ytell

I'm a little confused about some aspects of trimming an aerobatic model.

If your model is perfectly trimmed, so they say, you will be able to fly a perfect hands free lines on all basic directions: level, 45 deg up and down, verticals up and downs. Now how can this be true if each if for each type of line the wing is significantly loaded differently? e.g for level flight the wing creates lift which is exactly the weight of the plane anf in 45 deg. clibe lift and drag are completely diferent from a level flight?

If the plane pulls to the belly on vertical line, so they say, increase your wing incidence.
How can this be true if the elevator trim position is not changed? as fa as I understand, the elevator will keep the wing in the same AOA regardless of wing incidence change (assuming constant speed is maintained)

Yoav

Charlie P.

I'm pretty sure it's not possible to trim most models to be neutral in all aspects. It would have to be a near symmetrical (wing AND body shape) pattern type with retracts. Most flight trim techniques are for short duration tendencies, not prolonged flight in a given aspect. If it flies straight for two or three seconds hands off that's enough to tell.

In your example you are changing the wing attack, but not the horizintal stab incedence OR the engine offsets. Typically an engine is offset down a few degrees and right a few degrees to the datum line to counter the tendency to climb with added thrust and counter engine/prop torque. Even that ideal angle is dependant on throttle/thrust and will change in flight,; so you design for an envelope.

BMatthews

As you're finding out there's a lot more to trimming a first class aerobatic model where the flight maneuvers will be judged for style and accuracy. For example your "increase the wing incidence" rul for correcting the vertical pulling likely relates to the prop thrust as much as it does anything else. But just adding up or downthrust wouldn't do the same thing because you won't get the same vertical displacement of the thrust line with altering just the downthrust that you will with a change in the wing incidence. I sure don't pretend to know the ins and outs of precision pattern model trimming but I know enough to realize that when you alter the wing incidence you alter a lot of other things that at first do not seem related.

Another big trimming issue for pattern models used to be the vertical position of the horizontal stabilizer. Back before mixing was widely available in the Tx's pattern designers thought nothing of cutting into a model to shift the stabilizer mounting position up or down as little as a quarter inch to change how their models performed in knife edge flight. I don't even remember if it was to cure a pull to the top or belly or to cure a roll couple from the rudder. In one case a designer was up against the clock with little time before packing up to go to a major contest. Instead of the lengthy total remounting process he cut and added some anhedral to the stabilizer and made up a Y end for the pushrod to the elevators. That did the trick apparently since he won the major contest and for a short while anhedral stabilizers were all the rage with somewhere around a half dozen major players producing designs with anhedral stabilizers in the wake of that one quick and dirty solution. The case of this was written up in the article when the first guy to do this published the plans for his model. This was some time back in the early to mid 70's as I recall.

So yeah, fully trimming a precision pattern model is not about science so much as it is about the Black Arts. There's science behind it but I defy anyone to have come up with the solutions that are popular in the PP community from a purely analytical beginning. Too many of them are stuff like this "add incidence" trick.

Some of this has been lost to time as well. I gather it's normal to set up mixes and mode switches to provide such cross couplings that are needed for different flight modes and select these trim modes multiple times during each flight to suit the maneuvers being done.

Lnewqban

The wing is what flies, and the prop helps it; the rest of the airplane just follows.

Hence, increase your wing incidence = decrease the incidence of the rest of the airplane = same wing AOA

If the plane pulls to the belly on vertical line => decrease the incidence of the rest of the airplane = stab pushes "down" more and increases a nose pitch "up" that works against the "pull to the belly" tendency

da Rock

decrease the incidence of the rest of the airplane ????

The incidence of the airplane is the incidence of the wing. The AOA of the "rest of the airplane" would chance if the wing incidence is changed.

Incidence of the wing is the angle between the wing/fuselage centerline as is the incidence of the h.tail the angle to the fuselage centerline. Changing the incidence of the "rest of the airplane" gives the same change if you changed either part's angles.

And if you decrease the AOA (if you meant AOA instead of incidence) of the whole airplane (except the wing), you wind up decreasing the AOA of the h.tail.

If you "increase your wing incidence " you will change the AOA of the "rest of the airplane" but the result is to pitch the fuselage CL nose down more at cruise when the wing settles into giving the lift it has to give. And that's affected of course by the new drag on the fuselage and the new trim on the elevator you're going to need since the h.tail is at a different AOA.

Not sure what you want to say? Hard to follow stuff when new ways of describing things happen.

MTK

There has never been nor is there presently any Pattern or IMAC aerobatic model that has the qualities you describe. Some Pattern models are close but will never get any better than "close". There are some compromises required to get a more complete flight envelope. Sure one can add electronic mixes galore to an otherwise non-optimally trimmed model but that isn't a real answer to the Pattern competitor.

We try very hard to get a model trimmed very close to neutral in knife edge flight. That means that ANY amount of top rudder will not alter the model's direction, ie- pushing or pulling and no roll coupling. We try for a rudder only knife edge loop and rudder only flat turns in either direction; some models do get very close to doing exactly that in their trim. This is done primarily by choosing the appropriate CG (between 25% and 33% MAC), correcting the wing and stab incidence for that CG (as much as +0.75 degrees for wing and + or -0.5 degrees in stab), and lastly correcting engine off sets.

The better engine arrangement requires very little downtrust and about 2-3 degrees of right thrust for current Pattern designs, for the typically driven single propeller (not conta rotating))

Some Pattern planes are just dawgs and will never be trimmed right without major surgery (Contary to some people's belief, adjusting stab vertical location +/- 2" will not make a current design model fly appreciably differently; And a 1/4" will do absolutely nothing). The surgery may involve effective dihedral adjustments, either by building new wings or by moving the wing tube.

Once knife edge is trimmed right, we are about 80% of the way there. Snapping and Spinning the model are also keys but these are more based on the design of the model as opposed to trimming it. For example, a tail volume coefficient of about 1.0 produces a sluggish, non agile model that becomes really dawgy to fly. BUT... It's a groovy model and that's the reason pattern models of the 60's and 70's were designed that way (one simple maneuver per hi speed pass). Those designs would fly today's schedules really badly if at all. Most of today's designs use TVC of around 0.8 which is a compromise between grooviness and agility, except, in my opinion, a little too groovy

My designs use TVC of around 0.6 because I want my design to minimize my workload. I don't want to be banging the sticks all over the TX just to get a reaction. To get that, stab area, wing area, wing area distribution, and tail moment all play the key roles.... it could mean new wings, stabs and in extreme cases, making the tail arm long enough, which means hacking the fuse. Then it's a question of feel...how do we want to model to feel when, for example, we roll from up right to inverted. Slight adjustments to CG are often done to accommodate a certain feel.

We want repeatability in response. Repeatability generally comes from a more forward CG. But a more forward CG compromises feel. So we balance it.

It's seems very complex when one is new in the sport and doesn't know much and worse doesn't know who to turn to for correct advice. But after doing this for 35 years, this stuff becomes simple.

As DaRock I think stated, the aerodynamics involved can get tedious. The biggest problem we have is the rotating wave of high mass air hitting the surfaces at unequal angles. That's why contra rotating set-ups make sense in precision work

Apologies for the long answer but it barely scratches the surface of the subject. Hopefully I've given you enough info to appreciate what's involved

BMatthews

That was actually a really nice insight into what goes on with designing and trimming current precision pattern models and the reasons why the decisions are made. Thanks Matt.

Bax

My Feedback: (11)

To comment on something the original poster asked about...you cannot get a model to be "hands free" in all flight regimes. If the model will fly hands free in upright, level flight, you will not be able to fly hands free when in inverted, level flight. The same would be for uplines and downlines, regardless of the angle. The fact that the wing will almost always have a symmetrical airfoil precludes it from being trimmed such that it generates lift in all attitudes without pilot intervention. Hands-off upright flight means that you will still have to apply some elevator when inverted.

Even with a well-designed, excellently-trimmed model, you'll likely find that inside loops or snaps will take different amounts of elevator deflection than outside loops or snaps. The same with left-right maneuvers. It's all very, very complex. The best one can hope for is a model that responds the way you feel comfortable and that performs the maneuvers to the precision you're trying to get. Someone else will take the same model and wind up re-trimming the whole thing to suit them.

BMatthews

I took his "hands free" to mean that the model is actually neutrally stable in pitch over as much as possible of the flight orientations. But yeah, that's hardly a "hands free" situation. In truth small control inputs would be needed constantly to keep the model pointed correctly when it gets nudged around by turbulence or is in some attitude where it has a slight tendency to diverge from the selected path.

Rodney

Contrary to what has been said above, I have often trimmed out my big bipes to fly hands off in level flight whether upright or inverted at one throttle position (usually about half or two/thirds throttle). Now, if I change throttle position, trim will slightly change. I have also trimmed out several monoplanes (.40 and .60 size) to do the same thing. It is difficult sometimes, you have to play with both the CG position and usually have a little bit of up trim in both ailerons when the aileron stick is centered, especially on the monoplanes.

MTK

Upright and inverted flight are not the problem for "Hands Off" flight. Knife edge is, and it will not be "Hands Off" in pitch and possibly roll, when the model is trimmed as you describe. Trimming a modern pattern model to be neutral in pitch for upright and inverted flight, is a fairly trivial matter, and over a rather wide power/speed band. Our wing loading is low enough to allow that

Current top schedules particularly in F3A, demand a great deal of pilot attention.....it's easier on the pilot if he minimizes his workload with an optimally trimmed model for "hands off" knife edge flight. This requires a forward CG and BTW, spinning and snapping the model with precision will become easer. All other flight regimes will fall in place once that's achieved. Unless the model is so badly designed that it simply isn't capable of doing certain maneuvers.

To the original poster, we must compromise when trimming the model to achieve the best flight envelope the model has. And BTW, I abhor e-mixes, try very hard not to use them

ytell

Thanks for the interesting insight.

This is exactly where I'm coming from while helping my son trimming his Spark pattern ship. As one post above written the subject of trimming is much of a black art. There isn't much written stuff on the subject and a few sources including semi official sites could be quite confusing on this subject.

Recently I came across an interesting approach by Bryan Hebert (Triangulation Trimming, http://www.hebertcompetitiondesigns....tion.aspx?id=8)
That is supported by some known F3A champions. His approach sounds very promising to me in the sense that once you follow his approach accurately you may get rid of most radio mixes of your model by manipulating only CG and incidences.

My problem is currently that I find it difficult to apply a recipe of this kind that I can't understand each step to its exact rational and expected effect on the model's flying qualities.

Would appreciate clarifications on the following specific points:

1. Why an increase of wing incidence may cure a model tendency to pull to the canopy on verticals? Is there any difference (in this sense) between up and down lines?
2. Suppose your verticals are now perfect. How can a plane be trimmed to both verticals (wing is unloaded) and level flight (wing loaded)?
3. The tendency of an airplane to go to the belly on knife-edge is taking care of by moving the CG forward. Is there any difference between left or right KE in this sense?


Thanks,

Yoav

flyncajun

I am Bryan Hebert, the creator of triangulation trimming. hebertcompetitiondesigns.com
First you must understand the correct priority of trimming a competition airplane.

#1 is C/g this is always close to 25% if you are around 950 sq in.But if the wing is bigger, say 1000.= like your Spark you may be able to move it back to say 27%
altitude, also plays a role. If you are a couple thousand feet above sea level, then you can run the c/g back a little more.
It`s a matter of the of the main wings (size, airfoil ) ability to handle 11-12 pounds of payload (wing loading)

There are two ways to induce wing inc. with the wing by adding inc. ,or by moving the c/g back. The latter is one that gets you in trouble with the mixes poor snapping tip stalls , poor spin exits ect.

My inovative method uses the wing , and a more forward c/g than the traditional methods of the past 20 years.
The positive incidence allows your to move the c/g forward 10% or more giving your a more stable platform to fly and control in all conditions.

#2 wing Incidence,,, if your wing is at 1000 sq in. (spark) set the inc at .5 if it is 900 or smaller you will sometimes need to adjust it up to 1 deg.positive.

After your wing adjustement, fly and trim the airplane for hands off flying if the airplane goes to the canopy in a upline increase wing inc. keep increasing the wing inc. untill it will go up perfect under any power setting then stop. you will see that increase in wing inc. will make the airplane climb requiring a need for down trim ,this keeps the airplane happy in both hroz. and vert. lines ( 1 deg of down thrust 1 deg of right thrust is all that`s required for the motor setting ) because the wing is now directing the flight of the airplane through the elevator trim ,not thrust for the most part, the throttle will not effect your up line, or horizontal lines.

#3 after the airplane will fly hands off on a horz. line and up line.
check the knife edge , first do a right rudder knife edge it should track perfect without pitch , (assuming the elevator halves are perfectly set, this is a very important step! ) if it goes to the canopy move the c/g back a little ,if it goes to the belly move it forward.
Next try a left rudder knife edge ,,NOW! here is the tattle tail for the c/g placement. because of the prop slip stream it`s more sensitive on left rudder knife edges.If the model pitches to the belly continue to move the c/g forward until it stops ,if it goes to the canopy move it back a little .

Shortcut Tip,,, if you go to the canopy on the left rudder knife edge you are very nose heavy.
If it goes to the belly on a right rudder knife edge you are very tail heavy.

After this step is complete re check the vertical if you were way of on c/g this could have effected the required wing setting you used for the up line adjustment ,then repeat steps 2 and 3.


No black Magic ,, now that the wing is flying the plane with a forward c/g, and the thrust has less effect on pitch/yaw, the airplane is neutral. And with the cg forward, the elevator has more efective power around neutral.
Remember c/g forward the surfaces have more power around center,with the c/g back the airplane becomes sensitive around center with less power and control


After these three things are performed to your satisfaction (trianglation trimming you strart setting your airplane up for the throws in the class you fly , the c/g and wing inc. setting should not be different from Sportsman to FAI.

One big mistake ,and an old wives tale from the past ,is the airplane should fly hands off upright and inverted ,,this is not possible and is usually the cause of many problems ,trying to chase this legend!
You must not confuse radio setup with airplane setup they are not the same thing. get the airplane flying perfect in the three planes,then, set the radio up to fly your pattern with precision.

Re read the article on my website. All the clues are there, then when you perform what I wrote, your eyes will be open more, and you will be able to understand better what took place.
Really it`s just 20 years of designing ,trial error and observation ,correction

I went mad trying to make sense of all the trim articles of the past they are all wrong for the skills of todays modern Pattern requirments.

Don`t try to understand it till you use the method then it will be clear as a bell

ChiefK

Bryan,

Tried to check out your website, but couldn't get a connection. Would you address trimming issues for smaller "classic" pattern models? I'm currently beginning a "Deception" build and very interested in learning how to properly trim this bird.

Greg

flyncajun

Hi Greg
The site should be back up soon ,,doing a little work on it.

Trimming a classic pattern airplane is no different than a modern pattern plane.
Use the trimming guide as much as possible. the only big difference are the demands on todays designs and the capabilities of our modern planes verses the older models designed for simpler patterns.

The big difference is ,most of the old designs had one piece fixed wings ,,this is a problem and can keep you from
doing the required adjustments needed for pure flying unless you are willing to cut and glue a little ,add a shim here and there ECT
Sometimes you have to resort to the old trim methods which are a band-aid at best if you are not willing to make changes.

You will notice many of my rules and sayings filtering in to todays modern trim debates. rules I have been preaching for 20years to aero tech`s and arm chair trimmers, some in this discussion
Forget what you have read before,start with a fresh look.
Read and re read my entire article, and put it to use on your model to the best of your ability. it`s all common sense!
You will learn funtion , and reaction in response to changes, not theory or math , and in the end, have a better understanding of how your airplane changes with different settings and adjustments ,good and bad.

Bryan

rmh

The trimming quandry
A truly competitive design for todays maneuvers is basically zero zero zero
The reason being that a competitive electric FAI bird flies at almost constan speeds and the only real effect which requires compromise -is the twisting action of the propeller.
Huge fuselages which provide more or less constant lift in almost any attitude (transitional lift) and tiny highly tapered wings are all part of getting a design which will do 6 snaps -in a row - with little or no apparant track deviation.
I looked over Andrew Jesky's latest models (he lives here now ) and it looks to me like he has it figured very well - for the latest style flying
Compromised angles may help in some setups but personally I found they were not my cup of tea .

In playing with a lot of foam models for the past few years - I changed my mind as to what constituted a really good highly aerobatic setup.
The wings are now not much more than guidance vanes ( well maybe that's pushing it) )
The big ol fat fuselage ain't there for drag - it really accomplishs a lot of the lift required.
Time marches on

flyncajun

Well ,there you are Dick!

Good to see your still loyal to your old methods ,it makes me feel smart!

A foamy is a poor example of an aerobatic model.
There is no airfoil to produce lift ,so just like your methods on a pattern airplane ,you have to run the c/g way back usually 30-35% to induce incidence ,and set it up zero zero zero to get it to drag huh (fly) around. it seems fine ,till you unload the wings , Knife edges,up-lines or, overload them in slow landing,Snaps ,Spin entry`s then they tip stall because in reality your tail heavy.

Zero Zero Zero,,,,,This is why the old designs had so much mixing, in reality it`s Just poor thinking that seams to make sense.
Try a 1.5 snap on this setup and see what happens (you have to fake the entry and exit just like the IMAC guys do), try a reverse spin and keep the airplane on track try a knife edge loop on left rudder , you see we fly much more complex stuff now days. You would be very surprized at the mixing Andrew has on his airplane,only he can fly it well,I trim Sparks all the time, (not his design)
but, I can tell you factory settings are way off base.

The only way the above method would work well ,is if a 2 meter model could fly at 5 pounds , the only reason we can get away with it in the flat foamys, is they are very lightly loaded they are really just kites with motors super light , and way overpowered
and they mix even worse than a pattern plane.

Bryan

rmh

Hmm
"No airfoil to produce lift "
I never knew airfoils produced lift
I learned something there
Now to figure out what it is -.

flyncajun

LOL
Now you know!
Bryan

rmh

merciful heavens!

speedracerntrixie

My Feedback: (29)

Too funny...........Tell you what Dick, I will take a Dalotel over a flying Dolphin anyday!

flyncajun

Well if you ever want to fly in competion, you might want something more modern.
"Real" F3a airplanes may look like a fish but, "form follows function."as they say.

I know this forum is about ideas and theory of flight, I enjoy that as well, But I`m not thumbing through a engineer manual ,to answer commons sense questions ,Experiance,and practical application trumps arm-chair theory.
It`s easy to overthink things and look smart then make your conclusions fit the outcome you desire, but, in the real world of competion and pursuit of perfection we have to be able to back up our theory with results! not more theory. These results influince the new theory ,not the inaction of thought and intellectual belligerence.

Model airplanes are not bound to full scale rules Many things change when the wing loading is decreased.
This discussion started on the rational of trimming I gave the practical answers and it was aurgued with more theory.

Dick design a 2 meter Dalotel with a flat plate wing ,Then try to get someone to compete with it
it`s easy to deflect what can`t be aurgued with proposterous thought! and change the spirt and intent of the debate!
it`s another thing to stay on the same page and back your claims up!

Nat Penton and I had this debate for many years, till he tried my ideas and guess what, it changed "his" thought process!
After he could no longer deflect my methods with theory ,he tried my approach, and it changed his thoughts.
and his setup process.

anyway, here is what a modern design looks like
http://hebertcompetitiondesigns.com/...rks.aspx?ID=10

Bryan

da Rock

Bryan, that Alferma looks to be more airplane than fish. I never wanted to build a pattern plane because they look so little like full scale planes and I'm a scale guy deep down inside, but that sucker looks decently "real".

By the way, real planes aren't bound to rules in practice. They're bound to what works in practice.

Remember..... in theory, theory and practice are the same. In practice, they are often not.

speedracerntrixie

My Feedback: (29)

Bryan, I'm curious as to why you would assume I have never flown in competition?

That being said I do agree with most you have said. I have always set up my models with the wing at a slight positive AOA. My reason for doing so may differ some. I do tend to favor more scale appearing models, just a personal taste preference. I am currently designing and building a 2X2M Laser and am quite excited about the airplane. With each year it seems as if pattern airplanes are being re-invented and I am wondering what is driving these changes. Personally I feel that a skilled pilot with a well trimmed airplane is going to do well regardless of the airplane, within reason of course. I would never expect one to be able to show up with a Kaos and take home the wood but perhaps somone who has some IMAC experience could show up with a 1/3 scale Laser and do well in sportsman against all the "Real" pattern airplanes.

MTK

Funny thing about fish is that they don't have a funny looking screw bolted to their noses, especially one that goes 'round and 'round.LOL
Talking form following function, I was watching the Science Channel the other day and the program of interest was the Deep, 10,000ft below sea surface. There was a fish there that had the whole front of its head covered in a clear bubble, like a clear canopy of IMAC models. That was to protect its huge eyes. That far down many fish chemically luminesce faintly and if a fish is to feed, it has to "see" prey. Problem is it is as dark down that far as anything known, hence the huge eyes. That bubble canopy was fascinating.