Incident angle.

[Basil]

Well here goes, it is with a great deal of intrepidation that I ask the following. I have never been in a position when building where I have had to pay attention to the incident angle . Allways built where a reference article etc was available. Also only high wing , sits on fuse, so no real regard for such.

 

I have been scratch building an AOP9 from plans, without a detailed article . Bearing in mind that I am a novice, still learing.(Still learining to fly as well, when I can get back to it). On the plan , refeernce line on top of fuse, lower rib profile is flat for about 95%.Bottom of rib was ILW fuse.( Full size plane incidence angle , as far as I can tell is 2 degs.) I set the bottom rib line 2deg+.It was very time comsuming constructing the cabin wing supports, due to lack of experience, and consultation.

The build has progressed, and so has my knowlege, and that led me to measure the incidence against the centre line of the chord. That is about +4deg.

My dilemma now is do I tear the cabin top/wing supports apart and rebuild( A lot of work) or is the difference on a model of this type slow flying/stol type, of little consequence? Thats my question.

I have looked at a small number of plans of AOP9's, all have smaller spans than this (82") , all have what look like quite large incidence angle ,I would estimate as much as +7 degs or more.

I suspect that this is not an easy question as it will be related to the model design/characteristics, rather than a broad concept.

I suppose I should not have started to read books about aeronautics for modellers( Thanks to Peter Miller) that has made me question my own actions!!!!!!!

I presume that I must be prepared for the worst, not least of which is learning to run etc. But how else do you learn.

here goes, I close my case,pressing the enter button

Bas

 

 

Edited February 26, 2021 by Basil
text addition, omission

[Graham Davies 3]

Hi Bas,

 

I think you have thought far harder about this than many! Many models fly very well with varying degrees of 'adherence to original design'. I have seen some models that look like MC Escher sketches fly surprisingly. Sorry, I meant surprisingly well...

 

I think the mainplane incidence relative to the tailplane is key. If the tailplane is also +4degree, you effectively have 0degree incidence, albeit with the model carrying some drag due to the slight nose down attitude.

 

Do you know the tailplane incidence? It may be easier to pack the leading edge of the tailplane a little.

 

Graham

 

 

[EarlyBird]

@Basil,

 

As a novice I have been in the same position as you. Too much incidence causes the model to climb rapidly under power, which is not good. My solution was to pack up the trailing edge and fit wedges along the cabin top. How easy this is to do depends on how far you have got with the build. 

[Basil]

Graham, the tail plane is ILW the top of the fuse. It's a 3/8 sheet carved , equal angle , just sits on the rear of the fuse. The AOP9, looks like a bit basic, thats why I selected it, well thats what I thought.

Bas

[Basil]

Early bird, not that simple. The wing roots/mount are part of the top cabin structure. The 4 corners of the cabin support the wing and are firmly embeded into the root and sets the trent for the wings, when connected.

Bas

[Matt Carlton]

Can you pack up the LE of the tailplane, or alternatively, cut down the fuselage where it sits to put about 3 degrees positive on the tailplane? 

If you haven't built the cowl yet, you could add a bit more downthrust to the engine/motor to try to mitigate the tendancy to nose up under power. 

 

[kc]

Are you building the AOP9 from ModelWorld March 1986?

 

The zero incidence line on a flat bottom airfoil is not the flat botom of the airfoil

[Basil]

KC. yes it's that one, the article is not exactly full of info. Especially for me. I dont quite understand your meaning re the incidence line!!

Bas

[Martin Harris - Moderator]

I think kc has missed the bit where you stated that you'd realised that the angle of incidence is measured at the chord line.  (incident is something that might result from the wrong incidence by the way! ?).  I missed it the first time I read it.

 

Can you post a picture of the area and one of the plan in case anyone can come up with a helpful suggestion.

[Basil]

Yes , the true angle of incidence is taken on the chord line.

The reading I have done also says that for construction the incidence line is that between the datum and the bottom of the flat airfoil!!!.

That's why I set the angle against the datum, the top of the fuse.

My true question is what are  member/s opinions as in this state, will it be distastrious, I suppose no one knows.( It's only + 2 degs out)

On other plans of the SAME model, but with a smaller span,I would estimate  the angle was +5 degs!!!!. Mine is + 4deg.

Bas 

As a note I dont know how to add a picture. cant find any guidance.In fact I am having a great deal of problems, mainly relating to access to past posts, re searches. References on the net, that I used a lot, wont search currently!!!!!

Edited February 26, 2021 by Basil

[Graham Davies 3]

I doubt it will be disastrous Bas, but it will put you closer to the stall, so take care at low speed/ high angle of attack. But then that's always good advice.

 

What you are likely to find is that you will trim this out with some down trim at the speed you are comfortable at. When you land, you'll probably find that you have a couple of degrees of down elevator. Not the end of the world!

 

Graham

 

[kc]

This is a Keith Humber design and he had many designs published- it might be safest to assume the plan is correct for this size.    20 years later he also did an AOP9 for RCMW Nov 2006 at 54 inches.

The 82 inch is shown on the tiny plan in the March 1986 RCMW

[Basil]

KC that the plan 82". Looks like you looked back at your old copies!!.Cant find any references to the angle in his article, and thanks.

Graham, thanks for your comments.

I'll ponder a little before taking a decsion about cutting off the wing base.

Bas

[Graham Davies 3]

Have you fitted the tailplane yet?

 

I think I'd be tempted to make up some spacers to lift the leading edge of the tailplane to achieve 2degrees positive. That way you'll be aerodynamically not far from the plan and won't need to worry. Looks easy enough from the plan.

 

Graham

[Martin Harris - Moderator]

I’m slightly confused Bas. Are you building an own design or from a published plan. If the latter then it should be fair to assume that the incidence has been designed and tested correctly. 
 

If wildly out the model may pitch up strongly when adding power and drop the nose on throttling back if trimmed to level flight at significant power.  I would recommend getting an experienced test pilot to do the initial flights - my limited experience with model AOP9s is that they aren’t aircraft which “groove” and need to be flown constantly. 
 

On adding photos, forget the old forum’s quirky methods - most people have found it much easier on the new one with drag and drop or uploads straight from your device. 

[Basil]

Yes the reason I picked it was that it did not seem, on the surface that is,a difficult build, but  would stretch me, and was doable. Still think it is.

I've been giving thoughts as to how to adhere to the original angle  with out destroying the cabin/wing support parts. The wing supports/cabin wing profiles are slightly larger than the wing airfoil, just enough room to rotate the wing around the front dowel to allow me to loose  the  2 1/2 degs that it's over. ( I have not drilled out for the support dowels yet, lucky)Then a bit of fettling  and realign the rear dowel, will make it good I think. I'd rather have things as they should be. We'll see.

Thank you all for your comments. Learned a lot , as always from you Pro's.

 

I just found out how to add pics etc

Bas

Edited February 26, 2021 by Basil
Additions, after some more serious thought

[Jonathan M]

Don't worry what other models have - just build true to the plan you've got.

[kc]

Yes, that was an original RCModelWorld March 1986 from my collection.  I looked up the magazine a week ago with a view to letting Basil have a copy but it seems he already obtained the article with the plan.

If the design had wing bolts or bands it would be easier to change the incidence but with wing joiners it is much more difficult.   Therefore it would be better to assume Keith Humber got it right and build to the plan.    I once had a plane which must have had an error in the incidence and it always flew with a visible amount of up elevator - no problem it flew really well but everyone noticed that it didn't look right when on the ground.   So don't worry the AOP9 plan is probably right and if not then some small trim will probably put it right.

 

[Graham Davies 3]

KC, I've seen some models that look like Salvador Dali paintings. If the builder had used rulers or Engineer's squares during the build, he made them out of rubber or indulged in some non-prescription medication when using them. But the models still flew! In the pantheon of modelling no-no's, 2degrees of additional incidence on a model such as this is not at the top of my list. On a pattern ship or a warbird, I think I'd be patient and aim for 'cock-on'.

 

Generally, dumb thumbs have downed more gangsters than Al Capone. A perfect model is indeed a fine thing, but limitations of the build process are all part of the fun!

 

Graham

[PatMc]

The only things reducing the wing incidence will achieve will be causing the model to fly with the nose & the thrust line higher by the same angle as the reduction in incidence.

Increasing the tailplane incidence would make more sense except that it's the one thing that is designed to be adjustable anyway. 
The model has throttle & elevator control, just use them as intended to control climb & speed respectively. 

[Martin Harris - Moderator]

If I'm following this correctly Pat, Basil has modified the angle of incidence by departing from the plan and setting the wing at an increased angle, after using the bottom of the wing profile instead of the chord line so isn't contemplating reducing it from the designed angle, but returning to it.

[Geoff S]

You've picked a very tricky model to build.  I have a part completed 1//4 scale AOP9 (Airsail) which I haven't touched for years, mainly because getting the structure to support the wing right (it's a piano wire frame.)  Not sure if yours is the same but I'm sure you'll get it in the end.  I always use the tailplane as zero and have a wing incidence meter to measure the wing angle.  I also read a few books on aerodynamics but found the Alistair Sutherland one the easiest non-academic book.

 

I think the important thing is to get everything square in all directions (something I found difficult on the Airsail).

 

Geoff

[Simon Chaddock]

Basil

The AOP 9 like all Austers has a generous elevator so I would not worry too much about the wing incidence. If on its maiden it tends to climb readily just trim nose down a bit.

That is all the full size did. It had an adjustable trim tab on the elevator that caused it to ride down or up a bit as required. Nothing else was adjustable.

If after you have flown it a bit and you find the elevator is not inline with the tail plane in normal flight then you can considering adjusting the tail plane incidence. The position of its Centre of Gravity can have an influence as well. 

If you are a member of a club it is quite likely there will be an experienced pilot who would be willing to do the maiden flight for you and will be able to quickly diagnose what if anything is wrong, maybe before it is even flown, and what to do about it.

[Basil]

Morning all, just read the latest posts oops. Over night I decided to cut off the wing support from the pylon and remake. I normally strive to get things corrct, dont like a bodge.I started before reading the latest posts.Yes the full size had its own habits as I understand it, was prone to bad habits, wasn't granted a full permission to fly privately without restrictions, like to buck. I am still learning to fly, as I have said , so like to reduce the odds in my favour, hence the change of mind.The tear down is requiring some patience.

I will have to make an incidence gauge of sorts I suppose.

Thanks everyone

Bas

[kc]

If you draw a straight line on the plan to extend the tailplane line then measure to the chord position at LE and TE you can then measure on the plane to get same incidence as plan.    If the plane has a solid fuselage then a line along the balsa gives something to measure from,  but with a built up fus then you need to pin a spare bit of balsa instead.   Then a mark on the structure for future reference.    You don't actually need an incidence gauge.