CG How far back?

[Kaan]

Hi, hopefully within next week I'm expecting to finish a Shadow and fly it at weekend if weather permits. :D

In the form I have read many times about the CG of molded gliders. Some fly CG a bit foreward some say CG at very rear! I now it depends mainly the pilot, but I wonder how far back a CG can be as a %. For ex. is it possible to fly at %50 or more from LE?

Or if possible by tolerating it with down elevator, should a model have a minimum weight to fly efficiently? :confused:

Thanks..

Kaan

[rogerflies]

If the plane is tail-heavy, the stab is lifting. Increasing the speed causes it to lift more, the nose pitches down, and the plane goes faster. When you pull up, the plane slows down, the tail lifts less, and the nose pitches up some more. It's almost impossible to manually fly a tail-heavy plane smoothly, and it's certainly no fun. It can be done with a computer, though, like on airliners with relaxed static stability.

The plane only has to fly fast enough for the control surfaces to have the desired effect. It's better if the airspeed is correct for the airfoil. That's when you'll get the best performance, and the plane will be easiest to fly.

In short, put the CG within the specs.

I like my planes to be as light and as aerodynamically clean as possible. I've never had a problem going fast enough to get back upwind, so I don't use ballast on windy days like some people do. A little down elevator has always done the trick, and I still have my best climb rate when I find lift. LTFB :D

Roger

[Ollie]

See:
http://polecataero.com/articles/cg-location

The best CG position depends on your flying skills!
My advise is to position CG as far aft up to the edge of your smooth flying.

[Ollie]

See:
http://polecataero.com/articles/cg-location

The best CG position depends on your flying skills!
My advise is to position CG as far aft up to the edge of your smooth flying.

Try it. I think you will like it.

[schrederman]

Perhaps it is almost impossible for some people to fly with an aft CG. I always move mine as far aft as I possibly can. Mine often fail the "dive test" by tucking with increased speed... yet I fly pretty smoothly. I find lift when others don't, and I have less trouble flying upwind than many. My CG is usually back in the 42 to 45% range, though that varies with airfoil selection, and planform. My strategy is to remove 1/4 oz at a time until I can't fly it smoothly. Then I add 1/2 oz back. I've been accused of haveing my models balanced at 30% chord... of the stab! Your method and CG location will be one of personal preference. Do try moving back slowly. You will be rewarded with a responsive, cleaner model that will require less ballast.

Just my take on it...

Jack Womack

[OVSS Boss]

Jack has a very proven approach to getting to "your" CG, it is a very personalized thing and you just mess with it up to a no fun limit is reached.

Now, ballast. To me, ballast is like learning to fish for bass. There are crank baits, soft baits, blades, etc., and if you want to be good at it you have to learn to fish them all where appropriate. Ballast is the same. If you want to fly contests particularly, you need to learn to have ballast in the arsenal. As Roger points out, if your only concern is coming back, you may not really need to play with it, but on the other hand, if you are having to head upwind or cross a steady strong wind to get to air, it may be required. I am sure no master of ballast, but that is why I am starting to get involved in F3B, you play with lots of ballast there.

So, dependent on what you want to play, you may want to make a set up and see what it can do.

Marc

[ChuckA]

Best CG location is a very personal thing and depends on your personal preference, skill level, AND PHYSICAL HANDICAPS. I have had vision problems for the last 10 years and find that I need more stability than most. I always start with the CG at 30% MAC (that's mean aerodynamic chord, not root chord) and adjust as necessary to give a positive pullout from a dive test started with slow flight to a stall. I then fine tune the cg location by adjusting the CG to give best pitch response in a series of S turns. My goal is to have a model that will do 720 degrees of turn without rolling out of a thermal turn or rolling off into a spiral dive. That way, if I loose sight of the model in a downwind thermal, I can just wait until it comes back into view. Sometimes I succeed but some models cannot be adjusted to give 720 degrees of turn hands off. I usually need at least 50 flights before I am satisfied with the trim.

[POF]

I like my planes to be as light and as aerodynamically clean as possible. I've never had a problem going fast enough to get back upwind, so I don't use ballast on windy days like some people do. A little down elevator has always done the trick, and I still have my best climb rate when I find lift. LTFB :D

Roger

Just to clear a possible misunderstanding (sorry - a bit off-topic).
The reason to use ballast is not only to penetrate the wind - the reason is to penetrate with the best possible performance. Have a look here for a good explanation: http://home.att.net/~jdburch/polar.htm

[Tuomo]

The best CG position depends on your flying skills!


Many think that flying with aft CG is an indication of a good flyer. It is not.

Many also believe that aft CG increses flight performance. It may do so but the difference is, at best, marginal.

Trim your plane so that you are comfortable with it. One missed thermal turn negates all the spreculative performance gain of aft CG.

I trim my planes so that in thermal mode they pull out of dive. In speed mode they an become almost neutral, but never unstable.

To me it also important that my glider reacts positively to each elevator trim change. It is very difficult to penetrate home from distant downwind thermal with an unstable or neutrally stable plane.

I am not speaking for extremely forward CG. Just a good compromise. Usually the plane with less pilot work load does best in competitions :)

PS No use being so negative about balast. Some planes need it more, some less, but in any case it is needed when wind picks up.

[Chipwillis]

For what its worth, I have a V tail Shadow and I had my CG initially at I think 110mm.

I like my CG aft for this reason- Lift Detection. The further aft, the easier upset the plane becomes and for me, that works.

However, I ended up adding another Oz to the nose because it was a tad too "happy" and the Shadow will Snap/Spin easily. I am sure the X tail version is probably alittle more tolerant but I have reached my compromise.

It is, pilot prefrence, not a fixed location to be used by all.

If you are experienced and like flying all the time, then you can go aft more probably, but the Shadow has great legs and finds itself far away horizontally quickly, and the added stability is a plus. Something to think about.

chip

[Kaan]

Hey, thank you very much for the replies... it seems that I'm on the correct path :D

I also measured cg of my current models Eraser extreme and 2 visions.. All three models are approximately set up at %43, as understood from the replies its a bit back :)

The main thing is it takes quite a lot of time to set it up when we concern all weather conditions!

I will start building shadow tonight, and hopefully it will require less time to set up CG :D

I think I can post the building notes and some pics as I proceed...

Regards,

Kaan

[Tuomo]

I like my CG aft for this reason- Lift Detection. The further aft, the easier upset the plane becomes and for me, that works.

However, I ended up adding another Oz to the nose because it was a tad too "happy" and the Shadow will Snap/Spin easily. I am sure the X tail version is probably alittle more tolerant but I have reached my compromise.


The theory between lift detection is complex and works only in calm conditions. In normal conditions one needs to fly the plane more or less all the time. Little more stability does not make it track like autopilot on, it just reduces pilot workload.

This is why I doubt how much the lift detection suffers if you move CG little forward. Remeber Shadow is not a DLG, you cannot turn it on a every small turbulence.

Anyway, if the stabiolity looks ok in dive test, maybe you have too much elevator movement or too little expo.

[Chipwillis]

The theory between lift detection is complex and works only in calm conditions. In normal conditions one needs to fly the plane more or less all the time. Little more stability does not make it track like autopilot on, it just reduces pilot workload.

This is why I doubt how much the lift detection suffers if you move CG little forward. Remeber Shadow is not a DLG, you cannot turn it on a every small turbulence.

Anyway, if the stabiolity looks ok in dive test, maybe you have too much elevator movement or too little expo.

That is why I said it was pilot prefrence Tuomo. Adding nose weight does make stability better, and pilot workload less.

It should also be said that the pilot should know when to change his own preferred CG depending on the conditions.

We are talking about the same thing.

[Tuomo]

Yes we are talking about the same thing, and I guess the difference of our approaches is not that large. However, I am the kind of guy allways carries some pieces of lead to the field.

In competitions one very often desperately (and suddenly!) wants plane to have little softer handling. It was ok in home field on that calm Sunday. Marks small batches of lift nicely :cool: ... But in a competition, hands shaking, narrow thermal, looking into sun, hard turbulence, turning tight in marginal lift low down over those hostile trees -- that is the real acid test of the trim :confused:

[Mark Miller]

One thing to remember is not to look solely at the placement of the CG between different designs as an indication to if the CG is fore or aft. Just because one design has the CG more to the rear is not an indication of it's performance or flying traits. Remember that you are measuring the distance at the root. Depending on the wing planform and the LE or TE sweep, forward or back will depend where the CG should be at the root.

On my Evolution XL I have the CG currently at 105mm from the leading edge. I have OVSS Boss's Albatross with about the same root chord and the CG is 77mm from the LE. This is because the Albatross has a distince forward sweep to the wing. Likewise measuring the CG to the percentage of chord at the root gives no indication of if the CG it fore or aft.

Mark Miller
Isthmus Models

[ThermalBuster]

Mark is absolutely right here. Measuring the cg as a percentage of the root chord tells you nothing about 'fore' or 'aft' location. If you have a straight LE with a swept forward TE putting the cg at 43% of your measured root chord is disaster. 35% of the mean aerodynamic chord is probably more like 25% of the root chord!

From the posts here describing handling it is very difficult to tell if the given pilot has a forward or aft cg. The other question is 'forward or aft of what?'. I can take a plane with a cg at any reasonably neutral (whatever that is!) location and make it act tail heavy or nose heavy. In fact during a single TD contest round my plane will go from decidedly nose heavy to tail dragging tail heavy and I don't have moveable ballast.

The point here is that you should play with cg gently and carefully to find out where the plane handles nicely for you. Then leave it there and practice flying!

[Mark Miller]

I also hear questions why manufacturers do not give exact CG measurements. As mentioned earlier by many people smarter than me that it is all a personal and elative thing. As a manufacturer I like to give a more forward safe CG and let the flyer adjust to his liking and conditions.

One story I'll pass along to get the gray matter flowing. I built a CARA for 2 meter at the NATS last year. I originally put the CG where Ed Whyte said to put it and I felt that it was way to far forward after test flying it. I kept taking weight out of the nose for a few weeks. At the NATS I was still taking weight out of the nose during 2 meter day one. All in all I probably took out 2 ounces. It still flew like it was nose heavy ie. when I tried to speed up or penetrate the nose would baloon up and so you were always having to put in more down the faster you went. Back at the hotel after a few gin and tonics I put the Cara together and looked at the decalage between the wing and stab. It was way wacky. I could not measure it but there was a lot of down trim in to keep the nose level. I figured that this was not a good thing and so I added an ounce back. I reset the trim and went to get another gin and tonic. The next day i decidedto keep adding nose weight until I was about a zero decalage angle between the wing and stab. I ended up adding about an ounce and a half back of the 2 taken out. It still flew nose heavy but still flew well.

The question is, what was going on and why did it still feel nose heavy and fly well with a 2 ounce difference? I have no idea

Where is the CG? I have no idea.

Sould we shoot for a reasonable decalage angle instead of the holy grail of CG nirvana? I have no idea.

Am I going to change it next time I fly it? No. i lawn darted the thing in November after not turning on the receiver. To bad. It was one of my most comfortable planes to fly. Not to worry. I have more Cara Kits.

Mark Miller
Isthmus Models

[schrederman]

OK, I'll bite...

My models are always set up to start with with 1 to 1.5 degrees of positive decalage. I measure the stab chord and figure what the difference is between the highth of the trailing edge over the leading edge. It isn't much. I set the model up on a bench, and level the wing. Meaning the dead center of the leading edge radius is even with the dead center of the trailing edge. I then adjust the stabs so that they have 1 to 1.5 degrees of positive decalage... depending on the airfoil of the wing. I am very meticulous about this so I am starting my trimming venture with a known value. This doesn't get changed, usually. I then go about figuring MAC, and again, depending on wing airfoil, I set my cg between 33% and 38% of MAC. I usually have to move it back for my flying style. My CG is usually around 40% of MAC. I have used this method for about 40 years, as taught to me by an old free-flighter. It doesn't fail me, or at least it hasn't yet...

Jack Womack

[solo6796]

This man do fly his models purty good... I seen him in person... I'll never forget the time he let me fly the Grand Espirit.

The guys with the Hawks tell me I turn kinda like Jack, because my models tend to make some really small circles, if I want... Sometimes just rotating around the center of the plane. That's a rearward CG for ya.

The telling story is to check the stab alignment on landing after setting the trim for your best glidepath. If it's positive, you may need a lettle less noseweight. If negative, drop a little in. Play with it from there.

Jack and Don pretty much taught me how to fly.

AJ

[Phil Barnes]

I'll start by defining a term that I will call "reference speed". This is a particular speed that you choose to trim your sailplane to. This means that you have set the elevator trim such that the sailplane will fly at this reference speed with your hands off the transmitter stick.

Assuming that the sailplane is always trimmed to fly at this reference speed then there is a single decalage that goes with a particular CG. Or you could also say that there is a single CG that goes with each decalage setting. If you change the CG then you will need a new decalage setting to get the model to fly at your reference speed. Or if you change your decalage then you will need a new CG to get the model to fly at the same reference speed.

To get the model to fly the way you like in terms of stability or sensitivity to pitch controls you can choose to either set the decalage first and then trim the model to fly at your chosen reference speed by adjusting the CG. This is how free flight guys typically trim models and this is the method that Jack was taught to use by his free flight buddy. In RC soaring we more commonly set the CG for flying qualities and then we set the decalage to get the flying speed we want by moving the elevator control. This makes sense since the stabilizer is typically fixed in place on a free flight model when the model is built, but it is easy to change the CG while trimming the model. On an RC model the elevator is movable and so the decalage can be freely changed while trimming the model so it is easier to set the CG and then retrim for reference speed with the elevator trim.

If after setting the CG and decalage on our RC sailplanes we should see that the model flies at the chosen reference speed with a lot of unwanted deflection of the elevator relative to the fixed stabilizer then we may choose to reset the fixed stabilizer at a new angle (reset the incidence? I ask the aero terminology gurus) on the fuselage to make that deflection go away.

I hope that I have used the term decalage in the correct way in this post and I especially hope that the aero gurus out there will correct me (quickly ;) ) if I have not.

Edit: I was using the term decalage incorrectly in this post. See this post (http://www.rcgroups.com/forums/showthread.php?p=6956504#post6956504) for the correct usage of the term.

[CloudyIFR]

If you download Sailplane Calc (see link below) it'll calculate a starting balance location for your model. The airfoil makes very little difference on the balance location. The program will calculate the neutral point of your model and a static margin can also be specified. Then fly the model and perform the dive test to fine tune to your liking. It DOES take into account the tail moment and tail sizes whether Cruciform Tail or V-tail. The formulas are from Martin Simons book on Model Aircraft Aerodynamics and will soon be posted on the SoarTech website as well.

One day I'll take the time to write an article for RCSD but for now the instructions on the program should be sufficient.

Curtis
Helena, Montana
suterc@msn.com

http://h1.ripway.com/cloudyifr/files.htm

[schrederman]

Well said, Phil.

Jack Womack

[Phil Barnes]

The question is, what was going on and why did it still feel nose heavy and fly well with a 2 ounce difference? I have no ideaBecause it was still nose heavy. You should have kept removing nose weight and retrimming the elevator for your reference speed until the model flew the way you liked it.

Sould we shoot for a reasonable decalage angle instead of the holy grail of CG nirvana? A real aero guy could calculate this and even calculate the CG that goes along with that decalage angle. Just look at the Supra plans, you can bet that the CG and decalage shown on those plans were calculated long before the first Supra was built. In the absence of the ability to calculate the desired CG and decalage the easiest thing to do with an RC model is to keep moving the CG back from a known safe starting point, retrim for reference speed after each CG change and keep doing that until you get the desired flying qualities. If you find a wacky elevator deflection when you are done with that process then make it go away by resetting the decalage.

[solo6796]

...What he said...

AJ

[Mark Miller]

Not being an aero guy but being a licensed airplane mechanic (A+P) I cringe at the use of decalage in this way even though it is commonly used. Decalage is traditionally used when setting up a biplane and is the difference in angle between the two wings. I was always taught to use the term angle incidence as the angle of difference between the wing and stab. It seems that times have changed and Decalage is the term commonly used now.

I appreciate all of your input on this question. it is just when you commonly see zero to maybe 2 degrees of incidence difference between the wing and stab I get a funny feeling when I am seeing maybe 10 or more degrees in the wrong direction from normal. It was about 10 degrees down to push the nose down for level flight with the CG so far back.

It was very windy on day one at the NATS last year. After I got things looking more normal on day two my flights got better although better conditions may have helped more.

Mark

Mark

[schrederman]

When I went through A&E school... in the late '60s, incidence was defined as an angle in reference to a datum. Decalage was defined as the difference in the angles between any 2 aerodynamic surfaces. Could be the wings of a biplane, or between the wing and the stab. I was told that a biplane had a decalage angle between the wings, and between each wing and the stab, ie. 3 decalage angles. Not saying that's correct, but it's what I was taught.

Nordics have lifting stabs and a very aft CG, in reference to the wing. In reference to the center of lift, they're not that far back. If your 2-meter had that much down in it, it had a lifting stab as well...

Jack Womack

[Phil Barnes]

The official Mark Drela aproved definition of decalage that I am attempting to use:
http://en.wikipedia.org/wiki/Decalage

Jack's definition sounds like a more general definition with Mark's definition above being a specific case.

By the way, you might find this interesting:
http://www.rcgroups.com/forums/showthread.php?t=645299

I'm waiting for a response but may post some more urgings and pleadings in that new thread if need be to get the desired information :)

[Mark Miller]

I think the guy that was teaching rigging in my school was a bit off the mark although I'm sure he thought there was no question. I agree that Decalage is the term.

Mark

[Tuomo]

...I then go about figuring MAC, and again, depending on wing airfoil, I set my cg between 33% and 38% of MAC. I usually have to move it back for my flying style. My CG is usually around 40% of MAC. I have used this method for about 40 years, as taught to me by an old free-flighter. It doesn't fail me, or at least it hasn't yet...


Most current F3J models are available with both v- and x-tail. V-tail versions need generally more forward CG than x-tail versions. For the same percieved stability (in dive test) the difference can be as much as 10mm.

I hope you are settled with the terms ;)

[gustabmo]

Most current F3J models are available with both v- and x-tail. V-tail versions need generally more forward CG than x-tail versions.My opinion on most V-tail F3Js out there if that the tail is extremely undersize and the control surfaces are minimal when compared to the X-tail versions.

Having a smaller tail volume coefficient and much less tail action (= very small moving surfaces) means you need a more stable setup, i.e. a forward CG.

For the same percieved stability (in dive test) the difference can be as much as 10mm.What I wrote above is not the same thing as this last affirmative... What I wrote is that the V-tails need a more stable setup, i.e. a setup that would pull out quicker of a dive test.


Really don´t know how to explain that I agree with the first part of your message but disagree with the last...

[Tuomo]

What I wrote is that the V-tails need a more stable setup, i.e. a setup that would pull out quicker of a dive test.


Some time ago I started another another thread about x and v-tails. Lets keep this thread separate from that.

But anyway, please elaborate what do you mean buy your comment? V-tails need to be more stable than x-tails to feel stable enough? Get what I mean?

[gustabmo]

But anyway, please elaborate what do you mean buy your comment? V-tails need to be more stable than x-tails to feel stable enough? Get what I mean?No, I didn´t get what you meant... please explain.

Explaining my phrase... it needs context to make sense: on "V-tails need a more stable setup", by V-tails I meant what is referred to on the previous paragraph: "most V-tail F3Js out there with extremely undersize tail sizes and minimal moving surfaces". These planes need a more stable setup.

[Tuomo]

Why do you think that "a v-tail model should pull out quicker out of dive"?

This is the key point. I do not understand why you say so. My experience is that a v-tail plane needs for forward CG to pull out of dive in a same way x-tail plane does.

With less tail volume (as is the case in many not so well designed v-tail planes) one needs more forward CG for same stability. Ok with this argument?

[SoCalGliderFlyr]

Simple: Move the balance back till the plane flies backward and then had a 1/2 ounce to the nose.

[gustabmo]

Why do you think that "a v-tail model should pull out quicker out of dive"?
Answer: because a glider with less tail volume coefficient and less tail authority needs more stability.


This conclusion I gave came from the following thoughts:

1) A model with a small tail volume coefficient and little tail control authority needs more stability than one with adequate tail volume and adequate tail authority.

2) Many current F3J V-tail models have too small tail volume coefficients and too little tail authority.

3) Many current F3J X-tail models have larger tail volume coefficient and tail authority then their V-tail siblings.


With less tail volume (as is the case in many not so well designed v-tail planes) one needs more forward CG for same stability. Ok with this argument?I understand what you said and I don´t know if that´s true... in my understanding that should not be true but I don´t have enough knowledge about all things involved and I may very well have gotten it wrong.

One test that should not be so hard to perform to check this is to set the CG of a X-tail plane to a certain stability margin... then change the elevator to a larger (or smaller) one, bring back the CG to the set point since it may have moved with the elevator change and check the stability with the new setup.

That should be relatively easy since many X-tail models have easily removable elevators.


Back to the point where we disagree:

We both agree that many current V-tail F3J models have less tail volume than their X-tail versions, and need a more forward CG than their X-tail versions.

Our disagreement is that:

I say the more forward CG is needed because the V-tail versions need more stability.

You say the more forward CG is needed to give the V-tail versions the same stability than the X-tail.


I don´t know who is correct but at least I think I finally understood the question...

[Tuomo]

Our disagreement is that:

I say the more forward CG is needed because the V-tail versions need more stability.

You say the more forward CG is needed to give the V-tail versions the same stability than the X-tail.


I think we need someone to help us with this :confused:

Personally I use dive test to determine the trim of my planes. I trim all planes (v-tail and x-tail, no difference between them) to pull slowly out of dive (in thermal mode). In speed mode I like them to be close to neutral stability.

[WimH]

I think we need someone to help us with this :confused:

I have an article from Reinhard Lahde or so about V-Tail sizing. He says that for the same "control power" , V-tails can be up to 15% smaller in size than calculated ( from properly sized other tail variants). This does however sacrifice some stability. I never had any problems with properly sized V-tails, but many of them seem to be on the smallish side....Don't know is that's the help you're looking for...

[BMatthews]

So far I have not seen any of you refer to the CG being related to the aircraft's neutral point and how this is determined by using the tail volume coefficient. Kaan, the nuetral point is where the pitch response would be dead neutral if the CG was placed at that point. That point is set by the relationship of wing to tail area and the leverage arm between the wing and tail. It all comes together and is known as the Tail Volume coefficient.

For various reasons most RC gliders end up with CG positions in the 30 to 40% of MAC range by using tail sizes and moment arms that result in the nuetral point being in that range. However it is possible to design with larger tails and/or longer moment arms and end up with perfectly correct CG locations in the back half of the wing and even at the trailing edge of the wing and the model will still fly in a stable manner. This is very common in the free flight world. Especially in some old timer and nostalgia designs.

It has been mentioned by various folks that there really are no specific planforms like conventional, tandem wing or canards. Rather it's really more of a sliding scale where the CG is located forward for models with large front wings and small stabilizers and how the CG location slides back as the stabilizer size grows. Eventually the CG moves to a point between the two surfaces as we reach a tandem wing configuration and as the rear surface becomes even larger from that point the CG finally ends up just in front of the stabilizer or even on it and we have a canard.

Mark Drela had a nice chart showing this but I can't seem to find it just now.

So the bottom line is stick with what the designer calls for or you can use the online calculator shown below to get a starting point. But bear in mind that the acceptable CG location is more of a range and the optimum location will be partly based on model flying efficiency and partly on your own comfort level.

For best efficiency the CG should be at or just barely in front of the neutral point. But most pilots find they need a little more stability or "pitch auto pilot" built in and choose to move the CG a few % more in front.

Here's a link to the calculator I mentioned.
http://www.geistware.com/rcmodeling/cg_super_calc.htm

[gustabmo]

1) A model with a small tail volume coefficient and little tail control authority needs more stability than one with adequate tail volume and adequate tail authority.This weekend I was talking to a friend about that and he disagrees, or at least questions this affirmative...

He says neutral stability planes need less tail force to make it yaw or pitch so it could be that an undersize tail would ask for a rearward CG. I say a forward CG would less often put a plane in situations where corrective tail action was needed... I don't know who's right.

Also, this doesn't address at all Tuomo's observation that in some cases the V-tail version of a plane need more forward CG to achieve the same pitch stability as the X-tail version of the same plane...

Misteries!!!
(that's what we call what we don't understand)

[BMatthews]

......He says neutral stability planes need less tail force to make it yaw or pitch so it could be that an undersize tail would ask for a rearward CG. I say a forward CG would less often put a plane in situations where corrective tail action was needed... I don't know who's right.....

I think you both have a grasp on many of the details but haven't quite put it all together for a whole picture.

Gustabmo, you're previous posts have parts where I feel that you don't understand how it all relates and then you'll have another section where it seems like you have a great grasp on it. I suspect symatics is getting in the way as much as anything.

V tails have been the subject of much discussion. The rules for sizing them are a bit vague. In fact the sizing of any tails in any format are vague as there is no one right answer. The fact is that the "best" tail volume for any design is actually more of a range of values and is probably accuratley describe by a bell curve. As long as you don't fall off the sides you're OK.

Because of the fact that there's some fudging and rules of thumb when going from orthoganal (meaning conventional low, mid or T tails) to V designs it's probably wise to say that the final neutral point and thus the true effective tail volume coefficient is best found by dive testing and using that to place the CG dead on the neutral point. WIth that you can then truly compare and come up with a true equation for comparing orthogonal tails and V tails.

You mention that the V tails need the higher stability that comes from a more forward CG. But the CG location itself does not provide stabiliy. The final "best" location for the CG is determined by the tail volume and it falls where it falls. A model with a small tail volume and resulting forward CG is no more or less stable than one running the same stability margin with a more rearward CG.

I think it was Don Stackhouse who was one of the first to show us in book and by web site that V tails cannot be sized from the projected area matching of ortho tails. This is where a fudge factor comes in that says V tails need to be a little bigger to accomplish the same effect as ortho tails.

[gustabmo]

I suspect symatics is getting in the way as much as anything.
Symatics ?!?!?!?

[massara]

the nuetral point is where the pitch response would be dead neutral if the CG was placed at that point. That point is set by the relationship of wing to tail area and the leverage arm between the wing and tail. It all comes together and is known as the Tail Volume coefficient.

Bruce, I think the pitch response is "dead neutral" when the cg is set at the neutral point of the wing alone.

With that setting, the stab isn't inducing any lift (AOA zero), so its area and volume coefficient doesn't matter. You can't say the vol. coef. has influence on the NP...

About the tail size and cg.

If a plane has the cg forward, it will need down lift by the stab. In this situation, you can use a big stab and a low AOA (of the stab) or a smaller stab with a higher AOA.

Now what will happen at the dive test ? Both stabs will get a higher AOA, but the bigger one will induce more force and the plane will return faster.

My conclusion: For a similar response from dive test, bigger stab -> more forward cg

Sergio

[BMatthews]

Symatics ?!?!?!?

My finger hiccupped.... :D OK symantics it is then.... :D

Sorry Massara but the neutral point is an all airplane thing. The wing itself does not have a neutral point. Or at least it is not of any concern unless you're making a plank style flying wing. But as soon as you add a stabilizer on a tail then you have to consider the overall plane's planform to be meaningfull in describing how it should or will fly.

There IS one thing that bothers me about classic tail volume coefficients and neutral point calculations. None of the equations I've ever seen takes the wing section's pitching moment into consideration. This is a big negative to my thinking.

I know from practical experience that a symetrical airfoil requires very little stabilizing influence from a tail. I've see control line combat models glide like a feather and retain excellent stability and control authourity with as little as a 5% wing area stabilator on a short boom. But I've also flown one of the Great Planes Pokeys that has a huge % stabilizer, albeit on a very short tail moment, and the model is HORRIBLY underdamped on the pitch.

Anyhow, I'm wandering.

As to your stab size example we are not really comparing apples to apples. In your example the models have the same CG location but the model with the larger stabilzier will have a more rearward neutral point location so the stability margin would be larger if you consider the CG's being at the same % of the wing chord. And with that being so the model with the larger stabilizer would pull out of the dive faster because it's margin of stability is higher. To be more fair you'd want to say that both models should have the CG the same % of distance in front of the neutral point in order to acheive the same value of pitch stability. As to which would pull out of the same angle of dive faster at that point I'm not sure but I suspect it would be very close.

[massara]

Bruce, thanks, you are correct. It was a terminology mistake. When I said "neutral point" or NP in my last message I was thinking about "Wing Center of Pressure" or CP.

[Don Stackhouse]

...Sorry Massara but the neutral point is an all airplane thing. The wing itself does not have a neutral point. Or at least it is not of any concern unless you're making a plank style flying wing...

One way to descrbe a plank-type flying wing is to think of the reflexed portion as a horizontal tail with the tail moment so short that the tail's leading edge is coincident with the wing's trailing edge, and the amount of reflex relative to the rest of the wing's airfoil equivalent to the decalage of a conventional wing/tail combination.

... But as soon as you add a stabilizer on a tail then you have to consider the overall plane's planform to be meaningfull in describing how it should or will fly.

Absolutely correct. BTW, Massara, a larger stab means the neutral point is further aft.

There IS one thing that bothers me about classic tail volume coefficients and neutral point calculations. None of the equations I've ever seen takes the wing section's pitching moment into consideration. This is a big negative to my thinking.

That's because pitching moment is a flight trim question, not a static or dynamic stability question. You decide what speed you want to fly, figure out all the nose-up and nose-down moments acting on the airplan at that speed, such as wing and tail incidences, wing and tail pitching moments, pitching moments generated by the fuselage, prop, etc., moments caused by thrust or drag acting above or below the C/G, moments caused by the C/G location, etc., and get them all to add up to exactly zero at the desired flying speed.

The distance from the C/G to the neutral point (typically referred to as "static margin" and expressed as a percent of the wing's Mean Aerodynamic Chord, or "MAC") determines the static pitch stability.

I know from practical experience that a symetrical airfoil requires very little stabilizing influence from a tail. I've see control line combat models glide like a feather and retain excellent stability and control authority with as little as a 5% wing area stabilator on a short boom...

Those terms indicate you're referring to the model's static pitch stability. Anything can be statically stable in pitch if the C/G is far enough forward, and it can be trimmed to fly at any airspeed faster than the point at which the tail stalls if you have enough decalage/up trim/reflex. A plank does not have much tail area or tail moment either, but it can certainly have a very non-symmetrical airfoil.

On full scale aircraft, the aft C/G limit is typically determined by static stability requirements, while the forward limit is determined by the need to avoid tail stall during landing flair. The Cessna Cardinal is one example of what happens if you get it wrong. The plane originally had a tendency to stall the tail in the landing flair at forward C/G's, which rsulted in a lot of busted noewheels and props. Later they retrofitted the horizontal tail with some slots that delayed the tail's stall and increased the amount of downward lift it could make, which fixed the problem.

...But I've also flown one of the Great Planes Pokeys that has a huge % stabilizer, albeit on a very short tail moment, and the model is HORRIBLY underdamped on the pitch.

That's a case of having adequate static pitch stability (the tendency to return to the original angle of attack if the pitch angle is disturbed; anything can have positive static pitch stability if the C/G is far enough forward), but poor dynamic stability (the ability to damp out oscillations). The key in your example is the very short tail moment arm. Tail moment arm and tail area are both linearly related to static stability.

However, dynamic stability is linearly related to tail area, but proportional to the SQUARE of the tail moment arm. If you double the tail area and cut the tail moment arm in half, the static stability will stay the same, but you will have only half the dynamic stability.

As to your stab size example we are not really comparing apples to apples. In your example the models have the same CG location but the model with the larger stabilzier will have a more rearward neutral point location so the stability margin would be larger if you consider the CG's being at the same % of the wing chord. And with that being so the model with the larger stabilizer would pull out of the dive faster because it's margin of stability is higher. To be more fair you'd want to say that both models should have the CG the same % of distance in front of the neutral point in order to acheive the same value of pitch stability. As to which would pull out of the same angle of dive faster at that point I'm not sure but I suspect it would be very close.

Given the same static margin, the big difference is that the one with the larger tail would damp out oscillations quicker. Its initial return to the trimmed angle of attack would be a little slower than with the smaller tail, but it would damp out to steady flight while the one with the smaller tail would still be porpoising.

Dynamic stability can be more important to handling qualities than static stability. For example, our "Chrysalis" series sailplanes have very long tail moment arms, and extremely good dynamic stability. One time I deliberately moved the C/G aft on the prototype electric 2-meter Chrysalis till the plane was actually slightly divergent in pitch (so in other words, the static pitch stability was actually slightly negative), then launched the plane and handed the transmitter to a first-time beginner. Despite the static instability, the plane was so well damped that she was able to mentally stay ahead of this statically unstable setup with no significant trouble.

[Don Stackhouse]

Regarding the sizing of V-tails, there is a great deal of discussion of this in the "Ask Joe and Don" section of our website:
www.djaerotech.com

The short answer is that it's not that difficult. A V-tail should have the same TOTAL area (NOT projected area) as an equivalent conventional or T-tail.

The "Projected Area Method" (i.e.: making the top view of a V-tail the same size as the supposedly equivalent conventional tail's horizontal, and the side view of the V-tail the same size as the conventional tail's vertical) is very commonly used. It's also dead wrong. It results in a tail that is seriously undersized. It can make the same elevator force as its conventional counterpart, or the same rudder force, but not both at the same time. That could become a serious issue during certain maneuvers, such as recovering from a spin!

There is absolutely no reason why a properly designed V-tail should need a different C/G, etc., than an equivalent conventional or T-tail. If it does, then it isn't equivalent.

The bottom line is that for the same total amount of tail area, you can make an equally effective tail by any of the common layouts.

[OVSS Boss]

Don is right, if a v is correct things work fine, like a Zenith. I have flown two other ships, a Phoenix and a Pike Superior, that were a bit skimpy and you knew it pretty quickly. The first was an F3B ship and you could go fast and then have a turn that snapped right out on you. The Superior was better, but when you were really needing that tail to work for you, it might not. I was working a low piece of air in Dayton once and went from good at 50' to on the ground and out of bounds so fast it was a bit of a shock.

You can snap about anything with a rearward CG we fly, but v's can work, if big enough.

Marc