Frontal Area and Drag

[Mike Switzer]

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.

[steveinindy]

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.

[WLIU]

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

[Bill]

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.

[Mike Switzer]

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.


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.

[EDGEFLY]

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.

[Mike Switzer]

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.)

[WLIU]

"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

[rwanttaja]

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

[Mike Switzer]

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)

[WLIU]

Your ballast dilemma is not new. If you want to see how other engineers have solved the issue of moveable ballast take a look at the Lake seaplanes and the the homebuilt helicopters. A friend who owned Lake 250 I think had movable ballast for flying solo or with passengers. I recall that a couple of friends who built Rotorway helicopters had some sort of moveable ballast.

Of course, ballast that has no other function steals performance so if you can find a way to say make the main battery mount on a sliding tray so that it can be moveable ballast, rather than just using chunks of lead, you can minimize the penalty.

By the way, if you make the ballast removable, you will have problems hopping a friend over to another airport and then dropping them off to return solo. That's why ballast almost always is permanently attached somehow.

Best of luck,

Wes
N78PS

[Mike Switzer]

By the way, if you make the ballast removable, you will have problems hopping a friend over to another airport and then dropping them off to return solo. That's why ballast almost always is permanently attached somehow

I was actually considering some sort of arrangement to hold water, which could be drained or added as needed. If I make it permanent, the minimum weight & location required for solo flight with a 150lb pilot would cause cg problems when fully loaded - if i make it permanent in a location that is good for all loading scenarios more weight will be required. Of course at this point it is all just a swag anyway as I don't know the actual fuselage weight until I do the structural analysis.

[WLIU]

I was suggesting something like a sliding tray with quick release pins to allow it to be moved fore and aft. One of the new RG 35Ah batteries weighs something like 25lbs. The Odyssey dry cells weight 15 lbs. The FAA requires certified airplanes to secure something like a battery against a 9G forward accelleration, so a couple of tubular 4130 rails will easily handle those loads.

Water freezes in the winter so your flying year may be limited using H2O as ballast. And antifreeze is more $$ and upsets the environmentalists if you casually dump it down the airport storm drains. You will notice that the sailplanes that use water ballast go south for the winter or hide in trailers 4 months or so of the year.

Best of luck,

Wes
N78PS

[Mike Switzer]

I had already thought about the water freezing. If I put the batteries all the way in the tip of the nose it still isn't enough. I'm gonna mess with the numbers some more today.

[Flyfalcons]

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.

They need to keep room in the mag for Mac to drone on about flying IFR in his Baron.

[WLIU]

" it still isn't enough."

Sounds like Mother Nature is trying to tell you that you need a longer nose....

Best of luck,

Wes
N78PS

[Mike Switzer]

50lb of ballast that I can move from the nose to the CG will fix the problem. I hate the thought of carrying around 50 lb of dead weight.

[rwanttaja]

50lb of ballast that I can move from the nose to the CG will fix the problem. I hate the thought of carrying around 50 lb of dead weight.
And if you're expecting other folks to build and fly the aircraft, there's always someone who will forget to move the weight or decide it's not necessary.

Also, there can be effects that even the professional designers don't anticipate. Recall the crash of the BD-12 on its first flight:

"During the flight the pilot said the '...longitudinal handling qualities were found to be undesirable and a proper 'flare' aircraft attitude was not achieved prior to touchdown.' When the airplane touchdown occurred, the landing gear failed.

"The pilot said the center of gravity (C.G.) calculations placed the C.G. at the mid-range point on the C.G. envelope before flight. He said that he and engineering personnel from the company decided that the airplane had an aft C.G. during their analysis of the accident. The pilot also said that the airplane's fuselage had a forward aerodynamic center due to its elliptical shape. According to the pilot both factors created a longitudinal control problem with the amount of up stabilator deflection available."



The BD-12 had required ballast in the nose to get the CG forward to the point where the engineers thought it was mid-range. And, it apparently wasn't.

You're right to be considering the mechanical issues more than the aerodynamic ones, right now... the more you depart from the Long-EZ planform, the more risk you're taking.

If you decide to go with a side-by-side setup, you might look into the Cozy and see how they handle it the CG issues.

Ron Wanttaja

[Mike Switzer]

And if you're expecting other folks to build and fly the aircraft, there's always someone who will forget to move the weight or decide it's not necessary.

That is exactly what I don't like about it.


If you decide to go with a side-by-side setup, you might look into the Cozy and see how they handle it the CG issues.

It has been a while but I have looked at it - they didn't allow for much useful load increase over the Long-EZ, and if I remember correctly the Cozy & Velocity both add ballast to the nose. I suppose I should take another look at those plans, but I don't remember any significant change.

If I turn it into a twin I can make all the W&B issues go away, but I don't really want to go that route - we are back to adding drag, plus complexity. The whole point of this exercise was to make something easy to build. (if you know how to weld)

[Ron Borovec]

I take Ron Wanttaja's point regarding writing for Sport Aviation. He knows what he is talking about.

But I think our magazine, Sport Aviation, no longer has much room for serious technical content. A couple years ago at Oshkosh, after his seminar, I spoke with Neal Willford, the Cessna engineer. I thanked him for some of the great technical/design articles he had written. And also mentioned my point about the seeming lack of technical content in the magazine. He agreed. He said he had submitted another article to Sport Aviation but it did not get published. He said they offered to publish it online, but did not have room for it in the magazine. Neal said he just did not want to write articles if they would not make it into print.

I joined EAA in 1964, in what I now think of as the Paul Poberezny/Jack Cox era. Actual technical content was the main staple of Sport Aviation. Now they seem to think their main audience is more interested in human interest stuff. They may be right. I still prefer the old Sport Aviation.

Ron Borovec

[Bill Berson]

Neal Wilford was my favorite Sport Aviation contributor. He did a nice article a few years ago called "doing more with less".Wilford should resubmit his latest article to Mary Jones for the new EAA Experimenter digital magazine.

[Eddie King]

What about a Monocoupe ???

[steveinindy]

What about a Monocoupe ???

What's that? *scurries off to look for it on Google*