Author Topic: Aft CG limit (Read 3081 times)

Allan Perret · « **on:** July 01, 2011, 07:59:20 PM »

What is typical max aft CG limit in terms of % MAC, for full size flapped stunter ?

Air Ministry . · « **Reply #1 on:** July 01, 2011, 09:07:58 PM »

C.G. rear. Max.

Damian_Sheehy · « **Reply #2 on:** July 02, 2011, 07:32:56 AM »

Your question is like this:
I have a quart of black paint and a quart of white paint. If I start to add black paint to the white, at what percentage will it turn gray?

A stability measure of the plane is the distance between the CG and the neutral point. For stability, the CG has to lie ahead of the neutral point and the bigger the margin the more stable it will be.

Where is this neutral point?
On a combat plane, wing only, the neutral point is at the quarter chord point (25% chord).
Adding an inch of tail arm moves the neutral point back about an inch (*).
Adding a square inch of stab area moves the neutral point back about an inch (*).

Edit: This statement is incorrect. It is (one inch) times (tail volume coefficient) times (another small term). So it's way less than an inch. See Serge's post below.

(*) This is only just true up to a point. Obviously, as you pile on more and more tail arm/stab area the aft weight will start to dominate, but the plane will look very ugly before that happens. The "about an inch" approximation assumes "very light" tail construction, the heavier the construction the less the increment will be.

With that in mind, it is desirable to position the CG close to the quarter chord point and have a sufficient tail arm and stab area to provide stability. With the CG at the quarter chord point, you reduce the effects of undesirable pitching forces when the plane executes maneuvers.

I haven't answered your question, but I hope this gives you a better feel for the cause and effect.

Allan Perret · « **Reply #3 on:** July 02, 2011, 08:27:42 AM »

What is the definition of neutral point. Is it same as MAC ?

" On a combat plane, wing only, the neutral point is at the quarter chord point (25% chord) "

Is it correct that it is a higher % chord for a flapped wing. Some like 30~35 %.

Damian_Sheehy · « **Reply #4 on:** July 02, 2011, 10:14:09 AM »

Quote from: Allan Perret on July 02, 2011, 08:27:42 AM

What is the definition of neutral point. Is it same as MAC ?

" On a combat plane, wing only, the neutral point is at the quarter chord point (25% chord) "

Is it correct that it is a higher % chord for a flapped wing. Some like 30~35 %.

I think you meant is it the same as the AC (Aerodynamic Center, which is at 25% back from the leading edge).
Yes, the neutral point for the whole plane is analogous to the AC for the wing alone.

That's right, the neutral point is much further aft on a modern flapped plane. I never did the math, but I wouldn't be surprised if it were aft of the trailing edge or beyond.

When the CG is at 30% or 35%, the plane isn't unstable; it just doesn't fly as nicely. So it's handling more than anything else and I guess the folks who can best describe these handling effects may be busy preparing for the NATS.

Steve Helmick · « **Reply #5 on:** July 02, 2011, 11:59:38 PM »

Quote from: Allan Perret on July 01, 2011, 07:59:20 PM

What is typical max aft CG limit in terms of % MAC, for full size flapped stunter ?

Ted Fancher's rule of thumb is the CG should be located at the same % MAC as the stab/elev. area % of total wing/flap area...as a safe place to start.

It does make a handy and easy to remember rule of thumb, but location of max thickness of the wing airfoil should also have some effect, I expect.

Steve

Serge_Krauss · « **Reply #6 on:** July 03, 2011, 12:34:35 AM »

Quote from: Allan Perret on July 01, 2011, 07:59:20 PM

What is typical max aft CG limit in terms of % MAC, for full size flapped stunter ?

Edit: Composed while Steve was posting his response.

Allan-

The most agreed upon approximation I've found is Ted Fancher's rule of thumb for flapped stunters, which suggests that the c.g. be placed at a percent of the wing's MAC equal to the horizontal tail area's percent of the wing area. So the c.g. for a flapped stunter with a tail area 25% of the wing area would have it's c.g. at .25 MAC. I don't know how far, and under what conditions, one could exceed this percent, but assume that as flap area and deflection diminish, the c.g. must move forward so that it's in the area of 15% - 19% MAC without flaps.

I disagree with some of what has otherwise been posted. For instance, adding an inch of tail arm would only move the aircraft's neutral point (NP) back a full inch if the tail area were over twice the wing's and if were 100% efficient. For the "typical" stunt proportioned model described below, an inch extension would only move the NP a small fraction of an inch along a 10" chord. To add a tail to the combat wing, you'd naturally need several inches of tail length to begin with and then add to that.

The neutral point for the whole plane is not easy to compute accurately, since it depends on several things pertaining to the horizontal tail's area, position, and efficiency. I've seen several methods on the internet and in recent texts, each with it's own set of approximations or assumptions. You can do a search here and on SSW Forum and find a lot on this. These computations seem to be very approximate for us without CFD.

Anyway, because the tail is not as efficient as the wing, the neutral point of an entire aircraft with a "normal" stunt-proportioned stab/elevator is noticeably, but not extremely rearward of the wing's N.P. If you're interested, the RC Aircraft Proving Ground site has a calculator that does all the work for you. Just go here and put in your model's dimensions:

http://www.geistware.com/rcmodeling/cg_super_calc.htm

(Remember though that their method has some simplifying assumptions too)

For instance, I chose proportions of a typical modern stunter, minus the tapers. I used constant chord wing (10") and tail (5") of spans 50" and 25" respectively. These give 500 in² wing with a 25% tail. Using a tail-volume coeficient of .5, I went ahead and computed a tail length (between neutral points of wing and tail) of 20". That gave a distance between Wing and tail leading edges of 21.25". I just plugged these values into their blanks, clicked on their "button", and got my answers:

NP of the entire plane is 4.66" behind the wing's leading edge, or at 46.6% MAC. If you put in trial-and-error static margin figures to get the c.g. at 25% MAC, as in Ted's rule of thumb, you'll find a static margin of 21.55%, which is very much greater than full-sized aircraft, RC, or FF models. That seems to be a characteristic of CL controlled models and has been posted here before.

A simpler computation that assumes 75% tail efficiency and just weights the wing and tail moments by area gives a NP at 53.75% MAC for the same dimensions. Increasing the tail length by 1.0" moves the NP, computed this way, only .15" rearward. Using equal wing and tail areas (500 in² each) and the same tail lengths gives a change in NP position of 3/8". That would be 1/2" for a 100% efficient tail.

You can plug these numbers in at RC Aircraft Proving Ground and instantaneously get their presumably slightly more refined answers. No math is involved - for you. Just measure a plane and plug in the measurements. The answer comes out. Have fun - or else!

SK

Allan Perret · « **Reply #7 on:** July 03, 2011, 05:51:11 AM »

Thanks Serge, great answer.

Damian_Sheehy · « **Reply #8 on:** July 03, 2011, 09:05:50 AM »

Quote from: Serge_Krauss on July 03, 2011, 12:34:35 AM

<Snip>
I disagree with some of what has otherwise been posted. For instance, adding an inch of tail arm would only move the aircraft's neutral point (NP) back a full inch if the tail area were over twice the wing's and if were 100% efficient. For the "typical" stunt proportioned model described below, an inch extension would only move the NP a small fraction of an inch along a 10" chord. To add a tail to the combat wing, you'd naturally need several inches of tail length to begin with and then add to that.

My apologies, you are absolutely correct, Serge. This is folly and off by an order of magnitude.

(My mind was focused of the tail volume term where an inch of tail arm has the same effect as a square inch of stab area. Yes, in terms of the movement of the neutral point, that one inch needs to be multiplied by tail volume coefficient and then by another fractional term due to downwash (1-depsl/dalpha) and another fractional term; ratio of lift curve slopes. And, that assumes a weightless tail so it's smaller again.)

Thank you for pointing this out

Serge_Krauss · « **Reply #9 on:** July 03, 2011, 10:06:12 PM »

Damian-

Yeah, these things you point out make NP computations awfully difficult to nail with any real accuracy. Some authors seem confident in their analyses, but I too have my doubts with so many approximations. I think they're more useful for comparisons.

SK

Tim Wescott · « **Reply #10 on:** July 03, 2011, 10:16:23 PM »

I have found this page to be a huge help in designing RC aircraft -- it's always given me a good safe starting point for my longitudinal CG, that's neither so far back that I need to change my shorts when I land the plane, nor so far forward that I have to keep it going 100 miles per hour just to keep the nose up. As mentioned already in this thread, however, control line planes seem to like a farther-forward CG.

Igor Burger · « **Reply #11 on:** July 05, 2011, 11:06:41 AM »

Quote from: Serge_Krauss on July 03, 2011, 12:34:35 AM

Edit: Composed while Steve was posting his response.
The most agreed upon approximation I've found is Ted Fancher's rule of thumb for flapped stunters, which suggests that the c.g. be placed at a percent of the wing's MAC equal to the horizontal tail area's percent of the wing area. So the c.g. for a flapped stunter with a tail area 25% of the wing area would have it's c.g. at .25 MAC. I don't know how far, and under what conditions, one could exceed this percent, but assume that as flap area and deflection diminish, the c.g. must move forward so that it's in the area of 15% - 19% MAC without flaps.
SK

My MAX has tail close to 30% of the wing area and it definitelly does not work well with CG aft oft the 25% point. I would say that Teds rule works well wit tails up to 25%, but I think the CG position should not go to more than those 25%. I thik it has something with fact that elevator can be lifting in level (because the CG is behind of AC of the wing) while the lift direction must be changed in maneuvers (must overcome the flaps moment, accelerate mases etc) ... I am not sure with the reason, but at least that is my experience so far.

Howard Rush · « **Reply #12 on:** July 05, 2011, 02:15:40 PM »

The "consensus" I see above is kinda simple-minded, as is the RC calculator. Golly, guys, this was figured out 70 years ago and is in any textbook on the subject. The NACA stuff is all online now; you don't have to go to the stacks in the library anymore.

Serge_Krauss · « **Reply #13 on:** July 06, 2011, 03:18:52 PM »

I read it.

SK

Ted Fancher · « **Reply #14 on:** July 22, 2011, 11:18:56 PM »

Quote from: Igor Burger on July 05, 2011, 11:06:41 AM

My MAX has tail close to 30% of the wing area and it definitelly does not work well with CG aft oft the 25% point. I would say that Teds rule works well wit tails up to 25%, but I think the CG position should not go to more than those 25%. I thik it has something with fact that elevator can be lifting in level (because the CG is behind of AC of the wing) while the lift direction must be changed in maneuvers (must overcome the flaps moment, accelerate mases etc) ... I am not sure with the reason, but at least that is my experience so far.

HI Igor,

I'm in total agreement with you as far as the lack of benefits of a CG aft of 25%MAC. There just isn't any benefit for an airplane whose mission is to fly "pitch oriented" maneuvers under whatever conditions prevail. IMHO, the important factor is to be able to place the CG at or very close to the Aerodynamic Center of the wing so that changing pitching moments due to G loadings in maneuvers are minimized when conditions deteriorate. There is nothing inherently wrong with a tail larger than necessary to be stable at 25% MAC but there is no value in moving the CG aft of that point.

I"ve written a lot about this subject, much of it on the forums and/or in the various articles I've written for Model Aviation and Stunt News. Rather than rehash it here I encourage those who might find my thoughts of interest to track down that stuff.

The fact is that you can move the CG aft as far as you want as long as you move the Neutral Point of the entire critter aft as well. The thing that produces stability is the "Static Margin", i.e. the distance between the CG and the Neutral Point. The proof of the pudding is that as the tail gets bigger it eventually gets larger than the "wing" and we call it a canard. As the tail gets bigger yet the necessary CG will move far aft of the "forward" surface in order to maintain the same "static" margin. A canard will always have the CG well aft of 25% MAC of the forward surface.

The neutral point is effected most dramatically by the size of the tail compared to the wing but that isn't the whole deal. The NP is the sum total of the airloads on every square inch of the surface of the vehicle. That's why merely putting feathers on the back end of a stick will allow you to throw it with the sharp end staying forward. Put some weight on the front end will allow you to throw it hard and fast and call it an arrow! YOu've moved the NP aft with the feathers and the CG forward with the weight on the nose and it is now "stable" and will fly true to the target. Remove either and the accuracy will deteriorate. Remove both and you can't throw it out of your shadow!

Unflapped stunters suffer with regard to optimum CG location. If you move the CG back to 25% of the wing with a 25% tail the ship will be "stable" but it will be very hard to fly well and will feel "tail heavy" even though it technically isn't because it will still glide straight ahead. It will however, have very little control "feel" because there will be no "pitching moment" to overcome when maneuvering.

Pitching moment is the last peg in the design board. Pitchning moment comes from two primary sources in stunt ships (one if you ain't got no flaps). When the CG is ahead of the Aerodynamic Center of the wing (~25% of the MAC [use the average chord halfway out the wing for practical purposes) it (the CG) is ahead of where the lift is centered (the AC) and some up elevator must be used to keep the nose from falling earthward (like a teeter totter). Any attempts to maneuver the airplane need to overcome this "negative pitching moment" before the maneuver can happen. This moment results in forces at the handle that allow the pilot to "feel" the response of the airplane. If the CG and the AC are at the same longitudinal point there is no "feel" from the pitching moment and the pilot must respond based solely on observation of the flight path. Possible but more difficult...especially when you want to stop the turn precisely.

This is why almost any good flapless stunter will trim out with the CG about 15%MAC so that there is a "moment" between that and the AC which provides feel for the pilot.

With a flapped ship you've got a built in producer of "negative pitching moments", the flap. A symmetrical airfoil (as on the flapless ship discussed above) has no pitching moment, as we've discussed. A flapped symmetrical wing on the other hand is only "symmetrical at neutral". As soon as you give a pitch input the flaps go in the opposite direction of the desired turn and, by so doing, change the symmetrical airfoil to a "cambered" one (technically, a line drawn at equidistance from the top and bottom of the wing's surface will now "bow" at the hinge line like a bird's wing). The act of cambering an airfoil produces a "negative pitching moment"...the wing wants to pitch in the direction of the deflected flap (if the airplane to which it was attached had no tail we'd call it a "flying wing" and the "flaps" would be called "elevators"!)

The resulting "negative pitching moment" serves the same purpose as the distance between the CG and the AC in the unflapped airplane. The force necessary to be overcome before the ship "turns" in the desired direction provides "feel" to the pilot and makes the ship feel "stable" while maneuvering with good feedback for entering and exiting maneuvers and corners.

The ultimate value of the "built in negative pitching moment" of a flapped ship is that we can no move the CG back to the AC and eliminate the pitching moment that was "necessary" in the unflapped airplane.

The reason this is valuable for a competition flier is that pitching moment from flap deflection "needn't" be a factor when flying in bad or windy conditions. Because the increased G loads encountered when maneuvering in windy conditions "act at" the AC and because the CG "can" be at the same location as the AC the response rate of the airplane changes very little and you can fly it with pretty much the same control inputs in bad conditions as in calm air.

If, however, you don't design your airplane so as to allow the CG to be at the AC (roughly 25%MAC) you will then have to balance it forward and in bad air you'll have to overcome the negative pitching moments developed by the distance between the two. The harder the wind blows and the faster your ship moves in loops the greater the G loads and the more it will want to open up. This can, and has, resulted in stunt ships that can't turn tight enough to stay out of the ground in high wind...even though they might beat everybody on a day with "stunt heaven" air. This is where "Ted's rule" about tail size and CG location came from. No, I can't do a Bill Netzeband or a Howard Rush dissertation on the math involved. But I can say without fear of contradiction that if you build a remotely "normal" stunt ship with flaps, make a tail that is at least 25% of the wing area and put locate the CG at 25% MAC you will have a happy airplane that will fly well in good air and bad and be plenty "stable".

Ted Fancher

sleepy gomez · « **Reply #15 on:** July 23, 2011, 09:48:28 AM »

Where do you locate the CG on a flapless biplane with wing stagger? My Doubletime (710 sq. in.) seems to work quite well with the CG at 23% of the total chord (LE of the top wing to the TE of the lower wing).

Howard Rush · « **Reply #16 on:** July 23, 2011, 08:13:19 PM »

Quote from: sleepy gomez on July 23, 2011, 09:48:28 AM

Where do you locate the CG on a flapless biplane with wing stagger? My Doubletime (710 sq. in.) seems to work quite well with the CG at 23% of the total chord (LE of the top wing to the TE of the lower wing).

I actually found that in an ancient textbook once. No point looking it up, because you found the right spot experimentally.

When I was a kid, one of my friends built a Sterling Polish Fighter. We figured that because the Ringmaster instructions said to put the CG at the forward leadout, that should apply to all control line airplanes. This resulted in a lot of tail ballast. The Polish Fighter didn't last long.

Howard Rush · « **Reply #17 on:** July 23, 2011, 08:27:47 PM »

Quote from: Ted Fancher on July 22, 2011, 11:18:56 PM

HI Igor,

I'm in total agreement with you as far as the lack of benefits of a CG aft of 25%MAC. There just isn't any benefit for an airplane whose mission is to fly "pitch oriented" maneuvers under whatever conditions prevail. IMHO, the important factor is to be able to place the CG at or very close to the Aerodynamic Center of the wing so that changing pitching moments due to G loadings in maneuvers are minimized when conditions deteriorate. There is nothing inherently wrong with a tail larger than necessary to be stable at 25% MAC but there is no value in moving the CG aft of that point.

I"ve written a lot about this subject, much of it on the forums and/or in the various articles I've written for Model Aviation and Stunt News. Rather than rehash it here I encourage those who might find my thoughts of interest to track down that stuff.

The fact is that you can move the CG aft as far as you want as long as you move the Neutral Point of the entire critter aft as well. The thing that produces stability is the "Static Margin", i.e. the distance between the CG and the Neutral Point. The proof of the pudding is that as the tail gets bigger it eventually gets larger than the "wing" and we call it a canard. As the tail gets bigger yet the necessary CG will move far aft of the "forward" surface in order to maintain the same "static" margin. A canard will always have the CG well aft of 25% MAC of the forward surface.

The neutral point is effected most dramatically by the size of the tail compared to the wing but that isn't the whole deal. The NP is the sum total of the airloads on every square inch of the surface of the vehicle. That's why merely putting feathers on the back end of a stick will allow you to throw it with the sharp end staying forward. Put some weight on the front end will allow you to throw it hard and fast and call it an arrow! YOu've moved the NP aft with the feathers and the CG forward with the weight on the nose and it is now "stable" and will fly true to the target. Remove either and the accuracy will deteriorate. Remove both and you can't throw it out of your shadow!

Unflapped stunters suffer with regard to optimum CG location. If you move the CG back to 25% of the wing with a 25% tail the ship will be "stable" but it will be very hard to fly well and will feel "tail heavy" even though it technically isn't because it will still glide straight ahead. It will however, have very little control "feel" because there will be no "pitching moment" to overcome when maneuvering.

Pitching moment is the last peg in the design board. Pitchning moment comes from two primary sources in stunt ships (one if you ain't got no flaps). When the CG is ahead of the Aerodynamic Center of the wing (~25% of the MAC [use the average chord halfway out the wing for practical purposes) it (the CG) is ahead of where the lift is centered (the AC) and some up elevator must be used to keep the nose from falling earthward (like a teeter totter). Any attempts to maneuver the airplane need to overcome this "negative pitching moment" before the maneuver can happen. This moment results in forces at the handle that allow the pilot to "feel" the response of the airplane. If the CG and the AC are at the same longitudinal point there is no "feel" from the pitching moment and the pilot must respond based solely on observation of the flight path. Possible but more difficult...especially when you want to stop the turn precisely.

This is why almost any good flapless stunter will trim out with the CG about 15%MAC so that there is a "moment" between that and the AC which provides feel for the pilot.

With a flapped ship you've got a built in producer of "negative pitching moments", the flap. A symmetrical airfoil (as on the flapless ship discussed above) has no pitching moment, as we've discussed. A flapped symmetrical wing on the other hand is only "symmetrical at neutral". As soon as you give a pitch input the flaps go in the opposite direction of the desired turn and, by so doing, change the symmetrical airfoil to a "cambered" one (technically, a line drawn at equidistance from the top and bottom of the wing's surface will now "bow" at the hinge line like a bird's wing). The act of cambering an airfoil produces a "negative pitching moment"...the wing wants to pitch in the direction of the deflected flap (if the airplane to which it was attached had no tail we'd call it a "flying wing" and the "flaps" would be called "elevators"!)

The resulting "negative pitching moment" serves the same purpose as the distance between the CG and the AC in the unflapped airplane. The force necessary to be overcome before the ship "turns" in the desired direction provides "feel" to the pilot and makes the ship feel "stable" while maneuvering with good feedback for entering and exiting maneuvers and corners.

The ultimate value of the "built in negative pitching moment" of a flapped ship is that we can no move the CG back to the AC and eliminate the pitching moment that was "necessary" in the unflapped airplane.

The reason this is valuable for a competition flier is that pitching moment from flap deflection "needn't" be a factor when flying in bad or windy conditions. Because the increased G loads encountered when maneuvering in windy conditions "act at" the AC and because the CG "can" be at the same location as the AC the response rate of the airplane changes very little and you can fly it with pretty much the same control inputs in bad conditions as in calm air.

If, however, you don't design your airplane so as to allow the CG to be at the AC (roughly 25%MAC) you will then have to balance it forward and in bad air you'll have to overcome the negative pitching moments developed by the distance between the two. The harder the wind blows and the faster your ship moves in loops the greater the G loads and the more it will want to open up. This can, and has, resulted in stunt ships that can't turn tight enough to stay out of the ground in high wind...even though they might beat everybody on a day with "stunt heaven" air. This is where "Ted's rule" about tail size and CG location came from. No, I can't do a Bill Netzeband or a Howard Rush dissertation on the math involved. But I can say without fear of contradiction that if you build a remotely "normal" stunt ship with flaps, make a tail that is at least 25% of the wing area and put locate the CG at 25% MAC you will have a happy airplane that will fly well in good air and bad and be plenty "stable".

Ted Fancher

Got that, Igor?

Dennis Moritz · « **Reply #18 on:** July 24, 2011, 07:30:15 AM »

Got it. Canard. This discussion is a canard. Experiment. Start with ballast at a point of stability. Add tail weight (or decrease nose weight) until the plane fails to groove or gets scary.

phil c · « **Reply #19 on:** July 25, 2011, 01:36:18 PM »

I think you nailed it Ted. When you add flaps that move opposite the elevators it totally screws up all the static stability equations, so they don't mean a thing. The dynamic stability is another thing. The moveable flaps completely override the control feel and make it impossible to calculate the correct balance point. To add to the mess, in a maneuver the flaps are also under G loads and trying to drop further, while the elevators are being pushed back towards neutral, and the extra lift from the flaps reduces the effectiveness of the tail, reducing stability.

Maybe Howard Rush and Paul Walker can sneak in some runs on one of the dynamic stability models they use to design some of the fly-by-wire control systems and make some sense out of it.

I'll fall back on DownUnder's advice- add just enough flap movement to make the plane fly well through the "tight" corners!

Howard Rush · « **Reply #20 on:** July 25, 2011, 06:33:19 PM »

Stop that, Philip. This discussion reminds me of combat lines I've seen. They sorta look OK, but are just too tangled to mess with. Clip off the eyelets and use them to make a new set.

Dennis Toth · « **Reply #21 on:** July 25, 2011, 09:12:11 PM »

Guys,
This is an interesting discussion about moving the CG aft, over on the electric board there is a discussion about Paul's Nats ship a near the end of the thread the discussion turns to CG location moving forward on the electric ships. Paul and Bob H have both found that their electric powered ships have better wind penetration and are more stable with the CG less than 15% MAC. The electrics seem to pull through the corners even with the additional control deflection (and thus drag) and hold speed allowing a more forward CG then most of the IC setups. I had been working on my electric and was applying the "modern" approach of moving the CG back as far as possible to turn with the least amount of control deflection and thus maintain speed out of the corner. After reading this I thought I would give it a try since I can move the battery forward quite a bit to adjust the CG without adding additional weight. In the past I only move the CG 1/8 - 1/4" and kept thinking the aft CG would be better for corners. I was sceptical about this working on my ship thinking that Paul and Bob have purpose designed electrics and that's why it works for them. Well, over the weekend I decided to try a big swing of the CG on my electric. I moved the battery forward all the way which moved the CG forward to the 15% MAC. I made no other changes and I was surprised that it worked really well. The hop stopped on the bottoms and it flew the rounds smooth and on track. The first flight was in 3 mph wind and the next couple in 12 -15mph, worked for both conditions. On the second flight I tried a little lower pitch prop and it allowed lower lap times (5.35 vs 5.2 on 64') and still flew smooth through the wind. I do think the leadouts need to move forward and it will be even better. If you have an electric give it a try (this is really taking away excuses, it really is all pilot error now). BTW , for the wet guys, remeber to check you CG with a full tank, you might be closer to 18% MAC.

Best, DennisT

Igor Burger · « **Reply #22 on:** August 08, 2011, 03:22:41 AM »

I see I missed something here ... so late response to Teds, comment.

Ted, you are rght, but I will make it little bit shorter to show where that "opening" comes from exactly:

All moments on the plane are balanced (if I do not count mass inertia), if I take only those main moments which you mentioned (because there are several more, not so important like pitching moment of tail, warped surfaces due to circular flow in corner radius etc), then I can write:

1/
pitching moment from wing + moment of tail to AC + moment of CG to AC = 0 .... I note that I count all to AC of wing at 25% of MAC

The feedback to pilot comes from lifts made by wing and tail acting on movable parts ... means for example

2/
(wing lift * 0.2 * flap ratio + tail lift * .40) * something depending on linkage

and now:

you say that if I move CG forward, it will try to open radius due to GC moment to AC. I guess you mean the situation which will lead to higher speed in the radius. So how it looks in equations:

1/

Pitching moment from wing will be higher, because the speed is higher, but it will be well balanced by lift of the tail so we do not have any change in the equation

Moment of CF front of CG will be higher as you wrote because of centrifugal force which is higher at higher speed … so you are right, it must be balanced by little extra deflection of elevator … the mode is CG front of AC, the mode deflection is necessary

2/
That deflection really needs more force, because deflected elevator means also deflected flaps and most of feedback to pilot comes from hinge moment on flaps … little bit more deflected flap will change equation on place of that coefficient 0.2, because it will concentrate lift on flap little bit (let’s say 10%):
(wing lift (which is higher that that in calmbecause of higher speed) * 0.22 * flap ratio + tail lift * .40) * something depending on linkage

So that is what you wrote converted to equation, it shows how much more muscles you need in windy corner … but now comes trick … if you have such model, which needs more tail deflection, you will probably early see that it has too much flaps, and you will probably modify linkage so the flaps go less. And it will push you back, you will still see the same feedback from flaps, and the only difference comes from tail lift, which is only small fraction of whole feedback, because the tail hinge moment is 1/10 of the flaps hinge moment (example from my simulator of my older model without logarithmic device on flaps)

So shortly … I just wanted to write, that even forward CG leads to troubles you described, also model with little bit forward CG can be trimmed to fly well and easy in wind, example is my model from WC 2006 with CG relatively (not extremely) forward somewhere at 18-19% and still feels wind as an advantage in contest to other competitors. May be someone will remember that windy day on open circle (not in the stadium) when I got the best score of all of my 4 flights. That model had relatively small flaps (17% of the wing) and large elevator (50%) of the tail and 1:1 linkage between flap and elevator.

Howard Rush · « **Reply #23 on:** August 10, 2011, 10:04:58 PM »

For a given turn radius, all those moments should be proportional to speed squared. How much extra elevator it takes in wind (given the same airplane and controls) may be a function of sideslip or the difference between inertial speed and airspeed, but probably not of that other stuff. How much elevator you get for a given handle position or moment depends on the mechanics of the lines and the flexibility of the airplane and controls and can probably vary a lot with speed, as I learned at one breezy Nats as I was guessing how much control input it took to do the corners of a square eight with a 68-oz. airplane on 70-foot .015" lines. One of these days I hope to look into hinge moments and line mechanics and write a short monograph on the subject if I can figure it out. I think it's a big deal for stunt, and I'd like to understand it better.

Igor Burger · « **Reply #24 on:** August 10, 2011, 10:30:21 PM »

I have spread sheet fot those moments and forces :- ))) ... something like a "static" stunt simulator

Steve Helmick · « **Reply #25 on:** October 12, 2011, 10:10:19 PM »

By coincidence, a few weeks ago, I was over at Gary Letsinger's. He 'splained to me that sometimes you could move the CG forward and get a better corner, because the elevators and especially flaps would then deflect more.

This weekend, I was in Salem, and a fella was there Friday testing a ratty looking electron burner in an ugly wind. He explained that he had changed the flap/elev. to 1:1 control ratio and moved the CG forward, and found a better corner, better stop from rotation, better groove, etc., etc. Despite the wind on Friday, it seemed to work fine, tho commonly the CG and LO's would be moved back. He was from Deer Park, WA, having retired from Boeing recently. He even knew Gary Letsinger. Small World. Go figure.

Steve

PS: The fella with the ratty looking electron burner won Expert on Sunday in slightly drippy conditions with a flight of 595...only 10 of that being appearance points. Hmmmm. http://flyinglines.org/Follies.11.html