A question about Aquila XL vs. Grande airfoil

[ferincr]

Hi, here comes another one.
I have a semi kit of an Aquila XL, and I don't know if you know the plane but it has a flat bottom airfoil.
Now, does that makes de plane :

A- Easier to fly?
B- poor performance against wind?
C- slower?
D-not good for windy days?
E- All of the above?
My question is because I'm also builing the A grande ( which has a little round on the bottom of the Dtube) and I was thinking which one should I use to start flying until I feel more confident.
Thanks,
Fernando

[ejett]

Fernando:

I think the XL uses the same airfoil as the standard class Aquila. It doesn't have any Phillips entry so it won't penetrate as well as an airfoil like the Aquila Grande.

I think the best answer is "E - All of the above", but I wouldn't think that the AG is hard to fly. I have only flown mine one short flight, but I did not have any problems at all after I got the trim dialed in. I should be flying again this week if the weather gets a little better. Tomorrow is supposed to be pretty good, but I am on the road tomorrow. I'll give you an update after I fly it again.

Keep in mind that my flying field is about 10 acres of open space and I use a half size high start. My first and only flight was 4 minutes off of a 200 ft launch with me fiddling with the elevator trim the first part of the flight.

It flys much like the smaller Aquila but has a little sharper stall characteristic. This is an initial and subjective assessment however.

EJ

[ferincr]

Thanks for the info Ed, some how I new it would be you the one that could answer that question since you are kind of an Aquila guru. How many do you have?
Now, would it be worth to put Phillips entry on the XL???

[ejett]

I wouldn't say I'm a guru, but I am a fan. The AG is the reason I got into sailplanes. I saw one hanging in a hobby shop in Shreveport and knew I had to have one someday. After about 20 years, I finally got an Aquila. Now I have 3 standard class and the one AG.

EJ

[ejett]

Fernando:

Regarding putting Phillips entry into the Aquila XL, aerodynamically it would be beneficial, but it could be a hit or miss proposition. When Skip Miller modified the Aquila airfoil by putting the bottom sheeting on the original Aquila airfoil, it did help penetration and he went on to win the world championship with it.

But lofting a new set of ribs for the XL is not a trivial task. If I was going to do it, I'd just use the AG airfoil, but you could use any airfoil you wanted to.

Personally, if I had the ribs already made for the XL, I'd just build it that way for no other reason than to keep it original.

EJ

[Ollie]

http://www.gliders.dk/airfoil_history_class.htm
"A flat bottom airfoil was also the basis for the Aquilla Grande from Airtronics (hence the airfoil name), with which Skip Miller won the very first F3B World Championship in 1977. Skip did though modify the leading edge on his version; he raised the entry point and thereby lowered the effective camber. These flat-bottomed airfoils were the best we thought we had back then, but slowly as we got more experienced in thermal seeking we began thinking about aerodynamic efficiency and better airfoils. As Dave Thornburg put it in his construction article of Bird of Time in January issue of MAN 1979 “We have lift to burn” with which he meant that we had to use thinner airfoils with less camber to produce less lift to be able to fly faster and search larger areas for thermals to gain more flight time."

[ferincr]

Thanks EJ and Ollie for all the advice. It's amazing how adictive all this stuff is.
My story is similar to yours (EJ) I basically bumped into an AG in 1981 when I wanted to get started into RC planes.
I poped into a shop and I said to the sales person I want a power boat, a trainer rc plane an advanced plane and a glider , plus all the paraphernalia I need to put them on the air.
After a few crashes on the trainer plane I got bored to come back home and try to clean all that burned oil to see if I was able to re glue the pieces back together. The advanced plane I never flew it and the AG was my favorite (even though it lasted only a couple of flights).
And no surprise it lasted that short, at that time I didn't speak any English so I built it just guessing on the pieces and with my vast experience of building like two planes before that one.
24 years later all I have are the pieces of the AG and my old, (did I say old????) I ment OLD 6 channel futaba AM radio
Anyway, back to the point on the addiction I guess the best choice is to build as it is and then move to a full house more modern glider.

[Esprit2]

Fernando,

I agree with Ed that "all of the above" is probably the best choice.

In my opinion, the flat-bottom Aquila airfoil flies more like an Oly. Pretty gentle, not very fast and out of it's element in a stiff wind, but it will cling to the weakest thermal.

The Aquila Grande is still pretty easy to fly, but the plane likes to go faster and will force a low-time pilot to make decisions more quickly. But it should only take a few flights to get comfortable with it. Then the higher speed potential means you don't have to be so picky about the perfect day.

The only hang-up with the AG is that it wants to stall on tow if you place the tow hook as shown on the plans. That position is correct "IF" you balance the plane at the rear limit. If you use a more conservative forward CG for first flights and leave the tow hook back, then the AG will stall when it pitches up into a climb and wing-over into the ground. If you are ready for the stall, you can catch it with some forward stick pressure, but I've seen several first-flight AG's stall and go in hard.

Measuring from the aft CG point, whatever amount you move the CG forward by, move the tow hook forward by the same amount. Go for a few gentle first launches, get comfortable with it, and then back into more aggressive CG and tow hook locations in stages.

For the XL, I would think a flat bottom airfoil would be a competitive disadvantage... it's showing it's "vintage" design. The XL should fly well if all you want is a large sailplane, but a slow(er) plane will be chasing along at the back of the pack, so to speak, in a modern cross-country event.

Regards,
Tim

[ferincr]

Tim, (and verybody else)
thanks for your patience and the detailed explanation. It's amazingly helpful all the stuff I learn through this forum.
As for the planes I think I'll stick with what they come with.
I'm so eager to move forward that makes me wonder how can I improve on what I have, when in reality everything is like that for a reason. I need to learn to fly first but at the same time I want a high performance plane (what for?)
I'm not going to compete, I just want to hung around with the buzzards. So I think I have what I need for that. (Men if you see those suckers up there!!!!!), I bet the day my planes are ready to fly I wont be able to spot a single thermal!. But now.....
The only hang-up with the AG is that it wants to stall on tow if you place the tow hook as shown on the plans.
Been there done that, It's like deja vu, that was my second and last flight (1981).
I will be a lot more conservative with the GC and tow hook position this time.
Anyway it's good to know that you need to move both and not only the GC which makes a lot of sense (only when somebody else mention it to you), and normally that happen right after the crash.
Fer.

[Keith Johnson]

If you build with the stock flat bottom airfoil and you find that the wind is a little bothersome (it will not allow the sailplane to penetrate well) then try some ballast. Remember that you have quite a bit of wing area. So calculate the amount of wing loading you typically fly with, then calculate a new wingloading you would like to try. Then determine how much weight you need to insert into the sailplane to reach that new wing loading. Insert the weight in such a manner that the CG remains the same and so that the weight CANNOT shift while in flight (you can use velcro to hold the extra ballast in place or screw it in place). Remember that extra weight will cause the G force to increase faster if you yank back on the elevator and it will usually cause the sailplane to land at a faster speed (because of the extra MASS). So compensate for the extra weight by flying with a little more consideration for the extra mass. But sometimes the extra ballast can do wonders when the breeze picks up. You can make wing loading calculations for 5 oz/sq ft , 7 oz/sq ft , 9 oz/sq ft and sets of weights prepared ahead of time and clearly marked. What I shoot for is the amount of weight that allows the sailplane to cruise forward into the wind at XX mph. For me this usually means cruising forward at about 5 to 10 mph directly into the breeze. If the wind is 'gusty', it is not very relaxing to fly and land so there is a limit to appropriate choice of ballast to carry. Meaning, if it would take 11 oz/sq ft ballast to penetrate the wind as stated above I just prefer not fly. But sometimes adding the appropriate amount of ballast will really make a change in the way the sailplane cruises (it feels different) and you might prefer to fly with a little ballast even on the more calm days just to allow you to 'reach out' further per unit of time (cover more sky in XX minutes) in hopes of finding the 'mother of all thermals'. Of course more ballast requires additional pull on the winch line or highstart when launching. I have used the 'Miller Mod' on a 100'' Aquila (standard class) and preferred it because of the slight increase in wind penetration while at the same wing loading. I did NOT see a reduction in the ability to work weak thermals. You can always make the 'Miller Mod' after you have had experience with the standard airfoil shape.

[ejett]

Fernando:

Be sure to be careful with the towhook on first flights and start forward and move back slowly. I was lucky with mine because the previous owner had already determined the appropriate location for the towhook and the CG. Gave me a good starting point.

Weather is supposed to be beautiful for flying today so the AG will get another ride today. Probably several if I don't forget and leave the stabs at home like I did last time I went to the field. :o Temps in the low 60's, sunny and winds from the south at 5 mph. :D

EJ

[Esprit2]

Fernando:

Tow hook location is always relative to CG location, not to any fixed point on the wing or fuselage. If you move the CG, move the tow hook with it.

A tow hook position that is about 20 degrees forward of the CG is a safe, first-flight position. 15-20 degrees is a good generic range. Some planes/ airfoils will perform better with an even smaller angle (more aft tow hook location), but for first flights on an unknown plane, 20 degrees will get you in the air. It may be an unimpressive launch, but nothing will break and you have the option of adding some elevator on the way up.

I cut a guide out of stiff card stock. I started with a rectangle that was deeper than the fuselage. Then I cut one end at 20 degrees to the horizontal sides and the other end at 15 degrees. Balance the plane to put the CG where you want it. Then place the guide on the bottom side of the wing with the forward corner at the CG and the front edge angled down and forward. Were the forward edges passes the bottom of the fuselage, sight across to the center of the fuse... place the tow hook there.

The old Airtonics adjustable tow hook was great for making changes between flights... that's what I use. It beats the heck out of drilling a new hole for each location.

Regards,
Tim

[ferincr]

Thanks everyone for all the tips on the CG and towhook.
It seems like that was where I slacked the first time. I'll be a lot more conservative this time.

I have the original Airtronics tow hook but I was thinking in saving it for the Aquila XL.
I'm going to give another try to the servo actuated tow hook I put on the AG originally (sharing spoiler servo).
I never understood why it didn't released that day and I'm pretty sure it was my fault.
Anyway there will be many hand launched flights before I get into highstart again.
Thanks again to everyone for the useful info.
Fernando

[ejett]

Use the Airtronics towhook. Throw that releasable towhook away. There is a reason NOBODY uses them anymore. You would be better using the Airtronics towhook on the AG than the A-XL because the Airtronics towhook is not particularly strong and definitely not strong enough for the A-XL.

You would be better off with a standard heavy duty towhook made from a 6-32 or 8-32 or equivalent sized metric bolt on the A-XL.

See if you can't get an AVA style towhook from Barry Kennedy for the XL.


EJ

[ferincr]

So, that means that it might not been fault after all???
Airtronics tow hook it is.

[Keith Johnson]

'Esprit2 ' wrote 'Tow hook location is always relative to CG location, not to any fixed point on the wing or fuselage. If you move the CG, move the tow hook with it.'

I hear this general statement but I think it might be technically misleading so let me pose this discussion:

If I balance the plane such that the CG is directly under the thickest point of the wing (assuming the greatest lift is produced there) then place the towhook directly under this point, when I launch, the towhook will pull 'down' directly under the greatest point of lift. (In reality I would want the towhook slightly forward of that point but stay with me ... ) I would call this a 'stable' situation or 'reference condition'.

If I now place a nickle at the extreme back end of the fuselage, I have changed the CG but I have not changed the point on the wing where the lift is greatest and I have not changed the towhook position. If I launch I suspect that the towhook is still properly set because it offsets the 'overpowering' lift produced by the wing at that one point on the wing. The much higher forward speed of the plane and the enormous lift produced by the wing during launch probably are not much affected by the rearward CG location.

If, as is true when weight is moved to the rear of most sailplanes and then the sailplane is retrimmed for this change, some of the lift is now contributed by the horizontal stabilizer, the center of lift is 'shared' by the wing and the horizontal stabilizer and thus the combined center of lift has been moved back toward the tail. In this condition I can see that the towhook is now too far forward to offset the total lift produced by the combination of the wing AND the horizontal tail. So I can see that it would be proper to move the towhook back to the 'new' point where the lift is maximum. Now notice at this point I have not changed the CG, the nickle is still in the tail, but I HAVE changed the trim of the horizontal tail to produce more lift.

My point here is that I think the towhook is properly placed with reference to the combined lift point of all horizontal lifting forces combined, not the CG. The towhook is not controlling a point of gravity (or any change in gravity), but rather controlling the point of combined lift (or any change in the point of combined lift).

Another thing to think about is that gravity pulls straight down. If we angle the nose of our sailplane to 70 degrees (assume we winch up beginning at 70 degrees) the center of gravity of the plane is where? Has it not moved? Yet the center of combined lift and the towhook have NOT moved in their 'force vector' relationship, and that is what allows us to depend on abtaining a controlled launch. During the entire launch sequence (an arc) the relationship between the combined lift and the towhook do not change, yet the 'influence' or 'vector' of gravity is changing with relation to the lift vector and the winch pull vector.

So my conclusion is that to say we place the towhook with reference to the center of gravity I believe is not really accurate, although it is very convenient to look at it this way and to describe it as being correct (it is simple and pragmatic).

Any comments on this? I have thought about it off and on for many years but never found a discussion of it.

[ferincr]

ssswoooooshhh!!!
I definately felt the breeze!
I'll re read it later
I hope it is simpler than it sounds

[Keith Johnson]

'ferincr' the main ingredients to keep in mind when reading my post are:

1. point of lift from all combined surfaces
2. point at which the towhook/towring is located
3. center of gravity where (force toward the center of the Earth)

That will help understand my discussion. During a winch or highstart LAUNCH these positions deal with considerably different forces than when the plane is released from the winch line or highstart and floats free. At that point we only concern ourselves with the center of lift of all combined lifting surfaces and the center of gravity. The force of the towhook is gone, and typically the speed of the sailplane decreases (the energy in the winch or highstart is gone except the potential energy we aquired from it by increasing height and forward speed).

I would think that the towhook ultimately should be placed where it produces the perfect amount of vertical force (opposite the force of gravity) and at the same time the perfect amount of force opposite the pull of the launching system (away from the towhook). This should then take the plane up and at the same time forward. This balancing act is handled by the point of lift produced from the combined lifting surfaces.

I surely am not a physist so I am just trying to understand if there is something that is not clear to me. I have felt that I need to find the point of all lift then find the towhook position then change the cg (and retrim) in that order. The CG should obviously be 'pretty close' before experimenting, but the point is I think that during launch the center of lift and towhook position rule more than the CG, simply because of the excess forces generated during launch: launch line tension is huge, lift is huge but gravity stays the same. I would think that the two huge forces are what needs controlled during launch, thus the positional relationship of those two forces would be critical during launch, not so much CG.

[Bax]

Actually, you do want the hook placed in relation to the CG because the CG is the point the model rotates around. If the hook is too far forward of the CG, it will be difficult to pitch the model up enough during launch to get good height. If the hook's too far aft, it will tend to cause the model to pitch up too much, maybe even cause enough of a pitch-up that you can't correct with elevators before the model stalls.

Experience has shown this to be true. Change the CG, and you change the relationship of the forces, and you're be forced to move the hook. All the vectors involved, weight, towline pull, lift at the wing, tailplane downforce, airframe drag, etc, work in relation to the CG. If you move the CG, then you wind up changing the relationships.

[ejett]

Fernando:

Another 2 cents that I found laying about - No, I don't think the Airtronics towhook was at fault. The towhook placement was too far aft or you did not have enough tension in the towline. I have launched my AG off of my 3m highstart a few times and it seems fine. I'll measure it in a little bit and post the distance of the hook from the nose so you'll have a reference. You'll need 20lbs of tension in the line.

Another thing to do is to make sure you throw the plane forward with enough speed so that it is flying when it leaves your hand. What I mean is don't just let the plane go. For the first flights off the line, throw it level and let it rotate into the climb.

The releasable towhook will not prevent the plane from stalling on the line. It will just be something else to have a mechanical problem with. I would not use one myself. Of course, if it gives you a comfort factor then go ahead and use it.

I do have the Airtronics hook on both of my Aquilas and the AG, but in my opinion, they are not really strong enough. They will handle highstart launches and gentle winch launches. Might actually be a good thing, they might break before the wing. I hope I don't find out about that either way.

EJ

[ejett]

The CG on my AG is 12-1/2" from the nose of the plane and is about 5/8" ahead of the center of the wing rod. See the picture below. It is a conservative location, but safe. I have some work to do to my AG before I get to do any further flight testing, but the launches all went well.

EJ