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Old Dec 07, 2009, 12:01 PM
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Washout for sport/pattern plane?

Hello, everyone.

I'm quite new in original RC airplane design, however, this looks like challenging and extremely interesting lesson to learn.

One and half year ago I have scratch-built 1,6 m Extra 300S with 15 cc 2 stroke glow engine. It was real pleasure to construct it, but my flying skills were to rough at that time, and I crashed the plane after approximately 2 months of trying and learning. One of the reasons why it happened was the unstable character of the plane in abrupt maneuvres. I felt flying it like balancing the crystal ball over the glass table. Now, as I gained the experience, I would like to build this project again, but also make some improvements.

One of the possible improvements is to add the washout to the wings, so the interrution of the airflow over the airfoil would not start at the wingtips. I know about the geometrical and aerodynamical washout, and already put the last in to my new project (airfoil is starting with NACA 0013 at the root and ends with NACA 0015 at the wingtip).

The question is wether I should include also the geometric washout, turning the wingtip down at some angle? If this would be the trainer design, the answer probably would be "yes". How about pattern plane? Geometrical washout obviously would improve the stability in "head up" position, but will probably worsen the inverted flight characteristics. Whad you guys are thinking about this?
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Old Dec 07, 2009, 12:45 PM
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Washout isn't particularly good on a pattern plane if you intend doing inverted flying as well. You end up with wash-in when inverted.
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Old Dec 07, 2009, 01:41 PM
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Taktas,
I'll second Eflightray's comment.. Washout is not good for an aerobatic plane because it becomes washin (washin = bad) when inverted.

If you want to promote good stall behavior both upright an inverted then make the LE more rounded toward the tips and 'sharper' toward the root.

The other trick is to keep the model light so you are less likely to stall in the first place

Steve
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Old Dec 07, 2009, 02:54 PM
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Thanks. Absolutely, the geometric washout in pattern plane sounds as a nonsense to me now . And the aerodynamic one is the same as you are explaining, JetPlaneFlyer . I add it by increasing the procentage of the airfoil thickness towards the wing tip. The question is if this is enough (13% at the root and 15% at tip)?

I have calculated the polars of both airfoils, and the difference between the stall angle at Re 250000 is only 2.5 degrees (12.5 for 13% and 15.0 for 15%).

Does anyone have an idea if I have to go with further increase of the airfoil thickness at the tip? Suggestions for other combinations of airfoils are also welcome.

In fact, selection of airfoils depends much of the intended flying style. Well ... It should be suitable for pattern flying in local competitions (thus maintain good stability in square and triangle loop turns, fly inverted about the same easily as normally, etc.) In addition, I want to do basic 3D, such as hanging, harrier, inverted harrier, etc. So, a kind of compromise is needed

I'm sure, anyone would want to demonstrate your "excercises" between the competition flights ...
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Old Dec 07, 2009, 03:46 PM
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I'd concentrate more on rounding the LE than thickening the overalll airfoil.. But my gut feeling is that 2.5 deg would probably be enough for a plane that has only moderate wing taper. I've flown similar planforms (Su 26M) without any geometric washout and that had no bad stall issues.
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Old Dec 12, 2009, 06:42 AM
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Thanks, Steve for your suggestion. Rounding the LE basically means shifting the maximum thickness of an airfoil to the LE side. I did small research by comparing the three airfoils: NACA 0013 (green), NACA 0015 (red) and some "home made" airfoil with rounded nose. My impression is that rounded nose would reverse the desired aerodynamical washout, because it will develop higher lift at smaller alpha and stall at approx. 13.5 degrees. In contrast, simply thicker airfoil will behave more smoothly and stall at almost 15 degrees, as I described earlier. So, my choice is thicker airfoil instead of "customly rounded" LE. Am I wrong with this?
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Old Dec 12, 2009, 07:14 AM
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I cant argue with the graph.. go for it with the thicker airfoil!
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Old Dec 12, 2009, 01:23 PM
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Quote:
Originally Posted by Taktas View Post
Thanks, Steve for your suggestion. Rounding the LE basically means shifting the maximum thickness of an airfoil to the LE side. I did small research by comparing the three airfoils: NACA 0013 (green), NACA 0015 (red) and some "home made" airfoil with rounded nose. My impression is that rounded nose would reverse the desired aerodynamical washout, because it will develop higher lift at smaller alpha and stall at approx. 13.5 degrees. In contrast, simply thicker airfoil will behave more smoothly and stall at almost 15 degrees, as I described earlier. So, my choice is thicker airfoil instead of "customly rounded" LE. Am I wrong with this?
I just did an analysis that would simulate the rounding of the leading edge toward the tip. I took the NACA 0012 and rounded the front edge off the airfoil graphically. This shortened the chord by .25" (based on a 10" chord). Since the chord became less (but thickness remains the same) the effective thickness % does increase to 12.3%. The max thickness point also moved from 30% to 28%. The .25" change in chord reduces the Reynolds # by 6,000 (in respect to a 30 mph speed). You can see the results in the graph. The black line is the original, the red line just shows the NACA 0012 with the thickness changed to 12.3%, the green line shows the LE modification + thickness % change + the max thickness point change.
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Old Dec 13, 2009, 01:51 PM
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Thanks for joining, wyowindworks! Your results look interesting. However, I'm little bit confused about comparing different airfoils at different Re. As I do the comparition of the airfoils, I calculate Re for the mean aerodynamical chord of the wing, and use it for all airfoils. Therefore results I get are dependant on one variable and are easy to interpret.

I also have simulated my case in similar conditions as you do (i.e. different Re for root and tip airfoil), and find that difference between the stall angle of the NACA 0015 and "custom" airfoil with rounded LE is neglectable. However, the drag produced by the "custom" airfoil is larger, especially at small alpha. So, probably I'm still for the thicker, but not rounded airfoil
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Old Dec 13, 2009, 05:12 PM
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The question I was analysing is: Does rounding the LE from the normal shape to more round (gradually toward tip) improve the stall characteristics? I'm assuming that the airfoil is the same from root to tip and that the existing LE is manually sanded more round progressively from root to tip. I also assumed that any improvement that is gained would be a linear progression from the root to the tip. Thus, I only analysed the change at the tip. This will tell us if manually sanding more off the LE to make it rounder is effective. My analysis is NOT comparing the use of different tip airfoil, but rather the effects of manually sanding the LE of an existing airfoil to a more round shape.

3 things happen when the LE is rounded more than normal:
1. The chord becomes less since more material is removed from the LE. This reduces the RE.
2. Since the chord decreases but the physical thickness stays the same the effective thickness by % of the chord increases.
3. Since the leading is rounded more (more is removed) the max thickness point moves forward.

All three of these changes where made in my analysis. The polar charts mathematically display the effects of manually rounded more off the LE of an existing airfoil. Your analysis, in contrast, used a completely different airfoil.

BTW, my inspiration for my analysis was in response to the qoute by Steve:

Quote:
Originally Posted by JetPlaneFlyer View Post
If you want to promote good stall behavior both upright an inverted then make the LE more rounded toward the tips and 'sharper' toward the root.

Steve

Adam
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Old Dec 14, 2009, 05:18 AM
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Thank you, Adam, for your explanation. Can I conclude with following?

1. Manual rounding of the LE towards the wingtip has the potential of improving the stall behaviour, but introduce some uncertainty, such as where the LE is supposed to start become more "rounded" or what the side effects of manually changed airfoil will be;

2. Increased percentual airfoil thickness towards the wingtip has similar effect as a rounded LE (i.e. stall behavior improvement), but also behaves more smoothly at alpha values close to the stall angle and produces less drag at small alpha.

Please correct me if I'm still wrong.

Darius
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Old Dec 14, 2009, 11:20 AM
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Here is polar chart showing the contribution of each change: Thickness, max thickness point, and LE countour

Black - Original airfoil with no changes
Red - Thickness change, original max thickness point, original LE
Green - Max thickness point change, original thickness, original LE
Blue - Thickness change, max thickness point change, original LE
Orange - Complete Modified airfoil by increasing the LE radius

A manipulation of all 3 components gives the smoothest curve at this RE. As the RE decreases the curves become more sharp.

In answer to your question, sanding the LE is surprisingly easy to do on a symmetrical airfoil. Yet, if it was my plane and I was designing it I would use a different airfoil at the tip to improve stall performance. But, you're still going to have to shape LE manually unless your using CNC cut foam core wings with a composite LE (which still requires some sanding).

Also, I would make sure that the tip chord doesn't get any smaller than 50% of the root chord. I'd go larger to just be sure. Because of the planform shape the tip ends up being in an aerodynamic washout orientation because of the airflow (inverted as well). If I remember correctly the tip was 53% of the root on the full scale 300s


Adam
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Old Dec 15, 2009, 02:22 AM
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Adam, your further explanation helps me understanding the essence of analysis I have to do . I have calculated different Re for the wing root (390 mm) and the wing tip (210 mm) for the "critical" airspeed of 36 km/h. They are 224000 and 143000, correspondingly. By the way, thanks for pointing the 53% relation between the root and tip chords at full size Extra 300. It is almost 54% in my case, so I'm probably at the safe side. The polar charts below clearly show that customly rounded airfoil at the wingtip fails to perform as required (stalls at almost 12 deg. and produces enormous drag) I beleave, your case of rounded airfoil is different. At the same time, 15% NACA airfoil at the tip behaves nearly the same way as the 13% airfoil at the root. The question is if this is enough?

The foam core wing is a good idea, however this time I will go for classical airframe. I do not see why the precision of airfoil could be a problem. I will prepare LE templates for each rib .
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Old Dec 16, 2009, 02:29 PM
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Quote:
Originally Posted by Taktas View Post
Adam, your further explanation helps me understanding the essence of analysis I have to do . I have calculated different Re for the wing root (390 mm) and the wing tip (210 mm) for the "critical" airspeed of 36 km/h. They are 224000 and 143000, correspondingly. By the way, thanks for pointing the 53% relation between the root and tip chords at full size Extra 300. It is almost 54% in my case, so I'm probably at the safe side. The polar charts below clearly show that customly rounded airfoil at the wingtip fails to perform as required (stalls at almost 12 deg. and produces enormous drag) I beleave, your case of rounded airfoil is different. At the same time, 15% NACA airfoil at the tip behaves nearly the same way as the 13% airfoil at the root. The question is if this is enough?

The foam core wing is a good idea, however this time I will go for classical airframe. I do not see why the precision of airfoil could be a problem. I will prepare LE templates for each rib .
When I modify the LE of the NACA 0013 (I used the thinner 0012 in my previous simulations) I get a polar output that is very similar to yours. Using the 0015 looks like a good option.

I've been wanting to run a 3D analysis of your entire wing and see what happens at or near stall. I suspect that there is some aerodynamic wash-out that will occur and may not require the higher drag 0015 at the tip. Maybe tonight

Adam
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Old Dec 18, 2009, 01:13 AM
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That sounds interesting, Adam. How you do the 3D wing analysis? Can I learn it?
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