Wing/Tail incidence

[Jeffrey Cottrell 2]

Hi Guys, quick question.

Just about to start a build of a Watt Four (Wot4 clone) from the Colin Usher plan on Outerzone.
As downloaded, the plan does have a number of discrepancies, most of which are described in the accompanying text.
However I have come across a very basic oddity which I would like to resolve before cutting wood.
So:
Given this is a high wing sports model, I would expect the wing to be at a slight positive incidence compared with the tailplane. The wing incidence on my plan is in fact negative.
Being a semi-symmetrical aerofoil, I have drawn a line from le centre to te centre and treated this a chord line for incidence purposes. On the plan, the tailplane is level with the fuz top, so using this line as a reference, the wing chord at the le is some 3mm lower than the te, i.e.negative incidence.
First things first, am I measuring this correctly?
Obviously easily corrected before I start, not so much half way through the build !

Could use some advice, thanks

Jeff

[J D 8]

   Wing incidence can be a bit of a misleading term after all its angle of attack to the airflow can be altered by the elevator.  What we need to look into here is decalage or sometimes called longitudinal dihedral. The difference between the wing and stabilizer. For the aircraft to have longitudinally stability the forward flying surface has to be at a greater incidence, for a canard type it is the same.  For a symmetrical wing section the set up you have would not work.

However a semi symmetrical section will still be producing some lift when the chord line is at zero. 

  It may be the designer is trying to make the model neutral to trim changes at a wide speed range and in inverted flight.

Best I can up with on your odd set up.      Cheers,  John.

[Caveman]

I have just printed a copy of the plan and have drawn a line through the top of the fuselage sheeting.

 

As far as I can see the wing and tail have zero incidence.

 

 

GDB

[RottenRow]

It looks like a poorly drawn wing-section on the plan to me.

 

On page 2 it states the root section is NACA 63018 (actually incorrectly spelled NASA..) which is a fully symmetrical airfoil, yet the plan shows a droop in the leading edge, making it semi-symmetrical as you have found. Remove that droop and the longitudinal dihedral looks about zero degrees as Caveman above suggests.

 

Jeffrey, I would suggest that the line that you have drawn from LE to TE does not pass through the centre of the section (due to the incorrect way that the plan shows it). The C of G symbol looks to be centralised, so your line should pass straight through that; I suspect it is below it. Also the position of the dowels shown on the plan is not on the chord line, but below it, adding to the possible confusion.

 

It's strange that the plan gives figures for the tailplane incidence (zero) and downthrust (3 deg.) but makes no mention of the wing incidence....

 

I have just had a look at a Wot-4 wing (ARTF wooden version) and that looks fully symmetrical to me.

 

Brian.

[Jeffrey Cottrell 2]

Hi Guys, thanks

J D 8
Quite right, the correct term would be decalage. My bad.
Not sure what the designer was trying to achieve, but I've found so many issues with the plan I prefer to go my own way.

Caveman
Could be due to different printers producing different results, but since I will be working from my plans, will keep your diagram for info only.

RottenRow
Poorly drawn hardly describes it.
If you look at the bottom right corner of page 2, it says 'Killer Watt' which is the clone version of the AcroWot not the Wot4.
As such neither the span or the wing chord match with the fuselage on page 1. Also, as you say, 63018 is a symmetrical section, but the section shown on the plan definitely is not.
All in all, page 2 is useless.
I am using the wing section taken from the fuz side view on page 1, and designing my own structure.That is definitely semi-symmetrical so I wonder where the chord line should be, if not where I have it.
All in all, confusion relgns.

Jeff

[Jeffrey Cottrell 2]

Hi Brian
Been re-reading your post. Lots of useful info, thanks for that, but it does leave me in a little bit of a quandary as to how to proceed.
First of all, although I have decided to dispense with page 2 altogether and go my own way, I do notice that the rib shape as shown on that page is almost identical to the outline as shown on the fuz side view on page 1.
Also the rib outline as shown is very similar the that on my Limbo Dancer. This has flown very well for some years now, but its flight envelope is different to that I would hope or a Wot4 lookalike.
Rough measure says this is at 1/2 degree positive, which would indicate that a semi-symmetrical aerofoil does in fact still produce lift even at very low values of decalage. However it might also mean that this set up would produce a flight envelope very similar to the LD, which is not what I want.
So, how to progress:
First of all, go with the plan as is. That is with the semi-symmetrical aerofoil at its set negative decalage. Caveman has downloaded the same plan and his shows the decalage as zero-zero.
I believe this might be due to a difference in printer characteristics, but certainly seems a better option to my amateur eye.
So, second go with the section as shown but reduce decalage to zero-zero as above. Not sure this will give the the flight style I want.
Third. switch to a symmetrical aerofoil. The plan states NACA 63018. I have had a look at this one on the computer, but it looks similar to the upper surface of the plan section, albeit with the max thickness a little further back. but also a distinct curve to the rear edges, unlike the flat rear on the plan section. This would make construction more difficult, though not impossible, but I wonder if it would be worth it given the whole plot is experimental anyway. Rough measure from the screen gives 18% thickness
Note being symmetrical this would need a positive decalage to produce lift.
Fourth, use a symmetrical aerofoill, but of my own design (ahem).
Again this would need a positive decalage, possibly about 1 - 1.5 degree, but wouldn't be an issue at this stage. I produced an aerofoil using the lower section of the plan one duplicated for the top.
Sort of TLAR system, but doesn't need too much modification of the fuz outline, and gives a 16% thickness, which looks about right.
So, I guess my question would be 'where can I get a four sided coin' to make the decision.
Anyway, your thoughts?
Jeff

[RottenRow]

Hi Jeffrey,

 

I used a steel rule against the image of the plan on my computer screen, and this gives a line exactly as Caveman's (zero - zero) so I would suspect that your printed copy is a bit distorted. How do the straight lines around the border of your plan compare to your straight edge?

 

I can't really answer your question, but if I were building it I would probably go for a symmetrical section much like you have suggested. My Wot 4 (straight wing version) definitely has a symmetrical section, and the ribs are 'straight' from a point a little behind the point of maximum thickness to the trailing edge.

 

You don't need positive delalage to give lift, just positive angle of attack. Many F3A type aerobatic models are set up with zero delalage in an attempt to make them neutral... ie fly the same way upright or inverted. I think if you built the model as per (undistorted) plan, but using a symmetrical section wing, it should work ok. After some proving flights, if it were found that the best flying characteristics were achieved with some elevator trim added, then you could always modify the wing seat to change the incidence accordingly. Again, looking at my Wot 4, it appears to have zero - zero incidence, though I haven't measured it accurately.

 

I do think I would treat anything stated on the plan with a largish pinch of salt I'm afraid, and would be wary unless I knew that others had built this model and flown it successfully.

 

Otherwise, get one of these dice and make a throw....

 

 

Brian.

[Jeffrey Cottrell 2]

Hi Brian.
Thanks again for your help, much appreciated.
I did the same check as you, putting a ruler across the plan on the screen. Bit difficult to tell on that small scale, but I did find the same down angle on the wing as on the plan.
However, no matter, since I will be re-doing that section anyway.
I think we are both heading towards the same conclusion, from different directions.
Decision has been made to switch to a fully symmetrical section, probably by using the curve of the lower surface on the top as well.
Not sure that qualifies as a recognised section, but it works on the TLAR principal. Only thing I would change is this.
I see your point about setting it up zero-zero to start, but I would be inclined to have a small amount of positive, say a degree or so, at the start, on the basis that it would be easier to remove if necessary.
Other than that, we are on the same page.
Will let you know how it works out.
Jeff

[Andy Stephenson]

Unless the model is very marginal on power a symmetrical section should work just fine, after all there are plenty of models which fly perfectly well with symmetrical section wings.

[PatMc]

Zero - zero wing - tailplane incidence with symmetrical (or near symmetrical) wing section isn't uncommon in sports aerobatic models & works just fine. IMO best with a slightly rearward cg, generous tailplane area & distance between wing & tail reasonable length as in this model. 

BTW I checked out the wing & tail incidences as zero - zero using a graphics app (Paint Shop Pro). The wing chord shown on page 2 is slightly longer than shown on the fuselage & the section's don't quite match. 

Other than that I wouldn't anticipate any major problems with the design as drawn. IMO the quality as drawn is as good or better than most commercially available plans. 

 

PS note that the wings have some washout, if built as described in one of the notes, so that overall there will be some slight negative "decalage". 

 

Edited October 27, 2022 by PatMc

[Jeffrey Cottrell 2]

Hi Guys, thanks for your thoughts
Andy
Planning on using a spare OS40FP, so it won't be underpowered. If it turns out to be marginal I do have alternatives up to an Irvine 53. That should scare the horses.
Also, I won't be using the build structure as per plan.
Seems over engineered to me so there's plenty of scope for weight reduction.
We'll see.
Pat
Double checked the decalage both on the plan and on the screen, and they both still show some negative. Bit of a puzzle, but doesn't really matter since I will be changing it all anyway.
If you look at the bottom right hand of page 2 of the plan, you will see it titled 'Killer Watt' which is in fact the clone of an AcroWot, not the Wot4.
Also if you read the text on Outerzone, you will see numerous issues, especially with the rib outlines. I am assuming the note on washout only refers to the tapered wing, certainly can't see any purpose on a parallel chord wing.
Page 1 is usable, but I have pretty much discarded page 2.
When I start the build I will be incorporating 1 deg of positive, the reason being if that proves not to be necessary it will be easy to remove it.
Meanwhile, although it will not be a quick build, I will be keeping a running total of costs, to see how it compares with a commercial kit. Should be an interesting comparison.
Cheers
Jeff

[Masher]

On 26/10/2022 at 14:39, RottenRow said:

I built a Killer Watt about 10 years ago from the plan with no changes. Fitted with just an OS46LA it was my favourite model for a few years until I decided it was too oil soaked to fly any longer.  Unsurprisingly it flew just like an Acrowot! Well worth making.

 

[Jeffrey Cottrell 2]

Hi Guys

Masher

Can't seem to find this post on the forum. can you confirm date?

Brian

If this is right, can you confirm if it was from the Colin Usher plan on Outerzone?

Thanks

jeff

[Martin Harris - Moderator]

I've built models with flat plate wings, zero downthrust and no decalage which flew perfectly well.

[Masher]

9 minutes ago, Jeffrey Cottrell 2 said:

Hi Guys

Masher

Can't seem to find this post on the forum. can you confirm date?

Brian

If this is right, can you confirm if it was from the Colin Usher plan on Outerzone?

Thanks

jeff

Hi Brian. It was direct from Colin's website definitely 

[RottenRow]

I don't know where Masher obtained the quote above that was supposedly made by me..... I never wrote that, and have never built from a Colin Usher plan.

 

And looking at the Watt Four plan, it's unlikely that I ever will!

 

Brian.

[Masher]

My fault. I just wanted to post a reply but messed up,  just ignore 😔 

[ron evans]

All the models I have built for aerobatics have wing/tail incidence set at 0 deg, and has worked with wing sections from symmetrical to flat plate.

If you need more pitch stability, that's a different matter.

Incidentally🙂 the meaning of the word decalage is...The difference between the angles of incidence of the two wings of a biplane.

 

Ron

[J D 8]

             That is where "decalage" came from but later on also became the difference between wing and horizontal tail surface or fore plane in the case of a canard type.

[Andy Stephenson]

Classic pattern-ships were often set up with the tailplane at 0°, the wing at 0.5° to 1° and the engine at 2° to 3° down thrust.

[Peter Jenkins]

I believe that wing incidence will primarily affect the sit of an aircraft since you will have used the elevator to trim the beast to fly straight and level.  On some of today's F3A aircraft, the tail plane incidence is adjustable so that you can trim out any elevator input used for S&L flying.  I have one model that has that feature and one that hasn't.  Doesn't seem to make any difference to flying characteristics.

 

Most modern F3A aircraft have the TP at zero incidence, wing at +0.5 to +0.8 and motor at zero although contras are set at -1  but I've had to put a bit more downthrust on to stop a pitch to the canopy on a vertical upline.

 

I have a Wot 4 ARTF that has a very thick symmetrical wing that is great for keeping speed down on vertical down lines with its profile drag.  Never measured the tail plane incidence vs the wing incidence and never had any problem with stability.  It started life with an Irvine 53 Mk 1, but ended up with a Mk2 which is a significantly more powerful engine and then put a throttle pipe on it and that has given almost unlimited vertical but had to swap the 11x6 for an 11x9 to keep the revs down to 11,000 rpm.  A true hooligan's aeroplane.

[Peter Miller]

I always use a 0-0 set up on my designs, 0 wing incidence, 0 T/P incidence and NO up or down thrust on the power unit.

 

I like that set up and no one seems to complain about the way my models fly.

[Peter Jenkins]

Forgot to say in my post above that CG position is the most powerful trimming tool there is.  The CG shown on a plan may not be in the optimum position.  Clearly there is an aft position behind which stability will be neutral and further aft will make the aitcraft unstable to increasing degrees.  A slightly unstable aircraft may be flyable by a good pilot as things don't degenerate to wild antics when the CG position is just past the neutral stability point.  

It is worth experimenting with gradual CG changes to see how the aircraft's charchteristics vary and to pick the point that best suits you.

Wing and tail plane incidence, provided they are no more than a couple of degrees different will be resolved by elevator trim.  Remember though that the trimmed S&L flight is only so for that speed.  The CG effect is, to a large extent, speed insensitive.

Remember that the CG position is a 3D location.  Depending on where it is in relation to the thrust line and centre of drag will affect your aircraft to a greater extent if these positions don't intersect.  That's where adjusting down thrust and side thrust will reduce the immediate effect of power changes on the aircraft's attitude.  I say immediate effect as increasing and decreasing power in S&L flight will cause the airspeed to change and your aircraft's elevator trim will no longer be spot on. Obviously, the aircraft will then climb as speed increases and vice versa.

[kc]

Why possibly waste balsa on a design that has some uncertain details and no reports of how it flys!  

  Many clubs have someone who drew all around a genuine Wot4 kit parts and passed the info around............. you should ask around discretely.    The original Wot4 plan used to be sold by Chris Foss before kits were available and unofficial copies could be obtained on Ebay in recent years when you purchased a wing plan (built up wing and non Chris Foss  ) from that seller. 

Or get a known good plan like  Rival or Basic 3D from Outerzone.

Edited October 30, 2022 by kc

[ron evans]

Peter J.--not sure I could build to 0.5 deg accuracy 🥲. I've often wondered why top aerobatic models do not use all moving tailplanes. When trimmed for s/l flight the wing/tail incidence must be right, wherever you chose to put the c/g.

I've only had one model with an a.m.t., an aerobatic sloper over 40yrs ago. It flew well with smooth but powerful pitch control. Some weeks later I flew it in a very strong wind and pulling out of a high speed dive the tailplane fluttered, about a second or so of machine gun like noise than a bang as one half of the a.m.t. broke off.

Thankfully I still had pitch control so turned downwind to make a normal if hurried landing.

The fault lay with the long control horn being too flexible, so with that stiffened and repairs made there were no further problems. The model still gets an occasional outing today. As the model flew ok with half a tail, it had me wondering if I had made the tail too big 🤔

The a.m.t is a bit unnecessary on a club aerobat, but maybe just the job on the new breed of F3A.

 

Ron