Ribblet Airfoil For the B model

[Cameron Ramsey]

Has anyone else plotted out the 30-413.5 Ribblet Airfoil that goes on the B model. I've been studying Harry Ribblet before I even discovered the Bearhawk so naturally when I got my plans and figured out how log the cord is supposed to be, I plotted the 30-413.5 airfoil to see how it matched up with my Mylar copy. What I found was, the top camber matched perfectly but the bottom did not. The Mylar copy was almost 1/4 inch thicker at station 17. I've triple checked my numbers and measurements. This changes the Ribblet airfoil. It is minor, but Harry Ribblet designed these airfoils to be more efficient. My question is, was this airfoil made thicker for a reason or was it just a calculation error? Any help would be appreciated before I start cutting my form block.

[rodsmith]

Give Bob Barrows a call and ask, Mark may know but Bob definitely does. Please post what you learn.

[Bcone1381]

The post below is a short discussion about it, but with no conclusions.
https://bearhawkforums.com/forum/bea...241-wing-chord

What do you think you'll do? Use the Mylar that came with the plans or use your own calculations?

I am curious also....call Bob.

[Cameron Ramsey]

Thanks for the reply's. I was beginning to think I was on my own. After taking a day to think about it and restudying my Harry Riblett book, I think I will stick to the Mylar drawing. I suspect after re-reading my Riblett book that Bob slightly altered the airfoil to make it more aerodynamic. On the 30-413.5 airfoil that I plotted from the numbers straight out of Riblett's book, I notice that it has a slight "cusp" (Cusp= reverse camber) on the bottom side. This is that area where my drawing and the Mylar drawing are close to 1/4' different. This "cusp" helps the airflow on the bottom side of the wing stick to the surface and supposedly help give better aileron control. However, It does impose some drag. I suspect Bob may have taken the "cusp" out to get rid of the drag. I have been able to take a ride in a 4-place bearhawk with the Riblett wing (thanks to Mark Goldberg and Jared Yates) and the ailerons were smooth and had plenty of a authority so I think the Mylar drawing will be fine. I did send an email to the R&B Aircraft site and asked about this before posting on this forum but I was instructed to Study the plans closely and then ask others on the bearhawk forums and then, I could give Bob a call if that didn't work. So, that gave me that feeling that Bob probably doesn't want to be bothered by every little question that someone comes up with, which is understandable.

[Cameron Ramsey]

Bcone1381, thank you for the link to the other discussion. It helped greatly. I will stick with the Mylar drawing.

[nichzimmerman]

Cameron,
interesting discussion. I have no financial connection to Bob, but my impression is that he is always happy to chat. I've called him many many times over the last several years and he always entertains my silly questions. My guess is that the person who responded to your email didn't have the technical expertise to answer your question and typically Bob doesn't engage through email. My main point is that you (and anyone else who reads this thread) shouldnt be dissuaded to call him, though I recommend posting the question to the forum first, which you did. Good luck with your build!
-Nic, BH 1217

[jaredyates]

I agree, and also happen to know that Bob is out on a long trip and will probably not be accessible by phone for a few more weeks.

[Wyo Johnson]

Cameron,
I laid out the B model airfoil and then compared it to a scan of the airfoil...noticed what you are mentioned about the lower airfoil lofting. I'm laying out CAD drawings to cut out ribs with a CNC so that is on hold until I could ask Bob about it.

One other question is about wing twist. Not an Aero guy, but have worked with a number of them on projects professionally...and my understanding is that twist "softens" the stall or in other words makes the stall come on in stages so the aircraft nose drops "slower". I planned on calling Bob on the airfoil deviation and the twist, but just hadn't done it yet.

I don't have any reservations about the lack of twist as it is easier to build, but did want to hear what the engineering/aerodynamic justification was, so if you do get ahold of Bob you might ask.

Thanks,

Wyo Johnson (Andy)
1423B

[Cameron Ramsey]

Andy, You May research wing wash out and wash in. It may give you the answers your looking for. I believe that Bob kept the wing straight to make the build easier. I will say thay my time in the bearhawk with the riblett wing was very limited but the slow flight that we did seemed very stable. I’ll wait and call Bob in a couple of weeks so I can find out an answer to my question and pass it along to everyone that I caused to wonder about this rib design.

[Bcone1381]

Is it not because of the superiority of the evolutionary Ribblet Airfoil that drag inducing washout is now unnecessary?

[aachanis]

I plotted up the Riblett airfoil and compared it to the mylar drawing and noted that they didn't line up perfectly as well. I'm not sure that I got everything perfect, but it was different enough that I think the changes were intentional. Given the reports on the flying qualities, I'd stick to the mylar.

There are a lot of factors that go into wing design and wash out is only one of them. The wash out is supposed to stall the root before the tip to maintain better control. An airfoil with a sharp stall characteristic might drop out from under you before you are even aware there is an issue. Riblett airfoils are supposed to have gentler stall characteristics so that doesn't matter as much. The ideal lift distribution is elliptical which would lead to designing an elliptical wing, like the Spitfire. Of course, what you find out is that's a tough wing to build - even in a wartime economy with factory builders. Engineering is a set of compromises.

One note on Riblett, while I think he was on the right track, all his work was theoretical and only done in a computer program. He also had a way of coming across the wrong way and annoying people, so there are strong opinions about him as much as his work. Bob has actually built the wing and flown it in the 100% accurate real world wind tunnel, so I would trust Bob and the plans.

[Cameron Ramsey]

I agree aachanis. Thank you for your insight.

[Thomas Galloway]

I'm just curious what the plot comes out looking like. I took a 3' steel straight edge to my Mylar and found the bottom of the airfoil to be flat. The only Ribblett airfoil plots that i have seem have very noticeably curved undersides. Am i missing something?

[Cameron Ramsey]

No, your not missing anything Thomas. I actually plotted the ribblet airfoil and put my mylar on top of it. The top camber matches perfectly but the bottom side was about a quarter inch off. The Riblett was inside the mylar template. I researched and found the bob actually consulted with Harry Riblett and together they came up with a “modified “ 30-413.5 airfoil. Basically, they removed the “cusp” or curve on the bottom side. It’s just not advertised as a modified version of the airfoil.

[Thomas Galloway]

Good to hear. Thanks Cameron!

[nborer]

A bit late to this party, but one of the first things I did with my plans when I got them in January was to plot up the airfoils. I measured using a fairly crude system, but noticed that the ordinates I extracted were different (particularly lower surface) than what I was able to get from the Riblett book. I do recall chatting with Bob about the airfoil when I purchased my plans, and he mentioned that Riblett gave him a set of ordinates tailored to the Bearhawk's needs.