Why no flaps?

[N942VT]

And look where the air speed indicator is.

[Razncain]

I've been away, and just now catching up to all of the great stuff that has been posted. Really appreciate it, everybody. I enjoyed seeing the video of Bob landing in the snow. What a great plane. I was sold on the flapless a good while back ago, just even more convinced it's the only way to go for this little plane.

[kenryan]

I understand all the reasons for not putting flaps on the LSA, but if a guy was going to put flaps on the LSA would he want to stick with the LSA airfoil or use the Patrol airfoil, scaled to LSA size?

[rodsmith]

The LSA has a 6% camber airfoil versus 4% camber on the Patrol. More camber means more lift at any airspeed but limits top speed some, better fit for an LSA where you are limited by regulation on top speed.

[kenryan]

I read in an article that Bob was talking with Riblett about the Patrol airfoil, and Harry asked him why he didn't want to use the 6% camber airfoil. Bob said he didn't think he needed it with the big flaps, and Riblett agreed. That makes me wonder if a guy wanted flaps on an LSA, if he might be better off using the 4% airfoil?

[Mark Goldberg]

I think a guy would be better off if he built the plane like Bob designed it. Ask anyone who has flown a BH LSA if they thought it needed flaps. After flying it. Mark

[Chris In Milwaukee]

Flaps and their associated mechanics also add weight. LSA is a lean machine.

[Peter Bishton]

Gents I don't think the 6 stands for 6% camber. I have the Riblett book. The number is the lift coefficient at cruise.

My dad was liaising with Harry R in about 1996 and put a GA30 section on W8 Wittman tailwind- I built 27 of the 28 ribs for these new wings in 1998- I'd do one every night after work. For eg The GA30-413.5 on the Patrol is max thickness at 30% chord, lift coefficient at cruise is 0.4, max thickness is 13.5% of chord of wing.
the naming convention for the NACA airfoils is different and is the way the previous authors have been writing. For example the naca2412 used on the wittman buttercup (drawings by earl Luce), wittman big x ( I just bought the drawings from Randall Bray in Georgia they arrived last week to Australia ) and many cessnas, is 2% camber, max thickness at 40% chord and wing thickness 12 %.
Regards peter
plans built 4 place #1186

[rodsmith]

Somewhat confusing, direct from first page of his book, "4 camber levels, 2,3,4,6, indicating the approximate design lift coefficient in tenths". I hadn't caught that distinction in the several times I read the book. So I guess it doesn't mean % camber, still higher numbers mean higher camber.

[kenryan]

For you guys with the Riblett book, does he talk about flaps? Still trying to sort out if the Bearhawk LSA airfoil would be okay with flaps, or if it would be better to use a Bearhawk Patrol airfoil if one was going to use flaps. (Like I said, I already understand all the reasons not to use flaps.)

[rodsmith]

For you guys with the Riblett book, does he talk about flaps? Still trying to sort out if the Bearhawk LSA airfoil would be okay with flaps, or if it would be better to use a Bearhawk Patrol airfoil if one was going to use flaps. (Like I said, I already understand all the reasons not to use flaps.)

Here's Harry's comment on flaps " A good airplane design should start with the flaps, and then the rest of the airplane should be designed around the flaps" All Riblett airfoils are designed to work with flaps. If you have to build an LSA with flaps, not something I would do, either the 30-613.5 or 30-413.5 would work, performance of course would be somewhat different.

If you are really serious about this you owe it to yourself to talk to the designer. Bob is very reasonable about listening to peoples ideas. As an example, the first time I met Bob I told him I was thinking of building a Bearhawk with a Ribblett airfoil (this was before the Patrol). That would have been the end of the conversation with most designers. Instead Bob said go for it and let me know how it turns out, I was rather stunned. If he can't talk you out of flaps he may have ideas for the best way to do it with flaps.

[kenryan]

Thanks Rod.

Again, I understand all of the reasons for building the Bearhawk LSA exactly as designed by Bob. My question is not really about the Bearhawk LSA per se. I intended it to be more of a discussion around airfoils and flaps. Like you say "either the 30-613.5 or 30-413.5 would work (with flaps), performance of course would be somewhat different."

That difference is precisely what I am trying to understand. How would the same airframe (in this case the Bearhawk LSA) perform if designed with the 30-613.5 with flaps vs the 30-413.5 with flaps?

Another way of looking at it is asking the question, why did Bob choose to use the dash 6 airfoil for the LSA, rather than the dash 4?

Given that the LSA and the Patrol have similar missions, I'm thinking that maybe the reason Bob choose the dash 6 for the LSA is that he knew he wasn't going to put flaps on the LSA, and given no flaps he felt the dash 6 airfoil was more appropriate. It could well be that if Bob had decided that the LSA would use flaps, he might have stuck with the dash 4 airfoil. This is just speculation.

I do own the LSA plans, but I have not decided on building it. I am hoping that this next airplane might be of my own design, and I am looking at the LSA for ideas, particularly the airfoil, some wing details, tail and the landing gear.

[rodsmith]

"That difference is precisely what I am trying to understand. How would the same airframe (in this case the Bearhawk LSA) perform if designed with the 30-613.5 with flaps vs the 30-413.5 with flaps?"

My limited understanding indicates all else being equal the 613.5 airfoil will cruise a little slower and stall a little slower. The difference won't be big, maybe a couple knots on both ends

"Another way of looking at it is asking the question, why did Bob choose to use the dash 6 airfoil for the LSA, rather than the dash 4?"

The only one who can answer that is Bob.

"Given that the LSA and the Patrol have similar missions, I'm thinking that maybe the reason Bob choose the dash 6 for the LSA is that he knew he wasn't going to put flaps on the LSA, and given no flaps he felt the dash 6 airfoil was more appropriate. It could well be that if Bob had decided that the LSA would use flaps, he might have stuck with the dash 4 airfoil. This is just speculation."

I wouldn't consider it the same mission. The Patrol is a very high performance back country 2 seat aircraft. The LSA is just what it says, designed to meet LS regulations, and also has short field capabilities.

"I do own the LSA plans, but I have not decided on building it. I am hoping that this next airplane might be of my own design, and I am looking at the LSA for ideas, particularly the airfoil, some wing details, tail and the landing gear."

That sounds great, I would recommend getting Harry's book from EAA and some of the other airfoil/aerodynamic books that he references.

[jaredyates]

Another possible factor is the regulatory speed limit that the LSA was designed for, vs his priority for fast cruise speed on the Patrol.

[kenryan]

Thanks for the help and suggestions.

[bway]

I have found this discussion interesting as I have flown my LSA about 115 hours in a bit over a year. During this time I have made it a point to practice short field landings on actual short fields as much as I can. Frankly, the LSA can be landed so short that I would not want flaps on it. It can be hard slipped at minimum airspeed with full control and confidence. This allows for a very steep descent when landing over trees. The slip can even be safely carried into the flare, if I find myself with a little extra speed, as it can be eliminated instantly. I find that I get the shortest landings by touching down tail low ( close to or 3 point) and braking hard while holding the tail up with the elevator. This may sound risky, but it is very easy to control because of the compliant main gear and effective elevator. My airplane has a full panel with electrical system as well as conventional covering and full paint, so it is heavier than Bob's, and I'm a big guy at 260 lbs. still, it climbs well over 1,000 rpm and will cruise at 112 mph indicated at 75% on my O-200 with 3 blade warp drive prop. That said, I usually cruise at 100 mph indicated on slightly under 4 gph.

While the discussion of flaps for the LSA is an interesting intellectual exercise, I don't think flaps would add anything useful to the design. It is pretty great as it is. Of course builders can do what they want, but for me, the design is about perfect.

Bob Way

[Mark Goldberg]

Bob, glad to have you back on the forum especially with your experienced thoughts after flying your LSA. Mark