Yet Another Airfoil Question

[CalmAir]

I would like to generate a basic flat-bottomed airfoil that has good slow speed handling and will allow a 15 to 18% angle of attack. I want to stay with the flat bottom for ease of construction.

I use Profili to print them out, but with over 2000 foils to chose from I am frazzled. Is there one that would be in the list that works well in the low Rn model world?

Regards,
Dale

[Dave Robelen]

Hello Dale,
I wish there were a simple answer to your question. Part of it is the question "how low a Rn" . As far as airfoils in Profili with a flat bottom, you can do a whole lot worse than the Aquila section. If you wish to expand the speed range, I have had very good results with the Clark Y.
Regards, Dave

[CalmAir]

Hi Dave,

I really did not want to open a can of worms. There are so many in Profili, I just did not know where to start looking for ones with a fairly flat bottom, other than the Clark Y. Mainly ones that modelers have had decent results with. I am just referring to the normal models that we fly that run 10 to 20 mph with 4 to 7 ounce wing loadings. Nothing fancy, just playing around in case I would want to slap another slowflyer together someday.

I am looking at the Aquila's now.

Thank you.

Dale

[Don Cooper]

Dave,

I am not sure what you mean about expanding the speed range with the Clark Y. Could you expand on it a little? Thanks.

[Dave Robelen]

Hello Don,
What I was reffering to in regards to the true Clark Y was the fact that it has a fairly high maximum lift coeeficient, while at the same time showing an unusually low drag at lower values oMy f lift. This was show in the Princeton low Rn wind tunel studies, and it has worked out that way for me. My experience to date has been with a series of "wet" models frpm Norvel .061 to OS FP.25 size. All of them have an unusually large speed range with no bad habits, and excellent inverted performance.
Regards, Dave

[CalmAir]

Hi Dave,

Oh what the heck, lets pry open the can a little bit if you don't mind. I know what the theory says, but I was wondering what results you have seen on models with the following, still staying with the "flat bottomed" type airfoil.

The difference in a sharp leading edge compared to a blunt, more rounded leading edge?

The difference in a thicker versus thinner airfoil. Say 9% compared to 12%? Or is there much difference at all with our slower flying models?

I know that you could write about this for hours, and I don't mean for you to have to do that. However, I would like to know if you have found these differences to mimic the larger aircraft or what theory would suggest? Relative to models of course.

Regards,
Dale

[Don Cooper]

Dave,

Do you know anything about the 'WASP' airfoil listed in Profili? According to the Profili curves it has CL vs alpha characteristics similar to the Clark Y, but lower drag up to a CL of about 1.0. (I used a RN of 80,000 in my comparisons).

[Don Cooper]

CalmAir,

Profili will let you make those comparisons. I can't vouch for the accuracy, but the trends seem to be what I would expect.

[Dave Robelen]

Hello Dale,
To get down to cases, if I were going to build still another wing for my GW Sport, it would be a "french curve" faired section, about 10% thick, with the maximum camber at about 35% back from the LE, and a fairly small LE radius. my limited experience has shown that thicker airfoils, especially those with a large nose radius, are pretty draggy for the lift produced. The problem is two fold. One, the light small models do not do a good job of showing subtle differences, And relatively small changes in the model design and setup can overpower any effect from the airfoil.
Don, I am not sure which Profili you have, but mine is the older freeware version that only shows the profile. It does not give any data relating to the sections. I too have been intrigued wih the Wasp airfoil. It looks like a section that would be useful on RCHLG models, or the light electrics. So many fun things to try, so few hours, sigh---
Cheers, Dave

[CalmAir]

Okay Dave,

I understand what you mean, with so many variables and no way to gather the data other than through ones senses, it’s sometimes difficult to know if subtle changes make a difference or not. I do appreciate your insight.

On my RV4 I used a 16% thick NACA 230 and I believe it is too thick for a model this size. At cruise it is very quick, but at various angles of attack the wing seems to have a lot of drag. More than one would expect from the norm. It also has a blunt edge. If I was using a larger motor it probably would go unnoticed, The L/D ratio of my 1 ounce heavier GW Sport is much better.

Thanks again for your time.

Don,

I do have the registered version of Profili. I have had it about 2 months and have only made outlines with it. I need to learn how to use the other features in the program.

Thanks.

Dale

[Dave Robelen]

Hello Dale,
Some follow up. Another reason it becomes tough to compare our model's performance to scientific charts is the structure. assuming a framed wing with spars, a slice through the chord would reveal some very strange airfoils indeed. The data presented in the wind tunnel results as well as the computational work assumes a "pure" airfoil, something we rarely see. Also, our models fly through a range of Rn, your RV-4 probably has double the Rn at cruise than when at minimum speed. The 230 series of airfoils are well known for poor low Rn performance. A "laminar bubble" forms on the upper surface and really cranks up the drag. This of course is not the case in the world of full scale.
Something that is often overlooked is the pitching moment. The 230 series has essentially a zero moment similar to a symetrical section, while the flat bottom "foils" generally have a fairly strong nosedown force. With undercamber this gets even more pronounced. This is the main reason the CG needs to move aft as the airfoil gains camber, otherwise the model will show strong diving tendencies. Such fun.
Cheers, Dave

[CalmAir]

Such fun is right!

Using the GW Sport again, for an example, I'm assuming the center of lift would be slightly aft of the CG. This would seem to give a force that would want to drive the leading edge down. Why is it then on aircraft such as the sport that the centerline of the prop shaft is angled down a couple of degrees? Seems like that would add to this effect. Same on a delta wing with a pusher setup. The prop shaft is once again angled downward which would seem to drive the leading edge down. What am I missing?

Dale

[Dave Robelen]

Hi Dale,
Boy, you want get into deep stuff! Ok, first, the center of lift does not stay put, but roams around with the variations of AOA. Basically, this has nothing to do with the thrustline setting. In the case of the GW Sport, the downflow aft of the wing bathes the horizontal tail with air at an angle. Assuming the usual incidence angles and CG, this tends to give a noseup force. This is needed for a stable model, so that when speed builds the nose will try to rise. A little downthrust is very useful in controlling this effect.
The delta machines have some up trim rigged into the elevons for the same stability reason. Therefore, dowthrust is useful again. If you move to the neutrally stable aerobatic machines, all of this changes, and thrust offsets are a nuisance. The model should be rigged to maintain whatever angle the pilot puts it in.
Regards, Dave

[CalmAir]

Alright Dave,

Yes, I am analyzing this stuff too much. You just managed to clear up a lot of things for me in just a few paragraphs. I should have asked some of these questions before I drew up the RV-4.

Thanks,
Dale