and it won't be that much wider.

    I am attaching a pdf copy of the article - anyone have any comments? I am a mechanical engineer, not an aero engineer, so I hadn't thought about this a whole lot, I had assumed that less frontal area would result in less drag, but for the stated mission of 2 (or 2 1/2 people) plus baggage I am not so sure after reading this article.
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    steveinindy's Avatar
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    I guess the only argument would be whether you'd rather be able to have your passenger next to you or behind you. I'd prefer to have my fiancee sitting next to me rather than behind me but that's just my opinion.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



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    All of the points in the article are very true. The catch is that you really have to use composite materials to take advantage of what he is talking about. Take a look at the original Lancair 2 seater. The shape of the fuselage behind the cockpit is very important for keeping the flow smooth, reducing drag, and even recovering some pressure to essentially push the airplane and offset the frontal area. THe shape and the attention to the details of how the air flows around it allow the airplane to cruise at almost 200mph on 100hp. Look at Mike Arnold's little AR-5. The sexy RV wheel pants are another example of pressure recovery. The compound curves aft of the center of the wheel pant is very important. More recently, take a look at the fuselage shapes of the latest Cessna's, etc. The curve inward behind the cockpit is there for a reason.

    The problem with traditional materials is that they are hard to shape into the curves that address the issues in the article. At least in what passes for mass production for general aviation. Aluminum can be shaped into compound curves but the equipment is expensive in relative terms so Cessna/Piper/Mooney avoid doing that. A homebuilder can shape wood into nice curves with the right equipment but most homebuilders draw the line at that level of woodworking skill and equipment acquisition. Walter Beech sort of tried to shape the Staggerwing to get some pressure recovery aft of the passenger compartment and if you have ever seen a Staggerwing fuselage without any fabric on it, you will be at once in awe of the amount of wood stringers that form the shape, and horrified by the amount of woodworking that a rebuilder has to do.

    So when you think about aircraft shapes you need to think about how to coax the air into flowing around your shape and what material you can use to achieve the shape that you want. As Mr. Wainfan explains, the corners and edges make the air unhappy....

    Best of luck,

    Wes
    N78PS
    Last edited by WLIU; 09-01-2012 at 06:51 PM.

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    Quote Originally Posted by Mike Switzer View Post
    I found Barnaby Wainfan's article in the new issue of Kitplanes interesting.
    Why can't we have this kind of article in Sport Aviation? As a builder, I am really interested in frontal area and drag, and their effects on my airplane.
    Bill

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    Mike Switzer's Avatar
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    Quote Originally Posted by WLIU View Post
    The catch is that you really have to use composite materials to take advantage of what he is talking about.
    I think it can be accomplished with tube, fabric, & wood without any more work than it would be using foam core composite. Molded composite would naturally be easier for a large production run, but would be considerably more expensive to get started.

    Quote Originally Posted by WLIU View Post
    The shape of the fuselage behind the cockpit is very important for keeping the flow smooth, reducing drag, and even recovering some pressure to essentially push the airplane and offset the frontal area.
    So what do you do if the only thing behind the cockpit is the engine? I can keep everything pretty smooth from the tip of the nose to the rear of the cockpit.

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    Mike,Please share with us your analysis and conclusions about including changes to your design. Hopefully a bit more specific than I "decided on tandem vs sidebyside" . Many purchase decision are made on such a simplistic basis (including my own) but if you based a design decision on these (aerodynamic) factors, it would certainly be of interest to this group. I would also support Bills' point that this is a good example of what would be interesting material for Sport Aviation.

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    Mike Switzer's Avatar
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    Quote Originally Posted by EDGEFLY View Post
    Mike,Please share with us your analysis and conclusions about including changes to your design. Hopefully a bit more specific than I "decided on tandem vs sidebyside" .
    I'm not sure how much aerodynamic analysis I will be able to do, I only had the introductory Aero engineering class in college as I don't get along good with differential equations.

    I may end up buying some software to help analyze it if it isn't something I can set up in excel or Mathcad.

    Yesterday I spent some time looking at the different configurations side by side, and while I haven't reached any definite conclusions, there are some interesting tradeoffs. For one thing, with a light weight pilot (~150lb) and no other load W&B issues are easier to manage in the tandem configuration - with that big engine weight hanging off the back of the plane, the farther forward the pilot is helps somewhat.

    (The engine weight & position put me at a stopping point yesterday, I need to get some oil pan & water pump measurements as I may have an interference problem with the landing gear, and I need to get the engine as far forward as possible.)

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    "So what do you do if the only thing behind the cockpit is the engine? I can keep everything pretty smooth from the tip of the nose to the rear of the cockpit"

    Take a look at the Lightspeed Engineering pusher propeller spinner.

    The advantage of a pusher configuration is that the fuselage meets undisturbed air. Tractor configurations have the complication of all of the flow finesse having to be applied in air that has been accellerated and rotated by the propeller. That is one reason that some of the ways that you reduce drag are not obvious. Folks do not realize that the air is no longer moving straight aft once the prop grabs it. Pusher designs don't have that problem.

    So one thing that you might ponder is how Whitcomb's area rule might apply at the lower Reynolds numbers that most homebuilts fly at....

    Best of luck,

    Wes
    N78PS

  9. #9
    Quote Originally Posted by Bill View Post
    Why can't we have this kind of article in Sport Aviation? As a builder, I am really interested in frontal area and drag, and their effects on my airplane.
    Part of the problem is the legacy of a long-ago policy that no longer exists.

    Up until a decade or so ago, Sport Aviation was considered a member's newsletter, and thus members were expected to write for it without compensation. When homebuilding exploded in the '80s, several new magazines appeared (such as Kitplanes). Thsse magazines paid for content.

    So if you're going to spend a few hours slaving over a highly technical topic, your choice was to send it to Kitplanes, Homebuilt Aviation, Sport Pilot, etc. and get paid several hundred bucks, or send it to EAA and get two extra copies of the magazine. The decision wasn't that tough.

    I don't know when EAA changed this policy, but it was the target of a lot of rancor in the aviation writing community. Even today, I catch flak... "Ron, why do you write for EAA for free?" The fact is that EAA has been paying authors for a number of years, and their rates are about the best in the industry.

    The legacy today is that writers like Weir and Wainfan extablished a comfortable relationship with Kitplanes magazine and don't see any reason to change. Heck, *I've* got a comfortable relationship with Kitplanes, too, but am friends with some of the folks at EAA and thus don't turn them down if they want an article.

    Whether Sport Aviation would print a Wainfan-ish sort of article, I really don't know. In my opinion, there's generally a inverse relationship between wealth and interest in engineering topics. A guy without a lot of money is interested in efficiency; a guy *with* money just hires someone to build him an RV. There are those who believes EAA is trying to appeal to the latter.

    Oh, one last note: Barnaby's article is copywrited. It really shouldn't be posted to other forums.

    Ron Wanttaja
    Last edited by rwanttaja; 09-02-2012 at 04:21 PM.

  10. #10
    Mike Switzer's Avatar
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    Quote Originally Posted by rwanttaja View Post
    Oh, one last note: Barnaby's article is copywrited. It really shouldn't be posted to other forums.
    I had a debate with myself about that - but I couldn't figure a way to pose the question where we could have an intelligent conversation about it if half the people here had no idea what I was talking about. I won't argue if the mods think it needs removed, but then this thread isn't very useful.

    On another note, I haven't even got to the point of running aerodynamic calculations as I have been looking at the weight & balance differences between the 2 configurations. Basically, to keep the W&B within an acceptable range between fully loaded & just a 150lb pilot, I need to add 20 lb of ballast in the tip of the nose in a tandem seating configuration (with baggage between the rear seat & the engine), 60 lb (that must be removable) if I shorten it into the 2+2 configuration. (Unless I can figure another way around it)

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