Dehedral in a plank?

[nige]

I have a wing off a PC9 and it is in one piece and I am going to make a plank sloper out.

I was wondering if I should take the wing apart and put together flat flat or leave the dehedral in it?

The wing has 22mm either side of dehedral.

Nigel.

[felix]

in my thinking (and i could be wrong! ) for a plank or chevron it's probably best to have as little dihedral as possible.due to the general poor stability in yaw i would think dutch roll could become a serious problem at low speeds.

[nige]

Thanks Felix I will pull it apart and refix it with as a flat plank.

10 points for the quick reply.

Nigel

[thevon]

Nige,
I'm no expert but one point is that certain airfoils are made for planks and others for conventional wings. So I'd be a bit worried that the PC9 wing mightn't perform well. Also, if the wing has a fairly straight narrow shape it will probably be much harder to get it stable. If you look at your Ozprey, see how it's much wider in the centre - this makes them more stable I think. JW's are like that too. The narrower they are (eg Reaper) the more fiddly and unstable they are. It would be great if it did work but planks can be a PITA.

[nige]

Thanks Andrew I have recently experienced this PITA while trying to ballancing a plank sloper made out of an Aeromachi wing.

I have managed to get it flying pretty well. So I thought that I would try another one as it was just sitting there gathering dust.

Thanks for the warning, I will not put too much work into the finnish just in case.

Nigel

[felix]

Andrew is spot on about thin planforms and aerofoils.
One thing I learnt recently (thanks sean) is that aerofoils designated for tailess designs have a very low pitching tendency throughout their speed range and angle of attack (minimum center of pressure movement??).
Also a high aspect ratio wing has low stability in pitch (unless a large amount of sweep is used) and offer less airframe strength potential over a low aspect ratio.
What I think may be ideal for this wing is build it as light as possible,add some decent sweep, keep it a little nose heavy and add a fair whack of reflex. Will have a reduced speed range so trimming will be easier and the combination of an efficient high aspect ratio and light wing loading may make for a good light wind slope/thermal soarer .
Just my opinion though…..hats off to you for playing with a design (my favourite thing about this hobby).

[felix]

One final thought is something I have thought about playing with on my bee is maybe a small fixed tailplane on a boom could be an option if it proves to be a pig? (works for high performance hang gliders!)…..downside is to be effective it will probably set slightly negative so inverted flight could be limited.

….all this thinking and my head hurts now ;D

[nige]

Will your head hurt more if I tell you that I don't understand a word you said. Well some of the words anyway.

Now once again for the dummies please.

I m going to bed so don't rush.

Nigel

[drcuzo]

I am building a Raptor 60 from future slope designs. Paige has a good reputation for producing sweet planes. The Raptor is a high aspect ratio plank. Paige suggests having some dihedral in the Raptor.

It may be worth a post on RCG to get an opinion from Paige and the other Raptor flyers.

Steve

[nige]

Thanks Doc I will do that. I hope you will give a report on that Raptor. I have since pulled the wing apart and put it back together as a flat profile.

Nigel

[Hamburglar]

I put a little bit of dihedral in my raptor. Just enough so that it's flat across the top of the wing from tip to tip.

I can't comment on how it flies. I never got the chance before I left. It won't get a maiden till December.

Looking forward to hearing how yours flies Steve.

[drcuzo]

Sorry - off thread

I hope it flies really bad because I have built if for my adult son to learn on. It is a birthday present. If it flies well it will be hard to give it away, even to Tristan.

It has gone together nicely. I will post some pics when finished and Sean can give a critique when he maidens it (sorry to spring than on you Sean)

Steve

[felix]

Sorry nige I pretty enthusiastic when it comes to aerodynamics and I tend to rant a bit ;D
Not sure how to explain it more without drawing diagrams (a pic is worth a thousand words lol) but like the doc said rc groups would be the way to go for clearer aero dynamic theory and basic advice –I have some excellent books from flying full size that I could loan you next time your up this way.
As already mentioned a little dihedral like the bees is ok but I definitely would be reluctant to go any further than keeping the upper surface flat unless you have a large fin or drag inducers at the tips.
Keep us posted on how you go.

[nige]

Thats fine Felix I am only new to all the terminology. I am searching through Wikipedia to translate your post ;D I will let you know how I go.
Thanks for the offer on the book.

Nigel

[sean]

I will post some pics when finished and Sean can give a critique when he maidens it (sorry to spring than on you Sean)

Hehe! Would love to maiden it Steve.