RCU Forums - Multi Segment Fowler Flaps

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Multi Segment Fowler Flaps

I was going through my computer and found these files. I thought they may inspire some and at least spark discussion.

What these drawings represent are my study of a multi-segmented fowler flaps for R/C use.

I developed these about 5 years ago and actually built a wing for test flights. I flew the wing about 10-15 times and remember astonishing results. Notice the LE of the wing in the drawings has slots as well.

The drawings represent a Wing Chord of 19" (for size reference) The entire extention/retraction sequence is accomplished with 60 degrees of servo travel with a positive up and down lock (overcenter device).

I'll dig around some more to see if I can locate the actual CAD files (AutoCad) that I used to make the track/lever assemblies.

Enjoy,

Craig

Multi Segment Fowler Flaps

Thanks Craig.
If I ever get that ambitious, your drawings will help ease my pain.

Have you built something like that?

Hi Mike,

Yup, I incorporated this flap design into a wing and flew it (see first post). I think I know where the demonstrator section of the wing is. I'll try to find it and post pics. No ambition needed! All the hard work is done :-)

Craig

Cool stuff Craig.

I designed a double slotted flap for RC using external hinges when I was right out of college. Never used it on an actual plane though.

Anyway, I pretty much understand what you've got going there as far as the tracks & pins - but what are the red "rods" that zig-zag along the flaps and change orientation as the flap extends?
Is that in some way representative of the pushrods?

Anyway, I am wondering how you set up the pushrods to push the flap out evenly. Did you need to push the flap in 2 places to keep it extending smoothly and evenly? I would think using a single pushrod would be a little dicey as far as keeping everything straight in the tracks.

Red Thingees explained

Hi John,

The red “Thingees” are the actuating levers for the second segment of the flap. Look closely at the drawing and you will notice that the center (vertical) lever’s fulcrum is on the aftmost trackpin of the main flap segment. As the main segment is moved aft, the trackpins move aft as well. The top of the vertical lever is anchored to the TE of the wing via the top lever. This anchor, along with the moving pivot of the vertical lever, forces the bottom lever to push the second segment aft on it’s own set of trackpins and tracks.

As for actuating this arrangement, there are three of these track/pin/lever assemblies in each flap. One on each end and one in the middle. The servo rotates a brass tube in the TE of the wing. On this brass tube are three “control horns” that connect to the track assemblies via 4-40 rods. As the servo rotates the rod (via a standard servo wheel and “Master” horn) each track assembly is evenly and equally moved.

Believe me… It’s hard to explain, so I hope this made sense!? I’ll take some pics of the demonstrator. You can see everything better.

Click my "GALLERY" button below to see photo's of the demonstrator section in various modes of extension.

Craig

Multi Segment Fowler Flaps

The slotted flap mechanism described here by Modelman is an outstanding example of engineering and and craftsmanship in my opinion. It might be the very best choice for a competition scale model of a prototype that had a similar flap arrangement.

The double slotted flap, described in chapter 8 of Theory of Wing Sections by Abbott and Von Doenhoff, increased the maximum lift coefficient of the NACA65(subscript 3)-118 airfoil from 1.55 at zero deflection to 3.35 at 65 degrees deflection. How much of such a doubling in maximum coefficient of lift is realizable at model reynolds numbers is a matter of speculation unless or until wind tunnel measurements or computer simulations are done. Doubling the maximum lift coefficient (if it is achievable at model reynolds numbers) would result in about a 30 percent reduction in stalling speed. The same reduction in stalling speed could be obtained by halving the wing loading (no speculation required). Which would be easier to implement would depend on the particular case and the technologies available to the designer and builder.

Then there is the middle ground of a combination of a less drastic reduction in wing loading and plain flap to achieve the same 30% reduction in stall speed.

How many ways to skin a cat?

Multi Segment Fowler Flaps

Thanks Ollie!

I intended to have a test section made so I could get into the tunnels at the University of Illinois. I never had the time to complete the test section. Mike Selig is still interested in testing it for his Airfoils at Low Speed publications. maybe I'll shoot for a summer completion. This was designed and built years ago.

My estimate was a 25 - 27% reduction in stalling speed. Frankly, the primary reason for my design of this flap arrangement was scale effect. I was tired of seeing simple, non-scale flaps on "Scale" models. I originally developed a single fowler arrangement... Then, on a dare from a friend, I took it to a twin arrangement and built the demonstrator and flight test wing. Believe-it-or-not, I have a drawing that shows THREE segments as well. I never did anything with that drawing.

My education came from your neck of the woods...ERAU.

Anyway, thanks again for the interest!

Craig

Multi Segment Fowler Flaps

[QUOTE]Originally posted by Modelman
Thanks Ollie!

My education came from your neck of the woods...ERAU.

Craig [/QUOTE

You mean my neck of the woods :D BSAE, '90, Daytona

Multi Segment Fowler Flaps

All I know about aerodynamics has been acquired outside the classroom and laboritory.

Multi Segment Fowler Flaps

Quote:

Originally posted by Ollie
All I know about aerodynamics has been acquired outside the classroom and laboritory.

Nothing wrong with that. Much better to learn things you are fully interested in rather than 'cause there's a test next week.

Back to the flaps..

Quote:

Originally posted by Ollie
[B How much of such a doubling in maximum coefficient of lift is realizable at model reynolds numbers is a matter of speculation unless or until wind tunnel measurements or computer simulations are done. [/B]

I don't have actual measurements, but have run a couple of slotted models in the wind tunnel when I was in school. We observed that the flow thru the slot and over the flap maintained attachment even after the flow over the main wing had started to separate. That and my experience with them on a few different models has me speculating that there is no real difference in their effectiveness between FS and models.

I did a google search on slotted flaps & reynolds number - no results about the difference in lift at different Re. It did yield this cool site, with a bunch of how & why for multi-segment airfoils.

http://adg.stanford.edu/aa241/highli...liftintro.html

Multi Segment Fowler Flaps

John,

I looked at the Stanford site you recommended. As you said, it is interestiong. Now I know more about flaps than I did before. Thanks.

BTW, the very last sentance before figure 10 reads,"Wind tunnel tests are also difficult to interpret due to the sensitivity of CLmax to Reynolds number and even freestream turbulence levels."

My inclination would be to employ a healthy safety factor with the maximum coefficient of lift associated with fowler flap design for models.

Flaps are useful for landing and take off but we still do not know, quantitatively, how useful on models. Unless scale competition is the main objective, I still prefer wing loading reduction to flaps for the following reasons:

1. The results of wing loading reduction are well known and predictable rather than speculative.
2. Wing loading reduction benefits virtually every flight mode rather than just takeoff and landing.
3. Wing loading reduction takes less design and construction effort.
4. Some of the benefit of fowler flaps is consumed by the wing loading increase associated with the flap mechanism.
5. The complexity of the flap mechanism reduces the overall reliability of the model some.

Originally posted by Ollie

Quote:

BTW, the very last sentance before figure 10 reads,"Wind tunnel tests are also difficult to interpret due to the sensitivity of CLmax to Reynolds number and even freestream turbulence levels."

I knew I'd hear from you on that. :p -proves you read thru it. If you think about it though, that is more of a general disclaimer about wind tunnnel tests in general than about the particulars of the subject.

Quote:

I still prefer wing loading reduction to flaps for the following reasons:

My counterpoints interspersed :

Quote:

1. The results of wing loading reduction are well known and predictable rather than speculative.

As are the drawbacks - see # 2.

Quote:

2. Wing loading reduction benefits virtually every flight mode rather than just takeoff and landing.

I just do not see it this way. Check out the first paragraph on the web site I posted. The wing is THE major source of drag on the airplane. If you are designing for speed, whether it's a jet or a sailplane, to much wing is a bad thing. You only want enough wing that the airplane cruises in it's drag bucket - remember I'm talking about speed tasks here. If you succeed in reducing wing loading by weight loss - there will then be gains to be had by reducing wing area.

The problem with reducing wing area is increasing stall speed, making TO and landing harder. That's where the flaps come in. The better the flaps work, the smaller your wings can be, the faster you will go at cruise.
If you want to go faster, put flaps on your design
Sounds funny, don't it?

Also, there are some tasks for which too light a wing loading makes things "skittish" Pattern fliers want thier planes "groovy" and not too responsive to gusts. Light wing loading == gust response. Not a point for flaps - just saying that light wing loading isn't a universal good.

On the other hand, my Stinger has super light wing loading - and I would never put flaps on it. Not meant to go fast - I'm trying to do 3d with it. 3d is definitely a place where low wing loading is crucial, and landing flaps are unheard of - and flying in wind is a pain.

Quote:

3. Wing loading reduction takes less design and construction effort.

Maybe, maybe not. Reducing weight may be as simple as finding lighter radio gear. Nimh batteries are wonderfull things. However, if you are actually resizing the wing - design consideration needs to be given to strength, AR, etc. And if you are actively reducing weight in the structure of the plane, that is a critical design consideration and does take effort. Proper flap design is something to learn, but no more so than good structural understanding. It all takes (a little) effort.

Quote:

4. Some of the benefit of fowler flaps is consumed by the wing loading increase associated with the flap mechanism.

Actual fowler flaps do require the need for some complex mechanisms like Craig's, that will add weight. Simple single-slotted flaps though, aren't any heavier than plain flaps, and a minor addition in weight even over a hard wing. And yes, even the more complex designs make up for the weight with gains in lift and result in a slower flying airplane( TO & landing).

Of course, some of the benefit of added wing area also is consumed by the weight of the added wing structure.

Here's a design question for you: Two cases, flaps that decrease stall speed by 10% and additional wing area that decreases stall speed by 10%. Which one adds more weight to the base configuration?

Quote:

5. The complexity of the flap mechanism reduces the overall reliability of the model some.

Yep, it's another thing to look at and make sure it works. But the tradeoff here is something you don't get with a wing loading change. In flight, real-time adjustment and control authority. Being able to dial in a bit of flap for takeoff is a great thing, in that it increases the effective alpha of the wing and the downwash on the tail during roll. If you've ever seen a jet model without flaps, but with a really long nosegear, you know what I mean. It makes takeoff rotation much easier. The same effect works on my Cessna though. Also, being able to dump out a bunch of flap and really lower the L/D for landing makes spot landings alot easier on many planes, especially ones that like to float.

If you don't put much stock in flaps, etc - that's cool. I love to experiment with such things and have always had good results.

Next up - full span slotted flaps & ailerons on my F-18. Should be up & running in about 2 weeks.

Multi Segment Fowler Flaps

This hobby sport is about having fun and I would be the last one to claim that my way of having fun is better than anyone elses way of having fun. So John, if fowler flaps are your idea of fun, please enjoy them. I'm not against flaps in general and have used plain flaps on many of my designs. I have even recommended them to others. If I saw a payoff for the effort involved I would even go so far as to try fowler flaps. Let us agree that we have differences of opinion. I sincerely hope that I have not annoyed you in any way.

Hey, sometimes a point-by-point online debate is taken as an aggressive act of some sort. I hope it didn't seem like I was tearing your point apart for sport. I enjoy it and don't take technical opinions personally. If you have responses to anything I've said here, I'll look at them with an open mind. All with good humor.

I invite you to try a plane with plain flaps, then rebuild it with slots and see if there's a difference. That's what I did with one of my first gliders. Still have it around, at a friends house, sitting in the garage. I ought to break it out for some pics, if not flying.

Let me know if you are ever up in Orlando. I'm a member of the Buzzards sailplane club, but not very active. We could go out to the field, if it ever dries out.

Multi Segment Fowler Flaps

Modelman,
Hi. First, I'd like to say great work. I have been trying to figure out how to do Multi-Segmented flaps, for a scale model that I'm trying to scratch build. Your pictures were quite helpful. Did you ever locate the CAD files? If so, could I perhaps get a copy?
Thanks
Pat

Multi Segment Fowler Flaps

Modelman awesome drawing of the multi-slotted flap, now I am going to have to build one :D

Thanks,
Mike