SHORT SOLENT MARK IV – 2014 WINTER BUILD

wollins · February 22, 2015, 10:26:40 PM

Technical question for you technical guys.

Does the thickness of an airfoil affect its lift? (everything else being equal) An example ... would a semi-symmetrical airfoil with say ... a 15% airfoil create more lift than one with a 12% airfoil? (all other factors being equal) And if so ... where's the max point that you can take that thickness cause I sense that at some point it would probably either becomes "unworkable" or too inefficient? (too much drag etc)

Colin.

pmackenzie · February 23, 2015, 03:53:18 AM

You can play "what happens if I change something" games on the NASA airfoil simulator:

http://www.grc.nasa.gov/WWW/k-12/airplane/foil3.html

(You might have to play around with the Java security settings to get it to run, and of course you will need Java installed.
They have instructions of what settings to make)

You have to make sure that as you change thickness you don't increase camber.
Camber is a fancy way of describing the curvature of the the line that is the average of the top and bottom shape of the airfoil.
If the airfoil is more or less flat bottomed then changes in thickness do increase camber.

If you play around a bit you will find that Clmax is only slightly affected by thickness, much more affected by camber.

They stop thickness at 20%, so no way to test anything thicker than that.

piker · February 23, 2015, 11:08:49 AM

I can't add much beyond what Pat has said, but generally (simplistically) I would say that the thicker the section is, the more lift it will generate (assuming an efficient/appropriate airfoil shape), but at the cost of drag. If you want my opinion, I wouldn't go past 15%. 12% is very typical for models.

For reference, I recall the scale section for the Sandringham was 18% (or was it more?....) but I chose 15% for my model as a good compromise between scale appearance and drag.

Do you know the scale section of the Solent?

wollins · February 23, 2015, 12:36:25 PM

Thanks Pat!

Quote from: piker on February 23, 2015, 11:08:49 AM
If you want my opinion, I wouldn't go past 15%. 12% is very typical for models.

For reference, I recall the scale section for the Sandringham was 18% (or was it more?....) but I chose 15% for my model as a good compromise between scale appearance and drag.

Do you know the scale section of the Solent?

Don't know the scale section of the Solent but from what I gather from the other information concerning both planes (Solent and Sandringham) I'm guessing they would be very similar. In any event, I was considering thickening the airfoil of this model because just by eyeballing the plans, the airfoil seemed VERY slim to me.

Upon measuring it turns out that my suspicions are correct. The airfoil is 10.71%. Now that you have given me your specs I'm gonna bump that up to at least 13% to play it safe and better accommodate my intended AUW. I don't know if it's because Ivan's planes are designed to be built ridiculously light that he's going for a less draggy wing? Ivan's planes are famous for being great flyers but at their typical cubic wing loading of a kite I imagine anything would fly great!

In any event, since this plane is gonna be more of a landlubber than a flying boat in terms of practical use I've had to build it somewhat hefty (read "normal" for this size. lol!) because earth is a lot less forgiving than water! (A "thump" beaks a lot more stuff than a "splash".

) Since I'm pretty much redesigning this wing (more ribs, heftier spar etc) I figured why not manipulate the airfoil because if I can get a little more lift, at not too much of a cost in drag that can't be a bad thing, right?

Having said that I'm not so worried about drag since I'm building more of a lumbering giant here as opposed to something nimble so airspeed is the least of my concerns. Course I'm new to all this so this is just my "sense" of "what should be" given my circumstances, so feel free to advise if need be.

C.

sihinch · February 23, 2015, 12:48:36 PM

Colin - if you use 18A CC ESCs then it would be Piker legal!?

wollins · February 23, 2015, 01:23:41 PM

Yeah but I suspect there might be some carnage in the air around that circuit! lol!

wollins · February 23, 2015, 08:37:18 PM

Was I meant to build this plane?

(and no I didn't photoshop that in!

)

C

piker · February 24, 2015, 10:04:42 AM

Yep! Collin's plane

wollins · February 24, 2015, 01:21:52 PM

Ok fine, so it has an extra "L", but everyone makes mistakes!

wollins · February 25, 2015, 09:13:07 AM

I was worrying about getting my wing incidence right and then I saw this. I'm not worried anymore.

http://youtu.be/ZOtVyxwNHQg

wollins · February 25, 2015, 03:15:03 PM

So I finally started in on the wing today. (needed a break after the fuse marathon! lol!) I have to say in fairness to Ivan I don't think I'll call this an Ivan Pettigrew Solent anymore cause I've changed it so much that if she flies like a pig (or worse doesn't fly at all) it wouldn't be fair to him. His planes fly great! So why mess with a proven design? Well for one ... I just can't leave well alone with this darn hobby.

I even have to modify arfs!

In any event, the changes here started because of my desire to build it more scale ... than once you change some things ... they lead to other things that HAVE to be changed since the original design was not designed around the changes! Bit of a catch 22.

Having said that most of my fuselage changes were cosmetic ... but they had to have the structural support to "support" those changes. More structural support means more weight, more weight means more support etc etc etc. Again, a vicious cycle!

The second (and more important) reason is that Ivan's flying boat designs are not meant for flying off of land. (and rightly so since they are flying boats afterall.) Therefore they're really fragile. However since I fly off land 99% of the time, I have to beef up the design. (the CL-415 was the same situation)

You can see an example of how this really is now a design "very loosely based on an Ivan Pettigrew plan" in the pic.

- That spar you see is about 70% thicker and about 30% "higher".
- Airfoil/ribs about 30% bigger (mine's the cardboard one under the outlined one on the plan above it)
- Camber has subsequently changed
- Wingspan is gonna be about an inch wider
- Ribs have increased from 28 (that the plans call for) to 42
- Motors mounts will be completely redesigned since his design are based on "stick mounts" based on the old GWS type brushed motors! (remember those, guys?)
- I'll be attempting to add the scale Gouge flaps. (Ivan's plans don't call for any flaps)

So ... the wing is gonna be even more changed than the fuse and really ... is there much more to a wing other than the ribs (airfoil) and supporting structures?

Wouldn't it be great if she flies?

Colin.

P.S. So why the heck am I rambling on about this? (in other words shut up and just build the plane! lol!) Because a few people have asked me privately about this stuff, so I figured in case any one else wondered if I was crazy (or stupid) to mess with a proven design ... that's my story ... err ... reasoning and I'm sticking to it!

wollins · February 27, 2015, 04:24:42 PM

Since nothing (measurements and spacing of ribs, size of spars etc etc) is according to the plans anymore, I've decided to do a mock up (with foam board) of everything before I start cutting balsa. That way I can make my adjustments to the foam to make sure everything fits perfectly first before I commit to the real thing. This should help me to cut down on my wastage of valuable wood!

C.

wollins · March 02, 2015, 03:20:29 PM

Now I can start cutting some wood.

The dots in the first pic are reference dots that I used to find the high/low spots across the ribs. Since these were all individually hand cut and the airfoil and the chord is tapered it was difficult to get them just right.

To find those spots I used a piece of thread and laid it end to end horizontally along the ribs on the dots. However some spots were mostly 1 to 3 mil off which is not a big deal in of itself but would probably show up in a somewhat wavy pattern in the sheeting over top (forward of the spar) and the covering. (aft of the spar) These and other inconsistencies will be fixed in the actual balsa ribs.

So, some progress is being made.

BTW, this is just the center section of the wing but it's the most difficult part since it houses the nacelles and the flaps.

C.

sihinch · March 03, 2015, 09:32:14 AM

Wow, Colin, that's dedication. Building a whole mock-up wing before you started building the actual wing. I'd have been winging it as I go along!

wollins · March 03, 2015, 11:55:02 PM

Quote from: sihinch on March 03, 2015, 09:32:14 AM
Wow, Colin, that's dedication. Building a whole mock-up wing before you started building the actual wing. I'd have been winging it as I go along!

Yeah, it's a lot more work but it'll save me on any more "redos". Every "redo" uses more wood and I've done a ton of redos thus far so I literally can't afford to waste any more wood on this project. You don't want to know how much coin I'm into this just in wood alone!

Colin