CG or trim???

[RandyK1]

I have a problem finding the sweet spot when flying my planes. I cant tell if I need to trim or move the CG. I know how to do the dive test to tell if the CG is correct, but to do the dive test the plane has to be trimmed to fly "flat" with no stick input. This is where my problem comes in. How do I know if I have the CG set in the right place when the plane wants to porpose a little and when its a trim problem? I know that CG affects the trim, and I know that when the CG is in the right spot and the planes trimmed correct it should glide for ever (flat glide) (well almost for ever:)). How do I tell when to trim and when to move the CG? Thanks, ...R...

[sblckwd]

If it "porposes" the CG is too far aft. The glide test will show that. If you trim for a flat glide and look at the elevator, when you land, it should be flat in line. If it's "up" you have too much nose wieght and you'll get nose up in a dive. If it's down you,ll get a nose down in a dive.

[Ollie]

The CG location is the only adjustment that affects stability. Therefore CG location should be used to establish the desired stability and for no other purpose! To set the trimmed flight speed use the elevator trim setting on the transmitter or by adjusting the decalage (angle between the wing and horizontal tail). If the elevator neutral is too far off geometric center, then elevator offset can be replaced by a decalage change to get the elevator neutral closer to geometric center.

When the CG is too far aft, the plane can't porpoise because the flight path diverges and there is no correction to that divergence when the contols are neutralized. Porpoising results from stability. The more stable the plane the harder it will porpoise. Porpoising results from a gust or abrupt control input that disturbs the trimmed flight path and speed. A stable plane reacts to the disturbance by correcting but because of inertia the correction over shoots and a decaying oscillation results if the disturbance is small and the plane isn't driven to the stall. If the disturbance is large enough that the plane is driven to the stall, then the porpoising does not die out with the controls in neutral and the oscillation can grow without limit if the pilot does not intervene. If the plane is trimmed to fly too close to the stall, then even a small disturbance will drive the plane into a stall and the oscillation will grow if unchecked by the pilot.

If, in the unlikely event that an unstable plane is driven to stall, it will only stall once. It will not recover from the dive that follows the stall without pilot intervention.

Finding the best compromise between an aft CG which reduces porpoising but also results in increased pilot work load to fly smoothly is the objective. Where this balance falls is a matter of pilot skill, flying style and the flight mission. Take the extreme example of the LSF eight hour slope task. The ability to fly continuously for eight hours depends on a light pilot work load and adjusting with enough forward CG to do it is essential. On the other hand, it the task is seven minute precision duration that depends on finding and working thermal, then a more aft CG results in a plane that signals lift better and the additional pilot work load can be easily tolerated.

[BMatthews]

I see room for some confusion here. We need to define what amounts to two types of porpoising.

First there is the type caused by Ollie's description of the model being MUCH too stable and wanting to radically nose up with each little gain in speed caused by the turbulence of the air the model flies through. That type is caused by the CG being too far forward and requireing too much decalage as a result just as Ollie describes.

The second type is what I think we may have in this case. A slight and continuous gentle nose up, nose down flight path of steady little mini stalls or mushings that takes place even in smooth calm air. I've seen some folks describe this second one as porpoising or hunting.

This second one is strictly due to having too much up elevator trim in the model. When it does this you just need to add a click or two of down trim and let the model speed up a trifle to avoid that slight mushiness. Some models have it and some don't. It's due to the airfoil and the wing structural design affecting how the airfoil operates in the high lift, near stall region. If you have a model that does this then it'll do it over a fairly wide range of CG locations whenever you get "greedy" with the elevator trim and try to set it up for a minimum sink state. Just add a click of down trim to smooth the flight path out and then proceed with your dive testing.

The speed up, slow down, speed up, slow down that causes this hunting action is related to the near stall charactaristics of your wing. I think it's caused by the nose coming up, a separation bubble forming or becoming larger which causes additional drag which slows the model which drops the nose which speeds the model up which reduces the size of the bubble which lets the model go faster which makes the nose come up which makes the model slow down which makes the bubble get bigger which causes more drag which slows the model down.......... and 'round and 'round she goes. So how can you stop this? The down trim for sure. Just speed it up enough to avoid that point. Or you can try playing with turbulator strips to help engergize the air before it either forms that separation bubble or at least keeps it smaller and more stable during the slow speed operation. In any event there's a repetitive oscillation of the airflow taking place that causes that hunting. If you can stabilize it then the model will fly smoothly.

So Randy, it's up to you to tell us what sort of flight you've got here.

[RandyK1]

Originally posted by BMatthews
So Randy, it's up to you to tell us what sort of flight you've got here.

My planes (mostly e-sailplanes) all do this. I get a slow porpoise when in a glide. Upon landing the elevator seems to be at 0 (flat). When I adjust trim in flight (computer radio) by a few clicks up the porpoising gets worse, if I adjust the trim down be a click or two the nose wants to start dropping, I pick up airspeed and the planes dont get what I would call a flat glide. I would like to have everything set up for thermal hunting so the planes would show lift easyer. I know with the CG more forward the plane will get more stable, and with the CG back it will get more twitchy and also make the plane react to lift changes more easiley. I can still keep the planes in the air for a long time (a long time to me anyway) so far my record has been a 29 min flight from a 40 sec motor run, and thats with the porpoising. My main problem is that I am self taught and everything I know ether came from the e-zone or I figured it out myself. If I had anyone else around here that flys gliders maybe I could let them fly my planes and see what they thought, but I dont :(. I would like to get them to fly nice and flat when in a glide as I am sure that would help me break my own record. Dont get me wrong, I can fly my planes like this, and have, but I know I will be alot happier when I can find that "sweet spot" between CG and trim. Thanks for all the help so far, please keep it comming :) ...R...

[nuevo]

Have you gotten together with the Chicago glider club?

http://www.soarchicago.com/index.html

They have a lot of talented guys who might can help you with plane setup. It's always helpful for me to get together with others, and learn firsthand.

I know it might be a drive. I could not find a map of their flying sites.

[RandyK1]

I wouldnt mind the drive (around 60 miles) if I could get some help setting up atleast 1 of my planes, then I could do the rest by myself. I wish there was an e-mail address on that site so I could find out when and where to go. ...R...

[fprintf]

Originally posted by RandyK1
I wouldnt mind the drive (around 60 miles) if I could get some help setting up atleast 1 of my planes, then I could do the rest by myself. I wish there was an e-mail address on that site so I could find out when and where to go. ...R...

Man, you have *such* a resource so close to you. Jack Strother, past president of the League of Silent Flight; Tom Kellevang, current prez. of the LSF; Bill Wingstedt etc. etc. All names even I recognize from reading the pages of soaring magazines or lately reading RCSE.

First step would be try mailto:webmaster@SOARchicago.com

Second, try the LSF site and get in touch with Tom. Better yet, join RCSE or post to the RCSE column here on RCgroups about your interest in flying with SOAR.

[nuevo]

PM me and I'll send you some email contacts. I don't want to post other's email addresses here.

[Ollie]

If your model flies too fast when it is trimmed with enough stall margin, then the answer is to lower the wing loading by getting rid of the motor and heavy battery. Then you would have a pure sailplane that really floats. As a compromise you could switch to lithium polymer batteries which are much lighter for the same capacity.

[SoarNeck]

The porposing you're describing sounds more to me like a c/g that's way too far forward.

First off - what kind of model are you flying?

The reason I ask is that the dive test is only valid over the range of airspeeds that you're trying to trim for. Most people put their model into a dive that quickly exceeds their practical thermalling speed by a huge amount. I've seen people trying to learn something about their c/g of their Great Planes Spectra by diving at 60mph...when all they'll really learn is that the stab tips twist at that speed! If you want to see how structurally rigid the model is, dive it fast...but don't use that to set the c/g.

The correct variation on the dive test that most people aren't aware of is to coax the model into a gentle dive by first stalling it. Trim level for flight at thermalling speed, and gently bring the nose up into a stall. As the model recovers, it will be much closer to thermalling speed. Notice what happens...odds are the model will pull up in response to the stab incidence that is having to hold all the lead in the nose from returning to earth. Aim for an almost imperciptable pull-out...but don't let the velocity of the model exeed thermalling speed! If you let it dive too long, you'll be back to
the original problem.

Also, try flying the model inverted. Do the stall-dive test again after retrimming for level inverted flight, and you'll get a true idea of how far forward your c/g really is.

I tried this system this weekend on a new Eraser XE that I've
been trimming. I set the c/g conservatively for the first few flights, and was pretty happy with the way it flew and indicated lift. I knew it was nose heavy, so I started doing the above tests. I had added a full 1.5 oz to the tail before I had the c/g right where I wanted it! that would be an embarassing amount of dead weight to be lugging around, if it were in the form of nose weight.

The model flew so much better than before, indicating lift by quickly popping the tail up, and dragging the tail badly through sink. The model landed much slower, and I ended up reducing the previously maxxed-out control throw for the elevator.

The thing that sold me the most on the new c/g location was that I actually noticed a thermal while blasting back to the field on a 75-80mph speed run. As I came back across the field boundary, I noticed the tail wiggle a little. I popped the airplane up on its wingtip, and low and behold, there was lift back there! Noticing lift at all speeds is important, since it will help you to thermal in high winds, where the thermals are drifting quickly across the field and the model is really cooking to keep up with them.

All said and done, What you're really trying to do is reduce the load on the elevator to minimal levels. All the stab should have to do is keep the wing flying at a constant angle of attack...balancing out the fairly minimal tendancy of the wing to pitch forwards as it generates lift.

If the c/g is forward, the stab not only has to do this, but it
also has to keep all the weight in the nose from plummeting back
to earth. The load on the stab will be much higher, meaning that
the elevator is more likely to run out of lift before the wing does (smaller wings don't work as well as bigger ones).

Therefore, when you slow the model down, odds are the stab will stall before the wing does. If it has to generate a lot of lift, it has to work at high angles of attack (read: high control throw or big decalage angle). At higher angles of attack, the stab will stall at higher speeds (just like you can't fly around with the model's nose high in air for long before it stalls) Follow me so far?

What this all means in the end is that your model will be controllable to a lower minimal airspeed, and will be more responsive on the controls. You can reduce your control throws to get the responses back to where you want them.

Don't be fooled by thinking a nose-heavy airplane is "more stable". It's actually just less responsive, and it will actually stall more easily. Don't push the c/g behing the neutral point obviously...and be mindful of the desire pilot workload for special situations like Ollie described.

[BMatthews]

Originally posted by SoarNeck
.....You can reduce your control throws to get the responses back to where you want them

Don't be fooled by thinking a nose-heavy airplane is "more stable". It's actually just less responsive, and it will actually stall more easily. Don't push the c/g behing the neutral point obviously...and be mindful of the desire pilot workload for special situations like Ollie described.

Exactly... Or at least within reason. But you REALLY need to be pushing the CG close to the neutral point to have problems.

Do not be mislead into thinking that if the elevator lines up with the stabilizer that everything is "Right". Far from it. Stab mounting angles are a compromise at the best of times and shear guesswork at the worst. Ollie covered that above when he said to trim the glider without worrying about the final elevator position and only if it was grossly off center should you worry about remounting the stab to more closely set the angles to allow for a more neutral elevator setting.

I never thought about it but you could be so nose heavy that the stabilizer and elevator is actually STALLING in a cyclic way and that is what is causing your nose to hunt. Moving the balance back to where the tail does not need to do so much work may allow it to operate in a more happy part of it's lift zone.

If you do a dive test at, say, 30 degrees nose down and then release how many feet does it take to pull back to LEVEL, not nose up but the first point where it's level.

[RandyK1]

Originally posted by Ollie
If your model flies too fast when it is trimmed with enough stall margin, then the answer is to lower the wing loading by getting rid of the motor and heavy battery. Then you would have a pure sailplane that really floats. As a compromise you could switch to lithium polymer batteries which are much lighter for the same capacity.

I knew when I posted in the thermal fourm that someone would tell me to get rid of the motor & battery, but the planes I fly are made for the weight of the "heavy" motor and battery combo. I did not take a pure sailplane and convert it to electric, that would way mess with the wingloading. The planes I fly range from 67oz. 101"wing with a wingloading of 12oz/ft (the largest) to a 3.5oz 41"wing with a wingloading of less than 3oz/ft (the smallest) the lightest large plane I have has a 87" wing and only weighs 22oz. I dont know the wingloading on this one, but I know its light.BTW, I posted in the thermal forum because thats what I want to do (thermal) and I thought that I could find the best thermal info in the thermal forum :)

I will never use li-poly batterys because of all the problems with them. I will stick with Nicd & nimh batterys. ...R...

[RandyK1]

Originally posted by SoarNeck

First off - what kind of model are you flying?



Some of my planes can be seen in my gallery http://rcgroups.com/gallery/showgallery.php?cat=500&ppuser=11736 I dont have all my planes pics posted yet and most of the flying wings I have sold. ...R...

[RandyK1]

To all: I am taking all this info in, so next time I go flying I can try some of this stuff. I would think that its not as easy as I thought it would be, so far there is no one answer, in this thread I have seen "If it "porposes" the CG is too far aft" to "The porposing you're describing sounds more to me like a c/g that's way too far forward". My brain hurts :). Keep up with the good info please, I can use all the advice I can get. ...R... p.s. all my e-sailplanes are F5J type

[Ollie]

Try flying a plane with a 7 or 8 ounce per square foot wing loading sometime. You may like it or not.

All but one of us agree that your CG is too far forward. Soarneck and BMatthews are highly experienced and well informed. Believe what they say.

[RandyK1]

I will move my CG back and re-trim the next time I go flying (wed, I hope) and report back. The reason that I went with the e-sailplanes is at the time was I thought that a motor would get me up alot higher than a hi-start would, and the higher I got before I started the glide the easyer and more time I would have to find thermals. The Sky Sergio that I have 87" wings, 22oz has a wingloading of aroung 6 oz per sq ft (I think) so I know what you mean about light wingloading, I think its the one I enjoy flying the most. Keep all the great info comming. Everything I know I learned from the zone :) ...R...

[intheswamp]

I stumbled across this short thread today and thought I'd bump it back up the ladder a bit. It's got some very good info in it from some very experienced/knowledgeable guys. With thermal season coming and some newbies (like me) joining the hobby I though this thread would benefit some folks. It's a very good thread dealing with cog, trim, and porpoising issues.

Ed
Intheswamp.... :Dning

[Peytr]

There is a very easy to use utility on the i-net, which helps you to determine where your CG should be to start with. Otherwise you'll possibly be trying to compensate a wrong CG with a correction of wing incidence, with ill results.

http://home.arcor.de/d_meissner/schwerp.htm

CG and wing incidence are interacting which makes things a bit fuzzy when trying to trim a new plane, with no reference for the CG. I found it easiest to get the calculated CG (in fact the utility generates two for larger and smaller stability) and put it there. After this I trim the plane using wing incidence and stab incidence to get the nicest flying I can get from the plane. One must allways do these trimming flights in dead air and little wind. Then I switch to the CG again and try to find the most rearward CG I can handle and adjust the incidences again. Usually I need a few flying days to get to the point where I feel things can't be improved any further.

I agree BTW with most people in this thread that your CG probably is too far forward.

[Phil Barnes]

I too missed this thread the first time it came up. Randy, the best thing you can do for yourself is to stop thinking in terms of "sweet spots" for CG and especially elevator trim.

There has been some good information posted so far but there is also some information posted that is way too technical and "deep" to the point that I didn't even take the time to read it all (and have no idea how accurate it might be). You need a simple aproach which I will attempt to describe. You also need to steer clear of incidence sweet spot mythology which runs rapant in the RC soaring community.

I will assume that you are flying kit models. If so then you should set the CG to the manufacturer's recommended position. If no such recommendation is made then post a question to the group and find out where other guys are setting the CG for your particular model. If you get a range of answers then choose a CG position that is near the middle of the suggested answers or maybe near the forward part of the range of answers. Your model's flying qualities will be determined by the CG position. There is no sweet spot CG, as long as the CG is in some reasonable position then CG adjustments will just have some modest affect on how stable the model is vs. how twitchy it is.

Once the CG is set you should fly the model to determine the correct elevator trim setting. This can usually be done by just some hand launches. Simply adjust the elevator trim at the transmitter until the model performs a flat glide without stalling or diving sharply into the ground. If it stalls or porpoises it needs down trim. If it dives sharply into the ground it needs up trim. Once the elevator trim position has been established you may want to reset the model's decalage. Decalage is just the angle of the fixed portion of the stabilizer relative to the wing. If after setting the correct elevator trim as described above you find that there is a significant deflection angle between the elevator and the fixed portion of the stabilizer then you can eliminate this by resetting the fixed portion of the stabilizer (resetting the decalage).

Resetting the model's decalage will not have a significant effect on the model's flight characteristics or qualities (the only exception to this would be with some flat plate type stabilizers that really don't like large elevator deflection angles). Flight characteristics are determined by CG position and as long as the CG is in some reasonable position then stalling or porpoising should be due to too much up elevator trim. Diving without pulling out should be due to too much down elevator trim. Decalage or incidence adjustments will not have a significant effect on the model's performance. There is no sweet spot for incidence beyond just setting the elevator trim as described. Setting the elevator trim simply makes the model easier to fly since it will fly flat and level when you take your hand off the transmitter stick. This also makes reading lift easier if the model flies flat and level most of the time.

It is as simple as that. You set the CG as desired for flying qualities. This CG position determines the required elevator trim setting and may lead to a resetting of decalage simply to eliminate trim drag from an excessively deflected elevator.

Any thoughts of finding "correct incidence" or "correct trim settings" to enhance model performance and especially finding incidence or trim "sweet spots" are thoughts that are best not persued since they all lead to the land of incidence mythology.

Assuming that you have the model's CG set in some reasonable range then you should be able to establish a flat, non porpoising glide by simply setting the elevator trim correctly. banish all thoughts of trim sweet spots from your mind and I'll bet you will find this to be true.

[Phil Barnes]

Peytr's post was made while I was writing my last post and is deserving of some comment;
There is a very easy to use utility on the i-net, which helps you to determine where your CG should be to start with. Otherwise you'll possibly be trying to compensate a wrong CG with a correction of wing incidence, with ill results.

http://home.arcor.de/d_meissner/schwerp.htmI have not looked at this link but if it does what Peytr says then it would be very useful. You could use it as your method for setting your CG correctly.

CG and wing incidence are interacting which makes things a bit fuzzy when trying to trim a new plane, There could be terminology issues cropping up here but I would say that once you set the CG then stabilizer incidence or trim is fully determined. The interaction taking place is that CG determines elevator incidence or trim. The two things (CG and elevator trim/incidence) are not independent variables.

After this I trim the plane using wing incidence and stab incidence to get the nicest flying I can get from the plane.Here is where I wish I were more of a tactful person. I have no desire to offend or start a flame war. The problem is that I really dislike the propagation of mythology. Especially mythology that overcomplicates things. The only trimming that gets done after establishing the desired CG is to set the elevator trim for a flat glide again. That position is fully determined by where you set the CG previously. There is not a range of positions to choose from. There is one and only one place to set the elevator trim that will result in a flat glide. If you are unhappy with the flying qualities of the model at this point then you go back and try a different CG.

If we can all agree on this one simple point then we can all avoid the strangeness and overcomplication that comes from thinking of incidence or elevator trim adjustments as being separate variables from CG adjustments. This mythology of incidence having an effect on model flying qualities and being in need of fine tuning is by far the most pervasive and deeply rooted RC soaring myth. I believe this myth has and continues to cause untold amounts of grief and wasted time when people attempt to persue it.

[John Walter]

Randy:
I sent you an email with my home phone number. Give me a call and I can give you detailed directions to the SOAR field.

[Hostage-46]

This mythology of incidence having an effect on model flying qualities and being in need of fine tuning is by far the most pervasive and deeply rooted RC soaring myth. I believe this myth has and continues to cause untold amounts of grief and wasted time when people attempt to persue it.

Finally someone said it!!!! Too bad it took me so long to figure out for myself!

[Sparky Paul]

Phil wrote:
There could be terminology issues cropping up here but I would say that once you set the CG then stabilizer incidence or trim is fully determined. The interaction taking place is that CG determines elevator incidence or trim. The two things (CG and elevator trim/incidence) are not independent variables.
"
.
This is correct... for a single speed, such as a free-flight would use.
With power, trim changes as speed changes. Or ballast, for TD gliders.
It depends on where in the flight envelope you want the trim to be set for, and then change as suited for the speed being flown..
Weight will have an effect also, but most electrics don't change weight while flying.. but can change with different equipment installations.

[Phil Barnes]

I have no disagreement with anything Sparky Paul has said. He has clarified a point that I should have been more clear about. You set the trim for a particular speed.

He has reminded me of another point that should be added. There is a fundamental difference between a free flight model versus an RC model. A free flight model has a fixed stabilizer incidence. This incidence does not change during flight unless there is some mechanism that is used to adjust the incidence to two or three discrete positions for different phases of flight (boost and glide for instance). On a free flight model you would typically set the stabilizer incidence first and then trim the model for a flat glide with CG adjustments. My free flight background was with rocket boosted gliders. Those models would typically have two different incidence settings. The incidence for glide would be set such that the model was sure to pull out of any unusual attitude the model might find itself in after the boost. This required a certain relationship between the wing and tail which was derived by just trial and error. The incidence would be adjusted with a small movable elevator that would move down slightly for boost. This reduced incidence setting for boost would cause any looping behavior of the model to have a very large radius. A spin tab on the vertical fin would then cause the model to roll on boost so that any looping behavior would turn into a spiral climb instead of a loop into the ground. On these models it was very important to set the stabilizer incidence to a very precise position to attain the straightest possible boost. The stabilizer incidence was adjusted with the movable elevator on the back of the stabilizer. The correct position was determined by test flying the models. The exact position for the movable elevator (and hence the stabilizer incidence) was less critical for glide. More up elevator would just require more nose weight to achieve a flat glide but would provide more positive recovery from unusual attitudes.

The point here is that fine tuning of stabilizer incidence on a free flight model is a perfectly reasonable and even perhaps necessary thing to do. There is still a fixed relationship between incidence and CG. It's just that you set incidence first on a free flight model and then trim for a flat glide with CG adjustments. Or at least that's how I did it on free flight rocket gliders.

Now back to RC models; The fundamental difference here is that RC models are..........radio controlled. This means that the elevator is controlled and movable during flight. Whenever the elevator moves, the model's incidence changes. This is why it makes no sense to attempt to fine tune an RC model's stabilizer incidence for any other reason beyond the desire to set your trims such that the model will fly flat (at the desired speed, thank you, Sparky) when you take your hand off the transmitter stick.

By definition, decalage (the angle of the fixed portion of the horizontal stabilizer relative to the wing) on an RC model does not change during flight. You may be inclined to adjust an RC model's decalage such that the elevator is not deflected at normal glide speed. This would be done only for the purpose of having the elevator neutral at the most commonly used speed. Setting the decalage in this way will do no more than eliminate a little bit of excess trim drag that would result from having the elevator more deflected than it needs to be. This action of setting decalage should not be confused with setting stabilizer incidence. Stabilizer incidence on an RC model is constantly changing during flight whenever the elevator moves.

[Ollie]

Phil, right on!!!!!

[rand24]

I would very much like to simplify my trimming process. I have been at it since the beginning and I think one comment in this thread might just clarify the whole shebang for me.

"You set the trim for a particular speed."

Fine, perfect in fact, what I don't have any experience in is the "size" of the flight "envelope". When I set my dlg for flat glide in dead air I would guess the airspeed at 6mph, a relatively slow jog. If I am lucky enough to find some lift, and subsequently need to make my way back home at some point I will tend to drop the nose and double (or more) the airspeed. For me, this increase in speed usually results in a model that wants to pull up- and requires a touch of down elevator.

(CG to far forward you may say. OK, but believe me I have flown the thing with the CG way behind the acceptable range as well. I've flown it this way to show me what a too rearward CG looks and feels like - twitchy yup funky on launch - check.)

If I knew that a typical dlg model flew differently when "coming home" than during the flat glide portion of flight that would help me to trim for one speed (glide) and take whatever else came and just move on to becoming a better pilot. I suppose a 100% increase in airspeed (6mph to 12mph) might reasonably be expected to take the airplane out of it's envelope. I don't know. This my question in case I'm not being clear.

The pullup that comes with additional speed seems to be convincing me the plane is not trimmed (or balanced) correctly. Incidence/decalge/CG fiddling ensues.

Seperately, I've convinced myself that the better dlg pilots I've seen seem to fly search at a higher airspeed than I do - For whatever reason I like my plane to float along just above stall speed- with the tiniest elevator input sometimes causing stall. Any thoughts on dlg cruise speed? Again, seperate issue.

[SoCalGliderFlyr]

I remember one flyer using a servo controlled sliding weight to change the flying characteristics of his sailplanes.

[Phil Barnes]

When I set my dlg for flat glide in dead air I would guess the airspeed at 6mph, a relatively slow jog. If I am lucky enough to find some lift, and subsequently need to make my way back home at some point I will tend to drop the nose and double (or more) the airspeed. For me, this increase in speed usually results in a model that wants to pull up- and requires a touch of down elevator.No problem here. Assuming that you have an aileron DLG model that utilizes camber changing; When you flip your camber switch or move your slider from thermal camber to cruise or speed camber you should also have the transmitter programmed to add a couple/few clicks of down trim which will, in conjunction with the camber change, retrim the model for a fast cruise. The bit of down trim will eliminate the model's tendency to pitch up at the faster speed and allow the model to once again fly at this new speed with hands off the transmitter stick. That way you will not be "over-piloting" the model and will be better able to recognize a patch of lift that you might happen to fly into on your return journy.

(CG to far forward you may say. OK, but believe me I have flown the thing with the CG way behind the acceptable range as well. I've flown it this way to show me what a too rearward CG looks and feels like - twitchy yup funky on launch - check.)It sounds like you have a good handle on the effects of CG change and are well able to set the CG to your own preference. You should go ahead and set your CG where you like it and then leave it alone. The issue you are describing (trimming the glider to fly hands off at the desired speed) is not dealt with by adjusting the CG.


If I knew that a typical dlg model flew differently when "coming home" than during the flat glide portion of flight that would help me to trim for one speed (glide) and take whatever else came and just move on to becoming a better pilot. I suppose a 100% increase in airspeed (6mph to 12mph) might reasonably be expected to take the airplane out of it's envelope.Doubling the model's airspeed will only have the effect of taking the model away from it's trimmed airspeed. This only means that you will either have to hold down elevator while the model is flying faster or you will have to program some down trim for fast cruise flight as described above. You should still be able to set your trims for a flat glide at the higher return airspeed it's just that the faster trim speed will require a bit of down elevator trim relative to the slow glide speed.

The pullup that comes with additional speed seems to be convincing me the plane is not trimmed (or balanced) correctly. Incidence/decalge/CG fiddling ensues.Think trim and only trim. The trim in question is the elevator trim (either electronic or manual slider)that is a feature of every RC transmitter. If you want the plane to be trimmed to fly at a faster airspeed then you need a couple/few clicks of down trim. If you don't want to mess up the trim speed that you have set for slower flying then use a mixer to mix in a bit of down elevator for the faster cruise phase of flight.

Banish all thoughts of incidence/decalage/CG from this issue. They have nothing to do with it. Remember the following;

1) Incidence changes constantly on an RC model. Every time you move the elevator control you are adjusting the model's incidence.

2) Decalage adjustments (changing the angle of the fixed portion of the horizontal stabilizer relative to the wing) do not in any way trim the model or even adjust the model's incidence. A decalge change will always be followed by a retrimming of the elevator which negates what otherwise would be a change in incidence. If you were to adjust the decalage without retrimming the elevator then the model would no longer be trimmed for level flight, it would either be trimmed for a severe stall or a sharp dive into the ground. Retrimming the elevator after a decalge change to get the model to fly at your chosen trimmed airspeed again will reset the incidence back to where it was before the decalage change. In the end, the only effect of a decalage change is to reset the relationship between the elevator and the fixed portion of the stabilizer. You typically aim to have the elevator streamlined to the fixed portion of the stabilizer at your chosen trim speed. This is done only to minimize trim drag that results from a deflected elevator.

3) CG adjustments are done only to adjust the model's flying qualities. You have the choice of stable and less responsive (forward CG) or less stable but more responsive (aft CG). CG adjustments are never used to trim an RC model.

Seperately, I've convinced myself that the better dlg pilots I've seen seem to fly search at a higher airspeed than I do - For whatever reason I like my plane to float along just above stall speed- with the tiniest elevator input sometimes causing stall. Any thoughts on dlg cruise speed? Again, seperate issue.Better pilots will fly fast when in known sink. They might even fly fast when searching in unknown air. The really good, world class guys can read air even when flying at fast cruise and so they can read air even when penetrating back upwind at a very fast airspeed. Other guys (like me) have a hard time reading air at a fast cruise speed and so may be forced to choose between a fast return versus being able to read the air I'm flying through.

Floating along just above stall speed is OK if the air is relatively calm and smooth and you are not in sink. Such a slow speed is a bad idea in turbulent conditions and an even worse idea if you are in sink. If you are sucessfully reading the air and only slowing down to this slow speed when in lift then it may be OK. You would need to immediately switch to a higher speed (you should have a speed switch that puts the model in low camber with a bit of down trim) when you encounter sink and begin a search for better air. Wallowing in sink at a real slow airsped is the kiss of death and is something that novices tend to do. Novices will either not recognize that they are in sink or will notice that the plane is coming down fast and try to stretch the glide by pulling back on the stick. Of course, more experienced pilots will recognize the sink and immediately click on the fast cruise setting and get out of the sink ASAP.

[Ollie]

1982 F3B champs. Swiss planes with servo to lead wt. to adust CG in flight. It was a bad idea.

[Peytr]

Hello Phil,

No need for a flame war. We're in here for discussion in the first place.

First about the 0 deflection mythology issue, think of it as follows:

If you need the elevator up at cruising (or thermalling) speed, the stab is not working very efficient. If you have to give a slight up all the time, its better to change the stab incidence all together, simply because the up elevator induces more vortex and profile drag in the stab wing. By changing the stab incidence you can have the same downforce with a less distorted surface in the stab wing, resulting in less drag from it. It surprises me how many RTF and other kits fly with constant defection as a result from too little (or too much) decaleage.

Secondly about the sweet spot mythology:

We all know a middle of the road wing section has a negative pitch tendency. One of the basic tasks of the stab is to balance this negative pitch moment by providing a downforce at the other end of the NP. By trying to find the most rearward spot for the CG at which the plane can still be flown without getting to twitchy, this downforce needed can be minimised. If this is done with 0 deflection one hits the sweet spot which can make a discernable (though not dramatic, this is where the mythology start imho) difference in performance.

There is a third aspect to this: The fuselage. Please read on before flaming :D

If the wing is at 0 degrees to the fuselage the plane will be constantly flying with it's tale down to give the wing the right angle of attack. The fuslage is representing a very large frontal area this way. That's why all wings are at a positive angle to the fuse to get the fuselage flying at the right angle. When you build a plane (out of a kit, ARF or completely scratch which I do mostly) the wing allways roughly is in the right angle, but hardly ever at the optimum. Fact is that if you slightly change the wing / fuse angle you'll have to adjust the stab/fuse angle to prevent the need for excessive stab deflection.

There's no mythology here but I'll slip in my asbestos suit, just in case :o

[Sparky Paul]

Flying tails get rid of the small drag penalty a deflected elevator creates.

[jrerickson]

Randy,

In your photo section it looks like one of your planes is the Sky Sergio. Check the cg. If it is a 9" chord, the cg should be around 3 1/2" from the leading edge. This is about 38% of the mean average chord, a good starting point for cg. I suspect your cg point is way forward of this.

The smaller planes that have a higher wing loading need to fly faster. You may be more comfortable with a slightly forward cg, say 34%. You'll have to trim for a faster glide, but you should be able to find a spot where you don't need any elevator input at all.

As you move your cg back you will need a little bit more down trim, but your porpoising should stop.

John

[Phil Barnes]

Hello Peytr,

We are in near perfect agreement now. No need for a flame war (there never is) and no incidence mythology in your last post anyway.


First about the 0 deflection mythology issue, think of it as follows:

If you need the elevator up at cruising (or thermalling) speed, the stab is not working very efficient. If you have to give a slight up all the time, its better to change the stab incidence all together, simply because the up elevator induces more vortex and profile drag in the stab wing. By changing the stab incidence you can have the same downforce with a less distorted surface in the stab wing, resulting in less drag from it. It surprises me how many RTF and other kits fly with constant defection as a result from too little (or too much) decaleage.We are in complete agreement here. Nothing more than a simple misunderstanding or maybe confusion of terms to deal with. Everything you have written here, in this paragraph, is about setting the decalage to provide for the smallest possible elevator deflections during flight. There is some confusion of terminology when you say to "change the stab incidence altogether" what you are really attempting to say is to change the decalage. I think you agree with this since you correctly use the term decalage in your last sentence. I agree with you completely that there is some (small) benefit to be had by setting decalage so that there is zero elevator deflection at the most commonly used trim speed. It's just that there is only a very small gain to be had there which has only to do with a slight drag reduction and nothing to do with gross flying qualities.

Secondly about the sweet spot mythology:

We all know a middle of the road wing section has a negative pitch tendency. One of the basic tasks of the stab is to balance this negative pitch moment by providing a downforce at the other end of the NP. By trying to find the most rearward spot for the CG at which the plane can still be flown without getting to twitchy, this downforce needed can be minimised. If this is done with 0 deflection one hits the sweet spot which can make a discernable (though not dramatic, this is where the mythology start imho) difference in performance. This level of aerodynamics is over my head and so I cannot confirm or deny the bits about downforce, negative pitch moments and what effect they might have on performance. Your overall point is in complete agreement with my postings. 1)You set the CG as required for flight characteristics (include a tad of performance improvement for aft CG if you are correct), 2)you trim the elevator for level flight at your selected trim speed and 3)finally adjust decalage to remove elevator deflections at this trim speed. Once again I think we are in complete agreement. I just don't use the term "sweet spot", I don't analyze the tiny performance gains as completely as you do and (perhaps there is still one small area of disagreement) I think the tinkering stops with one itteration of the above three step process. I don't see the need for returning to step 1) again "due to interactions between incidence and CG".

There is a third aspect to this: The fuselage.

If the wing is at 0 degrees to the fuselage the plane will be constantly flying with it's tale down to give the wing the right angle of attack. The fuslage is representing a very large frontal area this way. That's why all wings are at a positive angle to the fuse to get the fuselage flying at the right angle. When you build a plane (out of a kit, ARF or completely scratch which I do mostly) the wing allways roughly is in the right angle, but hardly ever at the optimum. Fact is that if you slightly change the wing / fuse angle you'll have to adjust the stab/fuse angle to prevent the need for excessive stab deflection. No mythology here and I agree completely. This is just another case where you are more meticulous than I am about squeezing every little bit of performance out of a model. To my way thinking, the model designer is responsible for setting the wing at an angle that minimzes fuselage drag and I assume that he has done so. I would expect only certain guys to be paying close attention to fuselage drag to the point where they would be fine tuning the wing's incidence relative to the fuselage. These people would be guys doing F3B speed runs or maybe slope racing. As a simple thermal duration guy I don't get interested in fine tuning the angle at which the fuselage meets the air as long as it is somewhere in the ball park.

[GaryO]

I only found this thread, and found it very interesting. A couple of comments:

Someone earlier suggested the stab may be stalling. I don't understand the logic of this, as the wing should always be flying at a higher angle of incidence relative to the airstream, than the stab, not even considering the downwash form the wing.

Fuselage drag may be undersetimated, certainly for hlg and probably for thermal. It seems to me that in addition to the angle of incidence that the wing is flying at in the airstream, the entire airplane is sinking in the airstream. Dr. Mike Selig has apparently published some info on fuse shapes to minimize drag, and for a look at a plane that tries to use his theories, look at Harlely Genie (www.genie.com).

Incidently, if anyone has a reference to Selig's actual article, I'd certainly like it.

Gary

[rand24]

Thanks all and especially Phil for the info - it's a great help, to me at least, to know that just because my planes acts differently at different airspeeds it does not necessarily require CG adjustment. This thread will probably help me become a better flyer by allowing me to pay much less attention CG incidence etc.

Essentially here is what I'm going with:

CG near the rear part of an acceptable range as determined by the many who fly my plane (xp4), stab trimmed to provide my desired speed, as long as the elevator is pretty near "flat" I'm good to go. Radio (8103) programmed to help me get near hands-off handling at different camber settings/airspeeds - fly.

I'm not promissing I won't occasionally fiddle, but I'm going to try to forget the idea that the correct CG/inc et al. will change my plane from 1:20 in dead air to 1:50.

Thanks!

[Sparky Paul]

Rand, just keep in mind any specific elevator trim is good for a specific speed.
Go to a different speed, and the elevator must be retrimmed, or held off the trimmed setting.
Something I haven't seen mentioned.. elevator controls airspeed.

[BMatthews]

..... When I adjust trim in flight (computer radio) by a few clicks up the porpoising gets worse, if I adjust the trim down be a click or two the nose wants to start dropping, I pick up airspeed and the planes dont get what I would call a flat glide....

In re-reading this bit from one of your earlier posts I'm now of the opinion that you've set up your CG correctly and that it's probably a little further back than you really want it to be in order to have a USABLE trim range.

I think your porposing may just be a series of very shallow stalls. Certainly if you can go from this series of shallow stalls to actually speeding up and trying to tuck into a dive with only a couple of trim clicks then this is too sensitive.

Things to try in this order.....

Program your trims for less throw volume so your trim range is wider at the Tx. Perhaps each click is moving the stab too far so your trim steps are quite coarse. Total trim throw should be about 1/8 inch at the trailing edge total.

If the trim volume is within the above spec and it's still too small a usable range then I would try moving the CG forward in 1/8 inch steps until you have a good useable 7 to 9 click range of operation between the porpoising to the model going into a shallow dive. This will let you set your thermalling and high speed flat cruise over a 4 to 5 click range.

Moving the CG forward like this is actually making the model less efficient but it greatly increases the ability of the pilot to do the flying more effectively. It's a tradeoff but a good one. The most efficient flying model is no good if the pilot can't fly it effectively.

And be aware that as the model trims down out of that gentle porpoising action that the flying speed WILL be a trifle higher. After all, it's not mushing anymore. But you should find that when the porpoising is just barely gone that the model has a nice flat glide. You may just need to get used to the idea that faster can still be efficient. Particularly for the heavier loaded electric models.

[Peytr]

Hello Phil,

Interesting matter this is, isn't it. We fully agree, I can see that. I guess translation makes things more difficult than they should be. One more remark.

I don't analyze the tiny performance gains as completely as you do and (perhaps there is still one small area of disagreement)

You are definately right, the differences are small to very small. However, I have been tinkering with very small thermal gliders mostly ( around 1 meter or 3' span) which are very difficult to trim well. If you want nice flat land thermalling from a 1 meter plane one simply has to squeeze out every bit you can get from it. I guess it's this what made me a bit picky. I just refitted an old Electra (out of all things) and there simply isn't a reason to do the whole repertoire. It hangs in there with no need for this minor adjustments.
When trimming a 1 meter, on the other hand, I can't stop putting shims under the wing and refitting the stab to get the last drop of performance from it, which is one of the attractive aspects of mosquito class gliders to me.