Hubb

My Feedback: (1)

It doesnt look like I will get to build anything this winter, unfortunately. I would like to revisit my old back-up plane to see if we can make it fly a little bit better. the plane in question is a Lanier Staudacher. It's not flashy and expensive but has served me very well moving up from Sportsman to Advanced. last year I tried to fly some Unlimited with it, but put it away after some problems and characteristics I blamed on the design (not my lack of flying skills) anyhow, my primary plane is a Radiocraft 35 extra and will stick with it this season if I can. here is the set up on the Staudacher:

Lanier Staud
24 lbs (light as I can get it)
BME 105
1 Hitec 5945 each ail and elev
1 5735 giant on rudder pull-pull
twin NiMh Rx batts

modifications:

tail group pulled back about 2 1/2 inches from plans
flat plate tail sufaces sheeted and reinforced with CF to make stiff
foam wings cored to save wing weight

early last year I built and added a ridiculous huge new rudder, thinking I needed it for rollers and such. I have since cut that off and reinstalled the original rudder (still a bit larger than plans). elevators are not counterbalanced.

the plane has always had some weird rudder coupling. weird meaning that it seems to change with airspeed and attitude. a good mixed KE across the field would yeild a weird pitch at the top of an upline with rudder corrections. all pitching is toward the belly. somtimes severe enough to snap out - again at slower speeds like the tops of uplines. I would like to know if that pitching can be lessoned or eliminated. or should I give up on this plane?

things I can change easily:

CG, I am using two 1650 NiMh Rx packs. so shifitng them would make significant CG adjustments. I feel that I am on the nose heavy side of neutral. perhaps going more neutral / tail heavy?

I can somewhat change motor thrust. move thrustline slightly up or down on firewall. as far as right thrust I have about 2deg and slight downthrust.

I could break the wing mounts loose and adjust wing incidence.

I could not so easily cut a new rudder shape. more taper? less taper? is this significant in pitch coupling?

not so easily, I have stab and rudder cores from another 35% airplane. I guess I could hack the tail off and add the other (airfoil) tail surfaces, although I would rather NOT. I dont think I can drop the existing stab without rebuilding the whole tail anyway.

really not easily I could move the whole wing tube up closer to the thrustline

really, really not easily I could drop a grand$ on a whole new airframe. ( I might get thrown out of house for that)

so what do you say guys, can this old airplane be trimmed and made to fly the unlimited sequence? when do you just give up on an airplane or design?

Hubb

GRH

Hub,
Sounds like pitch coupling is ruining your day. I've written some things about this subject that I'll attach here. Does this airplane have a large rudder horn balance? If so you may want to cut it off fixing the rudder area infront of the hingeline to the fin. This would be an easy fix.

Having a forward CG will tend to reduce pitch coupling to the gear...don't move the CG aft or the pitching to the gear will worsen.

Pitch coupling is typically nonlinear so I like to use the multi-point mix curves. You also will typically need more nose up mix with left rudder than right because of P-factor.

The Staudacher isn't know for it's knife-edge ability because of it's lack of side area...this lack of side area forces the airplane to fly at either very high speeds or high sideslip angles to perform knife-edge flight . Note that pitch coupling is directly linked to the sideslip angle.

Honestly I wouldn't waste time with this airframe if you're having serious issues with its handling qualities unless you simply want to experiment. It sounds like you'd have to put forth a considerable amount of effort to make some of the changes you've mentioned. There are so many great 35% airplanes out there that it makes sense to focus on other things...however I know that money is always a major concern. Some of the 35% ARF are so cheap these days and fly so well. That 33% H9 Extra is a great example.

To fly the Unlimited sequence you need an airplane which snaps extremely well and has lots of power in order to maintain vertical speed in the uplines. If your Staudacher did well in Advanced it should do well in Unlimited unless it's underpowered. I can't see how these pitch coupling problems that you're seeing now didn't drastically affect the sequence in the lower classes. If you're going to seriously fly Unlimited a top-notch 35% airplane like the Radiocraft is IMHO the minimum that you can get away with. The larger 40% provides such a noticeable visual and pace advantage that it's hard to compete with.

I wouldn't change the wing incidence...does the airplane have a straight upline with the downthrust you've mentioned or does it pitch to the gear on an upline?

Before you do anything read this old article and see what you can glean from it. Perhaps it will help you figure out your pitch coupling problems.


****************** ******************
Hi All,
I’ve been thinking about this knife edge pitch coupling phenomenon for quite some time now and have come to some limited understanding of the problem that might help put together some of the pieces of the puzzle. First I should say that the pitch coupling isn’t dependant on whether you fly in a knife edge attitude but rather it is dependant on the airplane’s angle of attack, angle of sideslip and rudder deflection. I noticed that one person identified a situation in which the application of rudder caused the airplane to pitch toward the gear regardless of the airplane’s attitude. As most of us know the airplane doesn’t know if it’s upright, inverted, vertical, on its side, etc. Such is the case with the pitching moments that we experience in knife-edge flight in that they aren’t dependent on the plane’s attitude. The interesting part of all this is what physically causes the airplane to exhibit these cross-coupling tendencies. I’ve isolated at least three things that I feel are the main contributors to this pitch-coupling problem. I’m certain that there are more so I don’t want anyone to think that this list is exhaustive. Every time I look at this problem from a different perspective I find another possible contributor to the pitch coupling phenomenon…that’s what makes it so interesting.

1) PITCHING MOMENTS DUE TO SIDESLIP

By sideslip angle I simply mean the angle of attack of the airplane in a directional sense. In the Aero world we typically refer to the angle of attack and the angle of sideslip as “alpha” and “beta” (greek symbols). With this in mind I should say that most aero types also believe that the airplane only cares about alpha and beta. We like to nondimensinalize things so we don’t have to confuse the issue with things like wing loading, airspeed, air density, g’s etc. Aerodynamically the airplane doesn’t really know anything about those dimensional things. In the real world we can’t, as R/C pilots, go around proclaiming that my airplane was flying yesterday straight and level at a lift coefficient (CL) of 0.001 and get all excited…people would think we were crazy. However, we can and do get excited when we say, “You should have seen my airplane, It was going 250 mph…that’s cool!”. Everything has its place even dimensional quantities, but from the airplane’s point of view the forces and moments that affect it are only a function of five things: 1) angle of attack , 2) angle of sideslip, 3) Reynolds number, 4) Mach number, and 5) the airplane’s geometry. For now, lets neglect Reynolds number and Mach number by saying that we aren’t going too slow or too fast for either of these to be big factors. Now that I’ve explained what I mean by sideslip, we’ll proceed to why it affects pitch coupling.

A) FUSELAGE SHAPE If one considers the fuselage or body by itself you can start to see how some shapes might be prone to producing an aerodynamic pitching moment when the body is in a sideslip. Let’s consider a fuselage in which the aft section of the turtle deck is rounded on the upper surface but flat on the bottom. Such a shape while sidesliping would accelerate the air more over the upper surface than over the lower. Consequently the pressure would drop over the top of the fuselage’s turtle deck and cause a nose down pitching moment to be generated solely by the fuselage shape. I’ve played around with this in the wind tunnel using an old Calypso (Prettner design) fuselage and found that this nose down pitching moment could be measured and was nearly equal to 2 degrees of up elevator at 15 degrees of sideslip. Don’t make the mistake of thinking that my Calypso had a knife-edge-pitching tendency because it didn’t. You have to consider the total package before you make that kind of statement. As a complete airplane one could easily set it up such that you wouldn’t see any noticeable pitch coupling until very large sideslip angles. A good example of an airplane that has a large disparity between the upper and lower aft fuselage cross-sectional shape is the Cap 232. It typically exhibits a noticeable nose down pitch coupling tendency in knife-edge that is partly due to its fuselage shape. You can see this effect if you put a “aerodynamic fence” so to speak down the centerline of the turtle deck. When you fly this new configuration you will usually see a decrease in the amount of nose down pitch coupling because the fence is reducing the air’s acceleration thus reducing the pressure drop.

B) VERTICAL TAIL (sweep, span, projected area) This is a more difficult effect to explain but never the less it can be a very dominate one especially if the vertical tail is very thick, and has a large sweep angle. I should say that any airplane that has a vertical tail that only sticks out of the top of the airplane (which covers 99.9% of all airplanes in existence) would exhibit this tendency to some extent. Let’s see if I can explain why… When you sideslip, the airplane’s vertical surface acts in the same manner as a wing and generates lift because of a change the vertical tail’s angle of attack. This lift is created by a change in the pressure distribution over the surface of the vertical tail. Typically the leading edge area (L.E. to 25% of local chord) of a lifting surface has a fairly large pressure drop over the leeward side. If you look at your airplane from the front and the top and then estimate how much projected frontal area your vertical tail has from each view you will get an idea of how much ability your vertical tail has to create a pitching moment while in sideslip. In other words, consider that a rather large pressure drop occurs over those projected areas while the airplane is sidesliping. This pressure drop when multiplied by the projected area gives you a force whose vector is above the airplane’s center of gravity (CG). If you think about it and draw a few pictures you can see how the vertical tail’s height and sweep can increase the moment arm and thus increase the nose down pitching moment due to sideslip. I found this out while working on an airplane that had a rather large vertical tail sweep angle. I then started to look for it in other airplanes and found that almost all standard airplane configurations have the same tendency to pitch down due to sideslip and that the vertical tail is a large contributor to this effect. If one were to design airplanes that had symmetric vertical tails (one top and bottom) we would alleviate a large amount of this while also getting rid of the need for right thrust.

C) ONLY THE WING AND HORIZONTAL TAIL Even though you can’t actually build just a wing and a horizontal tail and fly them together without some sort of fuselage, you can still consider the problem and draw some insight to what you have before you ever add a fuselage or vertical tail to the picture. Consider a wing and horizontal tail with symmetric airfoil sections both at zero angle of incidence relative to each other. If you place this combination in sideslip you will get zero pitching moment if the angle of attack of the system is zero. What if the combination is held at some angle of attack and you sideslip at this constant angle of attack…suddenly a small amount of pitching moment will appear that changes as a function of sideslip…Why? The answer lies in the downwash change that the tail sees as the sideslip angle changes. The spanwise downwash distribution for most planforms isn’t constant (theoretically it is for an elliptic planform…but in reality this isn’t exactly true) and this is even more true when the wing is in sideslip. This effect is a function of the main wing’s lift coefficient, the vertical and longitudinal position of the horizontal tail and the center of gravity. The purpose of pointing this out is to show that even with the simplest longitudinal model you can’t get completely away from pitch coupling. This sort of simple approach shows the importance of the downwash field and how the wing’s wake affects the tail’s ability to perform. Keep in mind though that everything’s relative and if you operate at very low CL’s (light wing loading/high speeds) these effects are minimized. You also want to get the tail out of the wing’s influence by having it be far away from the wing…kinda sounds like a pattern plane doesn’t it?

D) VERTICAL POSITION OF THE HORIZONTAL TAIL (This is the one that everyone’s so concerned about!) There is much debate as to what effect this has… The general consensus is that if you lower the tail it makes the airplane more prone to pitch to the canopy and if you raise the horizontal tail the airplane will certainly pitch to the gear…is this true? I can say that if you measure this in the wind tunnel you will find the opposite to be true…to a point. As the horizontal tail gets further away from the wing, the downwash it sees is certainly less. As Mike Nauman once eloquently wrote “You can’t get away from a whirlpool by swimming towards it!” this is the exact same situation that the horizontal tail deals with while the wing is producing lift. The main effect of downwash is that it reduces the static longitudinal stability of the airplane. Technically the downwash doesn’t reduce the airplane’s stability but rather how much the downwash changes per unit change of angle of attack. (If) a 10 degree change in angle of attack produced a 10.1 degree change in the downwash where the horizontal tail is located the tail would cause the airplane to be unstable. Yes you heard it right, If you put a horizontal tail on the airplane behind the wing it could become less stable than with the wing alone…but only if the rate of change of downwash with angle of attack was greater than 1. Luckily, downwash’s change with angle of attack is almost always less than unity thus a tail is a good thing. How could you possibly get a configuration where putting on a horizontal tail behind the wing is destabilizing? If you had a very low aspect ratio wing with the tail located very close to the wing’s trailing edge you could get this sort of situation…I know it sounds crazy but it’s been done. We don’t have this problem in pattern but sometimes this example it helps in the overall understanding of how things work.

Typically pilots like the feel of a certain amount of longitudinal stability which I will call “static margin” (“Static” because we are only talking about steady-state situations and “margin” because it represents how much CG margin you have before the airplane becomes neutrally stable in pitch). To keep the same amount of static margin you would have to move the CG forward as you lower the tail. If you keep the CG in the same place as you lower the tail you will need down elevator to retrim the airplane and the airplane will become less stable in pitch. If you continue to lower the tail the downwash’s effect would lessen just like it would if you raised the tail far above the wing. This means that at some horizontal tail position far above and far below the wing the required elevator trim would be identical if you neglect the drag moment you get from having the tail way up high or down low. (not to mention the drag you would get from the vertical tail that it’s attached to) This idea also points out that having a high tail will cause a slight nose up pitching moment from the drag of the tail (for a low tail the opposite would be true)

Another effect comes from the fact that the fuselage and wing have a wake and boundary layer associated with them that extracts momentum from the air. If the tail is forced to operate in this type of environment because of its relative position to the wing, it would certainly lesson the tail’s effectiveness. This may be what’s happening in practice when people lower the tail and the pitching tendencies reduce greatly…who knows.

E) POWER EFFECTS IN SIDESLIP As with any situation on a propeller driven airplane you must take into consideration the power effects… Just note that the propwash is skewed greatly behind an airplane that is flying with a large amount of sideslip such that the propwash will eventually align itself with the freestream. Since the propeller only spins in one direction, its effect will change sign whether you sideslip to the left or the right. This is a very esoteric and unquantifiable effect but theoretically it should be there. In one direction there should be upwash on the tail and on the other there should be downwash. The same goes for the P-Factor. If you see that the airplane pitches one way when you fly knife-edge with right rudder and the other way with left you might be led to the conclusion that power effects are causing the problem.



2) PITCHING MOMENTS DUE TO RUDDER DEFLECTION

This is a bit different than the pitching moments due to sideslip because in this case we consider that the airplane is at a zero sideslip angle and we are only deflecting the rudder. Why would this cause the airplane to pitch? What happens in this case is that the close proximity between the rudder/fin and the horizontal tail can cause them to influence each other. This is highly dependent on how close the tails are to one another so the geometry of the situation is critical. Let’s pose a simple example for illustration: Consider a T-tailed airplane where the rudder is deflected 30 degrees. The rudder deflection produces the same effect as a flap deflection on a wing. The trailing edge control surface alters the pressure distribution around the whole airfoil. Typically one will see that the suction side is much greater than the pressure side hence the idea that the wing is more or less sucked into the air rather that pushed. This is extremely poor terminology but I think the idea is conveyed…take a look at any airfoil data with a control surface deflected and you’ll see what I mean. Whenever the pressure drops the flow accelerates and if this happens close to the horizontal tail, it will be influenced also. In our example of the T-tail the pressure drops more on the underside of the horizontal tail than on the top thus one would expect to see a slight nose up pitching moment from a rudder deflection. You can see this in the wind tunnel and after you think about it for a while you realized that the reason this happens is because the suction drop on one side of the vertical tail is larger than the pressure rise on the other. This pressure drop is what alters the pressure distribution on the horizontal tail. One can imagine how sensitive this effect would be to things like the rudder and fin planform and the vertical tail height. The rule of thumb idea is that a high horizontal tail position will cause a slight nose up pitching moment with rudder deflection and a low tail will cause a slight nose down pitching moment…but only if the rudder hingeline isn’t swept.

If the rudder’s hingeline is swept the rudder will act sort of like an elevator. This effect could overshadow the effect of the rudder’s deflection on the horizontal tail’s pressure distribution. Now you see how the overall result you get while flying is a giant melting pot of small effects. Sometimes it’s hard to sort out the true “cause and effect”. If your airplane doesn’t pitch in knife edge you can pat yourself on the back and proclaim yourself a genius until you have to help your friend who has a coupling problem. If he’s a fellow competitor I guess you just keep your mouth shut and play dumb )


3) CENTER OF GRAVITY!!!

This is the most important thing to consider when you’re confronted with a pitch-coupling problem. Rest assured that there is a CG position that will cure your knife edge pitching problem but you may open up another can of worms by making the airplane’s handling qualities go to pot in other flight regimes. The designer who can balance all of these effects and make each phase of the aerobatic sequence easy to fly is the true “genius”.

The well-known effect that the CG has on pitch coupling is:
If you move the CG forward you will have to trim the airplane with more up elevator. When you fly in knife-edge at some sideslip angle, rudder deflection (which we know can produce a pitching moment) and angle of attack, you will have a counter pitching moment that comes from the elevator trim. This trim setting can cancel a nose down pitching moment, which comes from the sideslip, rudder deflection, etc.

The opposite is true if the CG is moved back. Many people have adequately explained this over and over so I won’t belabor the point, but I would dare to say that the CG is equally important if not more than the vertical position of the horizontal tail.


If you made it this far I certainly appreciate your patience. I hope what I’ve said hasn’t confused the issue too much. I’ve thought about this for years and I still find it interesting. I hope that everyone continues to strive for that perfect pattern design and will be better able to do so armed with a little better understanding of why things work. Nobody understands it all…that’s what makes it so challenging and fun at the same time.


George R. Hicks

RC_Dave(ORCC)

Holy Crap!!!!!

Anyone got an spirin for my headache?!?!

Just kidding, while I won't pretend to understand all of that, there certainly is a fair amount that makes some sense when given an appropriate amount of time to digest.

Thanks, George

Cheers,
Dave

Anyone wanna go for a beer and design the "perfect" plane with me?

Hubb

My Feedback: (1)

Wow George! I appreicate you taking the time to respond! I was hoping for an interesting discusion, but your response was above and beyond.

after posting my question I did some more reading on the subject and came up with this quote I picked up out of the "pattern" forum.

quote from MTK in pattern forum...

I did not find reference in your response to the wing CP (unless I was too cross eyed from reading and missed it) I was all prepared to raise the incidence on my wings after reading that. BUT what I have gleaned from your research leads me straight to the design of the fuse side area of the Staudacher! it makes sense to me after reading about the sideslip effects on a fuse with a rounded turtle deck on top and a flat (square) bottom. I see that I may be fighting a loosing battle then, but I am still interested in playing with this airframe to see what effects of this pitching I can overcome or minimize. it may be worth the time spent to possibly make a few observations that could help later down the road. I guess winter up here in the north east can make you think of these strange things

Hubb

why_fly_high

My Feedback: (19)

I don't really have much to add to this one but one thing of note is that Diana Hakala (sp?) was on the U.S. aerobatic team a few years ago with a Stadacher, full-scale. She had a rather large fin added to the bottom of the fuse. I don't know if it was added for increased yaw stability or if it had an effect on yaw-roll/pitch mix. The plane was white with blue and pink stripes. All that said to point out that if you want to add something to the bottom of the fuse, it would still be IMAC legal.

Poking my nose into things I don't understand,

Dan

GRH

Hub,
No you're not cross-eyed...it's not in my article because I don't subscribe to that line of thinking. It's actually one of my pet-peeves.

This will probably sound bad but the quote you present here shows me that there is a clear lapse in understanding of airplane stability and control in the precision aerobatics community. I see some things in the quote that I've had issue with before with regards to certain pattern related columns in the magazines.

First note that saying that the rudder has a low CP (center of pressure) is a gross generalization. Even though the author doesn't state this explicitly, the logic presented assumes the the rudder deflection somehow severely biases the pressure distribution on the horizontal tail making the pressure on the underside of the horizontal tail much higher than on top of the horizontal tail. While the rudder can slightly alter the pressure distribution on the horizontal it has very little control over the horizontal tail's lift coefficient and consequently results in very little pitching moment. My computer models show that the the elevator required to trim this situation is about a 0.2 degrees for most common configurations. In other words CM_dr is very small. Wind tunnel tests I've run on various configurations also show similar results.

Note:
CM_dr = d(CM)/d(dr) = change in pitching moment coefficient with a change in rudder deflection


Second there is no natural nose-down pitching moment emanating from a wing...this is a fallacy that has also infiltrated the pattern community. If the airplane is stable (CG in front of the neutral point of the wing/tail/fuselage combination) then at zero angle of attack there must be a nose-up pitching moment otherwise you cannot trim the airplane at a positive angle of attack (AOA). If the airplane has no wing incidence or tail incidence then this will require some trailing edge up (TEU) elevator to trim the model at a positive AOA. Please note that if the airplane's aerodynamics are linear (and most good aerobatic designs are very linear) then a wing incidence change will ONLY result in a change in the angle that the fuselage flies...basically you use your wing incidence to tune the fuselage angle in flight. Changing tail incidence is somewhat absurd because you have control over it with the elevator...basically you change it's incidence all the time by moving the TX sticks.

Now we know that to fly in level upright flight (which is where we typically trim) the airplane must produce lift that's equal to the weight of the airplane, so the lift coefficient (CL) is not zero. We also know that CL increases linearly with AOA and that to be statically stable in pitch we need the nose-down pitching moment to increase (get more negative) as AOA increases. (CM_AOA < 0)

Now lets assume that that we're flying in level flight at some normal cruise speed (CL > 0) and we roll the airplane to knife edge. What should happen? Notice that the wing most go from producing lift that's exactly equal to the weight of the airplane to producing no lift in order to maintain a straight flight path. (Trimmed for knife-edge flight the AOA and CL are now zero). Notice however that we've done nothing to the airplane so it's still trimmed to produce the lift coefficient for upright flight and consequently the statically pitch stable airplane will start to pull toward the canopy if there is no pitching moment generated from another source. Now my little article is stuffed full of little sources that can generate a pitching moment in knife-edge flight but the real result is this: If the airplane were perfectly neutral (no pitch coupling) then it would always pitch to the canopy at zero AOA if the CG is set such that the airplane is statically stable and the airplane was initially trimmed for level upright flight...also the slower the speed you trim the airplane for (higher AOA/CL) the more severe the pitch toward the canopy will be.


FWIW, I think pattern fell into this wing incidence will trim the airplane concept because the airfoils they were/are using have a nonlinearity in the lift and pitching moment curves near zero AOA. You can see this in some of Michael Selig's UIUC data. Unknowingly they fell upon this and tried to explain it but came away with the wrong explanation...trying to explain why things happen is human nature but it helps to be rigorous and accurate. The bigger models that operate at higher Reynolds numbers don't exhibit these same nonlinearities and as a result the pattern theories have been, at least in my mind, disproven.

This discussion all really stems from pattern guys adjusting their wing incidence to straighten the downline which typically doesn't work on large scale airplanes...I call it the POVDL (Power Off Vertical Downline) debate...but that's a story for another day.

GRH

Dan,

Is that you? Good to hear from you.

I've found that the sub-fin eliminates the need for right thrust due to spiral slipstream. I also think it makes upright harriers more stable...but right now that's just a theory of mine.

George

Troy Newman

Hey George,

How's it going man?

Hope you guys don't get offended by a pattern guy jumping in on the discussion. I'm not nearly as versed in aero stuff as George. I really value your opinion on these things. Most of my stuff comes from experience rather than schooling. However I am an engineer and look at things from an analytical perspective I think!

Not all of us Pattern guys subscribe to the reasons stated in the pattern forum! <GRIN>

I too have found that a sub-fin reduces the amount of right thrust needed. I spent some time talking to Quique about this last year and he has seen the same thing. The wing incidence does just like you stated in my experience it sets the angle of the fuse in the air. I have found that as you increase incidence (Positive) the models will get more stable or locked on. My thinking on this that the horizontal stab is not more "loaded" the wing and stab are fighting each other a little more and as a result the model tends to be less sensitive to turbulence and so on. Kind of like a boat in the water. At slow speeds with the hull grabbing more water it can carve accurate corners in the water. At higher speeds with the less Hull in the water the boat will skip across the water a little easier. Of course this is way too simple an explanation as the shape of the boat hull play a critical role.... Same for the stab incidence elevator trim controls this all the time. Plus as you stated move the elevator stick and result is really just changing the incidence of the stab to change pitch. What I have found in trimming pattern models is the elevator trim will be speed sensitive and making the elevator control surface trail the airfoil of the stab inline at neutral is the most efficient and is the least speed sensitive. This makes sense.... Where I see the stabs needing changed is, as the model will slow on an upline. Under good power and normal speeds the model tracks correctly...But slow down and the model can tend to pitch to the belly is the elevator trim is "UP". Now thrust lines play a huge role in this too.... but I tend to get the elevator flying with the stab after CG and thrust has been setup. The big thing is we can get away with sooooo much stuff on a pattern model that really doesn't fit a rule. Or we are working in numbers of scale that just don't care. The changes would have to be so radical to really effect changes.

In the last couple years I have found that airplane setup in terms of CG helps the snaps more. A more forward (nose Heavy) CG is helping the snaps both in rotation stops, and heading changes. The More stability of the forward CG is helping to control the snaps. The only problem is getting enough elevator to get the model to break. As for coupling issues, my experience lately has been CG plays a huge role in this but I don't look to a zero coupling as a trimmed condition. The CG plays so many roles and has so many effects in spins, snaps, stability in corners and all sorts of other places...Even the vertical CG of the model will affect the rolls, and vertical up and down lines. I tend to not dial incidence to adjust for pitch coupling, though many pattern flyers do this. I try to get the CG right and when I have the model stable enough using the CG then I deal with the coupling via a multi-point mix. I think much of the pattern plane stability compared to the scale aerobatic models (yaw and pitch stability) tends to come from moments that are a little different from the scale models.... The lawn dart type of thing.

In today’s' FAI F3A patterns we are now doing things that require the models to do different things. Rolling Loops, Rolling circles, and snap and roll combos that have not been seen until recent years. These are things that are essential to the Scale Aerobatic sequences. I think pattern models are still changing from the days of Jet like "pure" performance. The pattern models at the top of the skill levels are now doing some of the things full size aerobatic aircraft have done for a long long time. The scale aerobatic models are emulating this full size schedule base. The pattern models are still in the model airplane phase of aerobatics and in this phase comes some really neat qualities. The level of precision tends to be higher, because the sequences are not as complex. Rather the skills are grown in the pattern sequences. They are grown on models that tend to be more pure than the scale models. What has happened recently is CPLR, Quique, Chip, Jason have become so proficient that the maneuvers are getting more complex and driving the skill set higher. Then these guys then apply the skills to the Scale Aerobats and they can overcome some of the coupling or scale reactions of the models...that are really getting so close to the real thing.

The Power off vertical dive thing...I don't think its possible to have the model stable enough for precision flying and do uplines (thrust line) correct and have the downlines not pull out slightly. So I use a mix of thro to down elevator at idle. I have found that adjusting the incidence on the wings will change elevator trim but it tends to cause problems with the way model grooves. Back to my bad analogy of the boat. I have found that the models like to fly at a certain angle or bias on the surfaces.... If the model has this stability or bias then it will track better.

Pattern guys will tend to run the CG back too far and they sacrifice stability to get model to perform a "test" better. The result is they drive the CG back, this tends to reduce the need for as much thrustline offset, and it also plays funny games with wing incidences and power on/off tracking. Sometimes the model will perform the test better and sometimes it won't care.... So the pattern guy changes the next thing. When he gets a result that "passes" the test He is done. And he lives with whatever stability he has. This then becomes a good model or bad model depending on his opinion of its stability and tracking. I honestly think that most of the models can be trimmed to track very well with very good stability but this doesn't always meant he model is "zero coupling"

The changes in pattern sequences result in the models changing to more scale like attributes and we are getting models that have good stability and yet can perform the rolling turn stuff and loop roll combos. My opinion is we get away with a ton in terms of aerodynamics because our wing loadings are so light. We are flying 10-11lb models on 1000sq-in wings some models even have bigger wings like 1100+sq. Take a Carden Extra 330 40%...2600sq-in wing and the models are around 40lbs....

This is about 50% lighter wing loading on the pattern models. Look at what we have gained in the Foamy world a lighter wing loading will fly the model sooooooo much better and do things that we could only dream about...and we are only talking ozs...

I didn't mean to hijack the thread a little. I really enjoy hearing what you have to say about trimming and aerodynamics. I think in many cases the pattern models evolve because of cut and paste changes. As a result you get contradicting aerodynamic things acting the same way. I have found that your approach to a model works and I tend to follow these ideas a bit more than say a trim chart. I have tried lots of different ideas to get a problem solved. In the pattern world I think guys look at cut and paste to solve rather than proper airfoil, and shape designs, which is really too bad. In the pattern world we can make it any shape we want. Maybe some really good design work rather than "we have always done this" can make a better flying pattern model. The things about curved turtle deck and flat bottom fuse causing pitching moments...I think play a role too. Many USA designed models have this feature and I don't like it.

I have actually found that flat bottom models tend to have different roll coupling with the rudder upright vs inverted vs knife-edge. My only explanation is that the fuse is not flying the same in terms AOA and sideslip. This to me is a prime example of fuse shape being wrong for the goal. This is why I don't fly flat bottom pattern models. I think the flat bottom fuse tends to play tricks on effective dihedral and this affects the roll coupling...They can also have funny things like this in pitch coupling too.

But why do we not see this so much in the scale aerobatic models? I don't have the large experience with big models like you guys...Is it an issue or not?

Any case...good topic...thanks for your very educated opinion. As a fellow Engineer I like data too. An idea can lead you down a path with the cut and paste method that may not be the best in terms of model performance. This even though the resulting cut and paste change has the desired effect on trimming. There is always another way to skin a cat.


Peace!

David Kyjovsky

GRH,

I have had some severe cases of belly pitch in models with very high mounted stab. Now, my theory of what happens is following (sorry if I replicate something you said in your "long" post, but I did not find it there). Maybe I am wrong... but this is what might be happening.
Imagine a stab mounted some way in front of the rudder (to avoid your "T-tail effect" ), on the very top of the fuselage (sort of like in the Cap). You roll, say, right to knife edge and apply left rudder. Now each half of the stab is in a different situation. The right half is "below" the fuse and "feels" nothing extraordinary. The left one, though, a different story: It is in the fuselage wake. But: the "top" surface of this stab half is in much "cleaner" and thus "faster" air, flowing around the curved fuselage top, where the lower surface is more "behind the sideslipped fuselage", in terms of inflowing air... in a turbulent, "stalled" air. Faster air over the top surface means positive lift on the stab half... resulting in the belly pitch.
The explanation may be a bit clumsy, but I hope that you can see my point.
Thinking of it, if all the above is true, it would also contribute to a "rolling to inverted" (to right in the model case) tendency (as the extra lift is generated only on the "currently upper", in my case left half of the stab).

David

flyintexan

My Feedback: (1)

This is a great thread, full of great information....now, if we could just get Nat Penton to post here. I think fuselage shape and stab position (in the wake of the fuse at a specified AOA) will soon become a more prevalent design criteria for pattern planes. We are already seeing more and more rounded, deep fuse bottoms, and sub-fins.

Forgive me if the following question is mis-guided...here goes: If the fuse is in k.e. at some aoa to fly without losing altitude at some given airspeed, then is it possible that the top half of the stab (above fuse side) is flying somewhat in the wake of the fuse and the lower half is flying in slightly cleaner air. Could this also contribute to the pitching much the same as an "above fuse only" vertical stab does in normal, level, upright flight? In another way, is the lower stab half now more subject to the prop slipstream......if the slipstream is disturbed by the aoa and wake on the high side of the fuse in k.e.? If this is true, I guess it does not contribute enough to counteract other pitch (to the belly) factors (the slipstream effect here in k.e. would contribute to a canopy side pitching tendency...right?).

-mark