Wash out

[Andrew Cousins]

Hi all after some advice from you guys that know about wash out.

OK I am building a piper cub (DB cub). I am building the first wing panel. The instructions tell you to pin the bottom main spar and the rear spar(trailing edge end) to the board along with the T/E stock. Then fit and glue all the ribs. the next step is to glue the leading edge on to the ribs.

OK all done it then asks you to unpin the wing from the board. You then have to place a 3/8th block under the T/E of the last rib. It then tells you to pin the root rib to the board and the last rib. I under stand that this is to give you the washout.

we are getting to the question.

You then fit and glue the top front and rear spars in and the leading edge top sheeting. You then assemble the tips. Now you can unpin the wing from the board.

And here is the question.

I understand the reason for washout but never built it into a wing before. My question is. After un pining the wing i would have expected the gap between the last rib and the board to stay at 3/8th but it isn,t it has reduced to half that. is the wash out still built into the wing or has something gone wrong?

Many Thanks in advance

[Peter Miller]

I am a little surprised that a Cub would even need washout.

I see that there is no mention of lower leading edge sheet. As described there is nothing to really lock the washout in. I normally desing a D box leading edge, i.e top and bottome leadign edge sheet with spar webs. These will never lose their washout.

One thing that occurs to me. If all the spar slots ore very tight on the spars that could cause the problem. Is the wing flexible? if it is you should be able to flex it while heating the covering.

[SIMON CRAGG]

I have had several cubs over the years, and none of them had wash out. I would not get bogged down with the subject. I would just build it all square, and after covering check carefully with an incidence meter. Test fly with everyhting at zero. You can always "twist" some wasout in at a later date, using the heat gun. Good luck.

[Andrew Cousins]

Thanks both Peter yes I was suprised as well. Yes there is lower leading edge sheeting which is fitted after removing the wing from the board. Yes the wing is a flexible much more than others I have built.

Simon I was contemplating not putting it in but not to sure so I put it in as the instructions said.

[Don Fry]

Why not speak to the bloke who owns DB Sports and scale?

[Andy Blackburn]

Posted by Don Fry on 25/04/2019 19:52:56:

Why not speak to the bloke who owns DB Sports and scale?

Good idea.

But if I were you, I'd do exactly what it says in the instructions. If you don't put the washout in, you'll effectively be increasing the wing incidence which is not what the designer intended.

BTW, I think you'll find that the full-size J-3 is normally rigged with about 1.8 - 2.5 degrees of washout...

[Martin Harris - Moderator]

My 1/4 scale Cub has similar construction to the full size with very small D boxes where the main strength is in front and rear spars and this ends up with quite flexible wings. However, the functional struts allow accurate adjustment of dihedral and washout - does your version have functional struts perhaps?

[Nigel R]

Interested to know why the washout is there by design on a cub. A rectangular plank wing inherently stalls root first I believe.

I've certainly never put wait in anything I have built with a plank wing.

[Peter Miller]

I will just add that the Sig Cubs never had any washout and they fly like dreams. I have built two of the 72" span Cubs and the Clipwing.

As Nigel says, parallel chord wingsnever need it. I only use it on my elliptical winged designs.

[Nigel R]

whoops, "wait" = "washout"... another auto "correction" by my phone.

[Braddock, VC]

DB cub was originally designed under the direction of the late David Boddington a man who really knew his onions.

Cubs are quite often mistakenly bought as trainers where the addition of washout can transform a plane like the cub, especially when flying low and slow if sudden evasive movements are suddenly called for.

Washout is not 100% necessary but if you lack experience it's not a bad idea, especially for the first few flights.

I have a Sig 1/5th scale cub and the plans and instruction book make no mention of washout but the kit is marketed as almost a scale model whereas the DB version clearly is not 100% scale.

Personally I'd follow the instructions.

Remember caveat emptor when assessing advice proffered on the internet.

[Andy Blackburn]

Posted by Nigel R on 26/04/2019 06:55:07:

Interested to know why the washout is there by design on a cub. A rectangular plank wing inherently stalls root first I believe.

I've certainly never put washout in anything I have built with a plank wing.

I assume that it's there to control how sharp or soft the stall feels to the pilot (I assume you're talking about the full size)?

[Nigel R]

VC, I would assume here it was just a case of copying the full size.

Andy, that's possible. I'm not up on full size design practices circa mid 1930s. For us, varying the CG also affects the stall 'feel' so we have another variable to play with, and it is easier to change in models than on the big stuff.

[Mike T]

Agree with all the comments about DB's design - stick to it!

If the wing has 'relaxed' after unpinning, don't worry - some washout is still there. Just make sure that the other panel relaxes the same amount. If it's a bit out, don't worry, this can be corrected at the covering stage, using heat if you are covering with film/'tex, or dope if you are going Trad. Brit.

[Tim Cheal]

I have a 2 meter cub and it is very prone to tip stalling (not a DB one though).

I think the real thing has washout, and this picture is great example of it.

Good luck with the build.

Tim

Edited By Tim Cheal on 26/04/2019 11:11:26

[Andrew Cousins]

Hi all thanks for all the replies. I have emailed DB to see what they say. I think I will stick to the plans and instructions with the washout. Just would be nice to know if the relaxing when it's unpined is normal.

Thanks all much appreciated

[Piers Bowlan]

Posted by Nigel R on 26/04/2019 06:55:07:

Interested to know why the washout is there by design on a cub. A rectangular plank wing inherently stalls root first I believe.

If the aircraft is straight and level and in-balance, this is my understanding too but introduce any yaw at the point of stall and the 'leading wing' will tend to stall first. I am talking about full sized aircraft here and washout will improve handling by making the handling more benign at the point of stall. At model sizes I think we are all agreed that for model aircraft with parallel chords, low wing loadings and aerofoils with good low speed characteristics (clark Y or similar for instance) washout is probably not strictly necessary.

But what about the mystery of Andrews disappearing washout? I haven't been able to find a plan of the DB Cub wing but presumably it has a D-box section in that the lower LE is sheeted and the ribs glued onto them. Then the LE strip added while the wing structure is flat on the board? The TE outer rib is then propped up 3/8in and the upper LE sheet added? My conclusion here (possibly wrongly!) is that as wing is built flat until the upper sheet is added with the washout, it will tend to 'relax' some of that washout as the rest of the wing will want to regain it's original untwisted flat shape. Presumably the designer (Boddo) allowed for that, so that the small amount of washout remaining is what he intended when he designed it. Personally when I build washout into a wing, as I have done with some flying wings I designed, I build it in from the outset, not half way through the build. As long as you achieve what you want, that is all that matters apart from building two wings with the same amount of washout (or no washout!).

This topic regarding the DB Cub washout was covered here a while ago with similar conclusions.

Edited By Piers Bowlan on 26/04/2019 11:35:51

[Piers Bowlan]

Posted by Nigel R on 26/04/2019 09:26:09:

VC, I would assume here it was just a case of copying the full size.

Andy, that's possible. I'm not up on full size design practices circa mid 1930s. For us, varying the CG also affects the stall 'feel' so we have another variable to play with, and it is easier to change in models than on the big stuff.

Moving the C of G aft will make the stall entry more abrupt and increase the likelihood of a wing drop (tip stall) and autorotation into a spin. An aft C of G will also tend to make the aircraft tend to spin flat, making spin recovery delayed or even impossible in extreme circumstances. The (full sized) C.172 for instance is cleared for spinning but not with the two rear seats occupied for this reason. In a flat spin the rudder tends to be blanked by turbulent airflow from the tailplane making it less effective to counteract the aircraft yaw, to stop it rotating. The ailerons are not effective as the wing is stalled.

Moving the C of G forward will not only make the aircraft more stable in pitch but make the stall more benign to the extent with some aircraft they will not even spin from level flight with a forward C of G but instead just enter a spiral dive. However, drop some flap and put them in a steep turn with power on and PULL and they will flick into a spin. - Well behaved unless provoked!

[Andrew Cousins]

Piers thank for the reply. Yes the wing does have a D section LE but not shear webbing. The ribs are glued to the front and rear spar and the LE. You then unpin the wing except the root rib, prop up the tip rip TE to 3/8 and pin the tip rib down. Then add in the top spars front and rear. Then the top sheeting is added, then the wing removed from the board and the bottom sheeting added. It was at this point I noticed the sag.

Thanks for the link I will have a read. Also thanks for your help.

Edited By Andrew Cousins on 26/04/2019 13:42:22