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Old Jul 23, 2012, 07:46 AM
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Joined Jun 2012
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My wing's angle of incidence. Please comment.

I'm ready to attach the 30" wing of my free flight model to its fuselage mount and I could use some feedback. My wing right now has an angle of incidence at the fuselage of 3 degrees and an angle of 0 degrees at the wingtips due to built-in washout. Is that okay? Do I have too much washout? Sorry about all the posts lately… I have many questions. Thanx.
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Old Jul 23, 2012, 10:47 AM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,310 Posts
A 30 inch wing model will be a real pain to transport in one piece. I strongly suggest you use some small shishkabab skewers fitted to the fuselage as rubber band dowels to allow you to use a few small rubber bands to mount the wing. And along with using the dowels and rubber bands comes the ability to shim the wing as needed for flight trimming. And another advantage of using rubber bands to mount the wing is that it makes the wing mount somewhat shock absorbing so you stand to get less damage during any not so soft "arrivals". So it becomes a win-win situation in every possible manner.

If it's not obvious how to best install the dowels for the rubber bands then post up a picture or two of the wing mount area of the fuselage and we'll offer up suggestions.

For that matter a 30 inch span model is a size which is very much thermal feed. Which means that you really SHOULD be using a dethermalizer setup of some form. And with the regular method for DT'ing of popping up the tail to a -45 degree angle you need a mount and more dowels at the tail. WHich means that the tail can mount with rubber bands as well.
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Old Jul 23, 2012, 09:40 PM
Balsa Flies Better!
Stamford, CT
Joined Oct 2000
6,704 Posts
In terms of the question you asked....let's turn the problem around...

The wing dictates the angle of everything else. At a particular speed, if an airplane is going to fly level, then the wing's angle of attack relative to the ground will vary depending on the weight of the airplane.

Tom Nallen found that a good rule of thumb is 1.8 degrees of decalage between the wing and the stab. Most of us add up to 5 degrees of downthrust relative to the wing. Hence, your 3 degrees is probably going to give you a bit of up trim, but might need a degree or two more of downthrust.

In terms of washout- that depends heavily on wing planform and airfoil. Skinny tips need a lot- elliptical wing tips need a ton...but a Hershey bar wing can get away with basically none.

PS

I agree with Bruce that having a removable wing does lots of good things- I wish the wings on my 30" scale ships came off.

Sam
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Old Jul 24, 2012, 01:28 AM
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Wing angle, decalage etc.

Thank you Megowcoupe and BMatthews. Your suggestions and expertise are appreciated as always.

I'm completely unfamiliar with dethermalization though I've come across the term and I plan to check it out. Thanks for mentioning it. As far as decalage, I plan to make my first trial glide with the horiz. stab. set at zero, where I've had good results. If the plane seems to pop up too much and too quickly then I'll shim an angle into it.

To see a drawing of my airplane, which also converts into a rocket, see my discussion thread titled "To: aeronca52 & yak52 - Re: Washout." It doesn't show the mount setup but the wing is removable and held in place with rubber bands, as you suggested. The drawing has one error in regard to the shape of the airfoil and a drawing of the actual airfoil is attached (hopefully) to this post. You might see why I'm concerned about wing angle, but I'm aware that most 'standard' full size airplanes usually have an angle of 6 degrees.

Thanks again. I learn something new and useful every time you speak and your comments are welcome anytime. Anyone who says this stuff is easy is crazy.
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Old Jul 24, 2012, 01:28 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
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That's a very thick and rather "klunky" looking airfoil for a 30 inch span model. Models of around this size do better with thinner airfoils and only moderate amounts of camber up to perhaps 4 to 5%.

For example something like this;


or;


The thinner shape deals with the viscosity of the air better at lower airspeeds. You'll see references to low and high Reynolds Numbers or Rn often quoted in connection with such things.

Don't fret though, the model will fly with the thick airfoil. But if you get a chance to do so try building a model with a thinner airfoil that has a proven free flight track record.

Trimming a free flight model is a situation where you're walking a tight rope. The best flying models are trimmed to be very close to neutrally pitch stable but with just enough stability to return to level trimmed flight if disturbed within a reasonable amount of altitude loss. The amount which is reasonable is related to the wing span. For a rubber model of 30 inch span I would like to see the model nose up and just barely skim the grass out about 15 yards in front of me if I launched it with a little less than proper airspeed and with the nose canted down about 5 to 8 degrees. Or if I launch it into a very mild stall again it should slow, drop the nose and recover to touch down or barely skim the grass about 20 yards away from the stall point. If it recovers sooner and with less height loss then it is TOO stable and I need to move the CG back a little and re-trim the wing to stabilizer angle for proper glide again. If it dorks in with too much angle then it's not stable enough and I need to move the CG forward a little and re-trim the wing to stab angle to compensate.

Note that this determination requires that the CG be moved and THEN the wing to tail angle is altered to compensate. This "decalage" angle is based on the CG postion and not the other way around. The CG location is the prime factor and then we compensate the other things to get back into trim.

So when I first test glide a new model I decide to move the CG or to alter the decalage based on my observations. If it goes into a stall I study the stall recovery period and distances and decide if I'll move the CG or alter the decalage based on the distances of the stall cycle. If it dives in straight away I'll typically move the balance back until I see some signs of a stall and recovery so that I can guage the amount of pitch stability and then make changes to the decalage or balance point location to arrive at the optimum trim.

This setting of the balance point to arrive at a stable but just BARELY stable becomes far more important when the power portion of the flight is much faster than the glide. The greater the speed difference between powered and gliding flight modes the more important it is to be on the ragged edge of safety. And since you're talking about fitting a rocket motor into the model for later this becomes hyper important.

Even then you will find that flying as a rocket with wings that the model will want to loop over and try to smack you in back of the head. This is why in high powered free flight models we trim for a spiral turn in the climb. It turns the tendency to loop into a steep banked climbing corkscrew like path. Or we use timer operated trim tabs or movable surfaces to reduce the decalage for the climb portion. But by doing so we turn the model into a neutral stability "lawn dart". With the shifting tail stuff if things go bad they go bad in a big way more often than not.

As you say, there's nothing easy about this stuff....
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Old Jul 25, 2012, 03:21 AM
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The windy west coast of Sweden
Joined Sep 2008
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Hi,

I wish to make a minor addition to the very good info above.
I'm aware that I MAY "bash an open door" here though....

We often forget - or forget to mention - the fact that if the stab e.g. is attached linear with the fuselage datum line (see top scetch), the 'decalage' will NOT be "zero" degree if the wing is "put" on same line (like resting on the building board), due to the fact that the true wing section datum ("zero") line starts, roughly, at the point where the airflow divides up/down....
This setup (top scetch) gives a decalage of roughly 2° positive...
Quite often, people (wrongly), call this "zero"....which obviously messes up the calculation...

A true 0° setup is shown on the bottom scetch....

I thought I'd just mention this....



.
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Old Jul 25, 2012, 04:19 AM
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UK
Joined Jan 2009
1,248 Posts
Oracletwo, three degrees of incidence will be fine. In many ways it doesn't matter too much about the wing incidence relative to the fuselage. This is mostly about setting the fuselage attitude in flight (for low drag and looks) as the wing will find the right angle of attack anyway.

The important thing is the 'decalage' or relationship between the wing incidence and tail incidence. As Bruce has shown this will affect the amount of longitudinal stability or the ability to recover from a dive. I like to have some way of adjusting decalage, ie moveable elevators or a stab slot that can be shimmed up a degree or two. This means you can set the CG position in the right place for stability and then set the model speed with the decalage. A low powered model can be flown at 'one speed' and can have more stability... Bruce's comments about barely stable are spot on for a higher performance model but if this is a first attempt there's nothing wrong with high stability (forward cg) and modest power.


One point further to Gluehands post: it is correct to say that the reference line for incidence is as shown - from the leading edge to the trailing edge. This is the datum line. But this is different to the zero-lift line which is what the air really sees. The effective decalage will be between the zero-lift lines of the wing and the stab. Increasing the camber of the airfoil will give a more negative zero-lift line. So greater camber will mean greater effective decalage, even where the datum lines are set the same. I've shown this on your original drawing Bosse... So even with the wings set flat to the fuselage there will be some decalage effect with this high camber airfoil.


Jon
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Old Jul 25, 2012, 05:04 AM
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AOI, decalage etc.

Thank you so much, BMatthews et al. Your knowledge and the insight you've provided me with is amazing and I'm going to make sure that I save it for reference. I know that my wing is rather blunt and if I ever make a new one I'll try to thin it out but even the current one was very difficult to make and I need to fine tune my construction process.

I checked the wing angle on my computer and noticed that I made an error regarding my angle of incidence; It wasn't 3 degrees but 1 degree, with 0 degrees at the wing tips, so I guess I have room to tilt the wing more. When I put a ruler under the wing along the fuselage the angle looks extreme compared to what the computer shows, based on the fore and aft heights of the wing mount.

By the way, this airplane flies as either a rocket or a plane, using the same fuselage, but not both at the same time, though I've seen airplane models that are powered by rockets. I'm sure that you're also familiar with 'em. I'm a babe in the woods compared to you all.

I'm consoled by the fact that my plane does fly and my new model is almost half the weight so I should get some good results after a bit of tweaking. Again, thank you for taking the time and trouble to share your obviously vast pool of knowledge.

PS: Thanks also to Gluehand and yak52 for your tips about decalage. I noticed that you somehow even managed to use my airfoil in your graphics. Pretty sharp.
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Old Aug 02, 2012, 09:06 PM
Culper Junior
eastern pa
Joined Feb 2007
2,186 Posts
I'm a babe in the woods compared to you all.

We were all babes at one time until we learned from others. Follow the footsteps until you can make your own.

Or something witty like that.
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