Holy CG Batman! He dared to post about the "Dive Test."

[Steve P]

Ha! I realize this is asking for trouble with this post, but I'm not smart enough to live a quiet life.

I was on another forum recently correcting advice that the "test" is fool proof and lead in the nose makes the nose come down! I know I can't fix stupid, but I can share good thoughtful information. While most all of what I'm sharing below is for competition thermal duration sailplanes, it's guidance that I've lived with and has made me a much better pilot. While we won't be tweaking nose weight in 1/8th ounce increments, if you are willing to read on you'll find it was well worth your time.

This first piece below from Ben Clerx I've held saved in my computer and cannot remember the original source. The second by Brain Agnew was published in RC Soaring Digest. Bold emphasis is mine.

YMMV!

Center of Gravity and Dive Testing
by Ben Clerx

Much confusion exists over what the “dive test” is and what it tells you. It is not a method to trim your plane for max performance. What it does tell you is the decalage angle (the angle between your wing and horizontal stab) and the resulting longitudinal (pitch) stability of your plane. From this you can infer your plane’s relative CG position, which is the piece of information we are looking for. Notice I said that the “dive test” tells you (indirectly) where the relative CG position is, not where to put it. If you want to know where to put it for max performance, read an excellent article by Brian Agnew.

To perform the test, start from launch height, dive, release the stick and watch the flight path of the plane. If it pulls out of the dive quickly, your plane has a relatively large decalage angle, has a forward CG, is pitch stable and will want to fly at one airspeed – the one it was trimmed for in level flight with the stab trim level. This setting is good for free flight gliders and student R/C pilots.

The noseweight has the same effect at all airspeeds. The large decalage angle (up elevator) has a tail lowering force that increases with increasing airspeed and hence the rapid dive recovery. Likewise, if you get too slow, the heavy nose (forward CG) and lack of a tail-down force will lower the plane’s nose to increase the airspeed to its “trimmed” value. If your plane makes a good gradual pull-out, you are somewhere between very stable and neutrally stable. This is the region I prefer.

If your plane remains in a 45 degree dive (do remember to pull out prior to reaching ground zero), it is neutrally stable. This is a good position for slope racing and F3B speed runs, because the plane goes where it is pointed instead of ballooning every time the stick is released or the plane is rolled out of the pylon turn. The pilot, however, must be proficient at “pointing” the plane. This means you’ll have to “fly” the plane and constantly change or adjust your pitch to maintain the proper airspeed. You’ll need a good view of the plane to fly it, and as such, this is probably not a good setting if you like to fly two miles downwind and have only average eyesight. I like to trim to this region then add a half ounce of noseweight and a clevis-turn of up stabilator.

If your plane tucks under by itself in the dive or keeps nosing up when you pull out of the dive, the plane has negative stability (or divergent stability). Whatever the airspeed trend is, it will tend to accelerate that trend. This yields a very maneuverable plane, but requires a fly-by-wire computer (like the F-16) to stop the divergent trends. Nobody wants to fly in this region, but if you like to fly with an aft CG and the plane doesn’t want to trim out and “groove,” you’re probably slightly into this region.

Again, the dive test tells you about the decalage, stability and CG position. It doesn’t tell you where it should be for max performance

[Steve P]

Trimming Your Sailplane for Optimum Performance...
by Brian Agnew
Published in R/C Soaring Digest, October, 1997

Well, for the fifth time in as many months, I've just finished reading an article in RCSD on how to properly trim a sailplane. All of the articles are based on the "Dive Test" and all were too technical and/or too confusing for the average pilot. I've been flying R/C sailplanes for about 18 years, and what success I have had is strongly related to what I have learned about properly setting up a sailplane. If you decide to finish reading this article, you may learn what took me almost 13 years to figure out.

My father taught me to fly when I was ten. We both loved sailplanes and regularly attended contests. We both became accomplished pilots - doing well, hitting our landings, but we were inconsistent in our flight times. When it came to setting up our sailplanes and reading air, we were the blind leading the blind. For 13 years we balanced our sailplanes per the manufacturer's specifications (usually, the middle of the spar), and flew with the assurance that the manufacturers know what they are talking about or they wouldn't be manufacturers, right?

In the summer of 1988, while preparing for the AMA Nationals, I built an LJMP Meteor and an Airtronics Sagitta 600. Both planes had the Eppler 205 and both were well-suited for the thermal duration. At this time, I ran across an article written by Larry Jolly on "How to Fly the Eppler 205." Wow! Here is a world-class pilot telling me exactly how to set up and fly my sailplanes. According to Larry, the magic point on the 205 is 38% of the chord. Talk about eliminating the guess work! I immediately got out my planes and found out that I was flying them near 35%, a 3% difference. No big deal, right? I moved them both back to 38% and headed for the field.

I cannot express the difference this made. It was like I had been hitting tennis balls on the edge of my racquet all my life only to find out the racquet actually has a "sweet spot." My first impression was that all of a sudden there was an abundance of good air. The sailplanes no longer plowed through the air, but were actually "light on their feet," reacting to the subtlest of movements of the air and controls. My planes were jumping in light lift the way they previously did when I happened into a boomer.

Now, I knew how to fly the Eppler 205. Now all I had to do was convince Larry to write an article for every other section I might be flying. Either that or figure out a way to be sure that I know when I have found the "sweet spot" for a particular section. Before I tell you how to go about doing this, let me say as little as I can about the "Dive Test." I know the theory behind the Dive Test, and I know a lot of guys swear by it, but I have to say I believe that, for our application, it is nearly worthless. I do not know of any serious competition pilots that rely on this test to find if their sailplane is neutrally stable. Never-the-less, I watch pilots use it and I read about it to the point of frustration. There is a very simple flaw in the dive test. This flaw is more apparent in sailplanes with fixed stabs than those that are full flying. The response of your sailplane to the Dive Test is going to be directly affected by your elevator trim setting at the start of the dive. The same sailplane trimmed for best L/D or minimum sink will respond much differently to this test regardless of the C.G. point. So as not to totally offend all of the die hard dive testers out there, let me just conclude by saying that the dive test is far too subjective and inconsistent to be used for our purposes here - finding the optimum location for both C.G. and elevator trim. Whew! Try the following, and I think you'll feel the same way.

Assuming that you've purchased your world-beater 1000 and have spent the last several months piecing it together so that it's ruler straight and beautifully finished, let's find its "sweet spot." Go ahead and balance it according to the manufacturer's specifications, as this is always a good place to start. However you balance it, make sure you remove nose weight 1/8th oz. at a time.

Charge your sailplane and get to bed early because we're getting up early enough to be out at the field 1/2 hour before dawn. If we're going to test our sailplane, we need the deadest air Mother Nature can provide. By the time you set up your winch and plane, there should be just enough light to launch. It is imperative to get consistent launches, but if you can't zoom consistently, don't, just let the line fall. Time every flight. Each flight should be as hand's off as possible and in straight lines to the limits of your vision. Go straight out and straight back 'til touchdown. Record your time. After each flight, change your elevator trim to maxing your time. Once the optimum elevator setting (longest flight) is found, remove 1/8th oz. of nose weight and start over, again. Every flight should be flown as close to minimum sink as possible. This is closer to a stall than you probably realize. It usually takes 3 - 4 flights to find the best elevator trim after removing weight. If the air is dead and you are launching consistently, your flights are going to get longer and longer as you remove weight from the nose and you are going to think, "There is no end to this process," until all of a sudden, your timer peak will start to suffer. This pattern will happen regardless of whether you are flying a Sink Buster 1500 or a Gentle Lady. What happens is simply that as the performance of your sailplane increases, your sailplane's stability decreases. This is the trade-off. You don't get something for nothing as they say. You will notice that as you remove weight from the nose, the performance (dead air times) increases, but at the same time you are having to put in more control input to keep the sailplane flying straight and at minimum sink. Eventually, the airplane requires so much input that the drag from the constantly moving control surfaces brings your Thermal Wonder 1500 down to Earth sooner. Put weight back into the nose until you reach your maximum dead air flight time and call it good. You will never have to wonder about your C.G., again, only your elevator trim.

Incidentally, I did this test on my Meteor and Sagitta 600 and, in fact, found that Larry was right. The optimum C.G. point for the E205 is 38%. How accurate and reliable is this test? Let me put it this way. I did this test several years ago on my Phoenix unlimited ship and on my Mariah 2-meter separately (both use the S4061). By the time I was done, both planes were balanced at not 42%, not 41%, but at 41.5%. That sold me.

Now, regardless of the airplane or section you are flying, you will be able to set it up for optimum performance. What this trial and error approach does not teach you is how to read air. I'll give you another pointer. If you want to learn how to truly be able to read air, stop flying at cloud-base. Buy yourself a good hand-launch sailplane, set it up for optimum performance and let the learning begin.

While I have probably oversimplified all of this, I should add a couple of points. As you move the C.G. back, you should move your tow-hook with it. I tend to put my tow-hook 1/16" forward of the C.G. This is conservative, but I'd rather give up a couple of feet on launch than pop-off.

The last point I would like to make is that regardless of how good your sailplane is or how well set up, the only things that will put you in the winner's circle is your skill. How far back you will be able to move the C.G. when testing your planes depends on your ability to keep the plane stable in its increasingly unstable state. 10 people at different skill levels will come up with different optimum C.G. locations for the same plane. This is the great part about testing your sailplane. This way, it allows you to find the optimum balance point no matter your skill level.

One final note. In competition, I fly a minimum sink the majority of the time. It is only when I know where lift is or when I am in sink that I fly at the best L/D. I always fly assuming I will not find lift and savor every bit of altitude - every point per second.

I am sure a lot of pilots will disagree with my opinion of the "Dive Test," but all I can say is that this system has made all the difference in the world in my contest flying. I wish that I had read an article like this when I was 10 and my father and I were just starting out. I hope it helps those pilots out there looking for the edge to get them in the winner's circle.

Good Luck!!

[pete914]

Good post thanks Steve. I could do a lot better with my fine tuning...

[Steve P]

I'm with you. I've gotten lazy with my scale ships.

[lenb]

Once again, excellent information from a seasoned pro and multiple time completion sailplane champion. Thanks Steve.

[stevekremer]

I agree, Steve is champ at completing sailplanes. They are also quite beautiful when completed.

Steve K

[Stew2]

I don't give two poops to Pinola about what people think about the dive test, meaning I don't care to enter a debate about something I don't have an opinion on and certainly don't know any answers about, I'm just scared poopless of flying a tail heavy airplane! I know that I am outstanding at building or at least balancing airplanes nose heavy and I am absolutely inexperienced in flying tail heavy airplanes. Has anybody gone through the testing outlined above with a scale moldy? I would love to, I just need to get over my chicken-ness to try. Is the tipping point a cliff? I also wonder what weight increment is reasonable. 1/8 of an ounce at a time in a 32 lbs. glider has to be different and probably drastically different than in a TD ship. Someday, maybe someday, maybe a Lancaster day, Steve and I could edge each other on to trying to find a sweet spot for each of our gliders. But before we go there, lets talk about what NOT to do with a tail heavy airplane or what trouble is easiest to get into with a tail heavy airplane? Will the stalls be harder to get out of, requiring more altitude to recover in? Will the airplane tip stall easier? I guess I better just get off tow and slow down and stall the airplane and see if it is that different? Do the minimum sink flying to "the same" altitude each time and then get in the pattern, keep the nose down and land the thing.. Is there a slightly modified testing that will work well for scalies and though not wring out every last ounce of performance, but at least get us "performing well".... In all honesty, I think the biggest advantage of tuning our scale stuff will be gained in the lower landing speeds. I suspect that has to come at the price of knowing the stall speed better and NOT going below it in the pattern. I don't expect scale stuff to fly like TD stuff nor do we fly them in TD contests or anything like them, but I sure think we can gain from tuning and look forward to trying. I'll start with the Ventus....

[DanR]

Scott, I agree. I've felt that my ASW-20 is nose heavy but I haven't had the nerve to move the CG aft! At the Sled Works aerotow last month, Mark suggested I do the dive test. The plane pulled out of the dive pretty quickly. He explained to me that was nose heavy. He suggested removing the seat pan that has some weight in it. I wouldn't do it.

The plane hasn't been assembled since leaving MN and won't be until next week in Winamac. Hopefully I will get up the nerve to move the CG back a little (maybe to the aft end of the suggested range) and see what happens. Tim are you going to Winamac next week? Will you be able to bring along your CG balance machine?

Will the plane thermal better / easier with not being so nose heavy?

Dan

[Steve P]

Will the plane thermal better / easier with not being so nose heavy?

Essentially, yes. It will indicate lift better (you'll see the tail rise in lift, it will float more and feel lighter on this sticks in good air) rather than the plane just plowing through the air. Hey, nose heavy certainly works as Scot pointed out.

I didn't get to trim my new 7M Nimbus 4D at Cumberland, so first flight at Lancaster will be as is, next I plan to pull the first of two 3 ounce chunks I have in with velcro next to the main weight. And if that goes well, take out the next 3 ouncer and she how it behaves. I knowingly maidened it at least 5-6 ounces too nose heavy.

[TomTerrific?]

And if your ever at the controls of a hugly unstable tail heavy sailplane... wondering in microseconds how am i ever going to get this bird down safely... DON’T do as i did once and throw the flaps down thinking that’l stabilize a bad situation cause it won’t... it’ll actually increase instability (my ventus quickly spiraled into the corn that day).

Instead- remember, as Master Lens says- quickly apply the SPOILERS and vary them to help control your landing glide path.

[lenb]

Scott, I agree. I've felt that my ASW-20 is nose heavy but I haven't had the nerve to move the CG aft! At the Sled Works aerotow last month, Mark suggested I do the dive test. The plane pulled out of the dive pretty quickly. He explained to me that was nose heavy. He suggested removing the seat pan that has some weight in it. I wouldn't do it.

The plane hasn't been assembled since leaving MN and won't be until next week in Winamac. Hopefully I will get up the nerve to move the CG back a little (maybe to the aft end of the suggested range) and see what happens. Tim are you going to Winamac next week? Will you be able to bring along your CG balance machine?

Will the plane thermal better / easier with not being so nose heavy?

Dan

Dan,
Don't be afraid to take some weight out of the nose, The plane will not FALL out of the sky unless you pull out 4 pounds and try to snap roll the thing. Take out a couple ounces and fly the plane. Of it still feels nose heavy take out another one or two. You'll get plenty of warning when it starts getting tail heavy.