rmh

Ed's pipes are the best - he lives a few miles from me and we tried lots of combos - others tried to do tuned cf pipes - some were really crap .
I did resolve l my header /coupler issues but others just could not get the hang of a zero side load on the connections .
my 80/160 stuf was the best - solid terrific power.

my ZDZ's ran very well -but then I had a lot of em!
the YS- no thanks -
The electrics fit my present needs best .
I even drive a Honda Odyssey

alasdair

MTK and rmh
How do you choose a CG point for your pattern models? I guess you adjust it until it feels just right, trial and refinement?
That's what I do anyway, I adjust until when I roll inverted from trimmed level flight it needs just a touch of down elevator to hold level inverted (on full power).

What would interest me is how this compares to the usual CG calculators. Or rather one in particular - for consistency.
http://www.geistware.com/rcmodeling/cg_super_calc.htm
This one is listed on the "resources thread" of this Forum
Could you maybe enter the parameters of your pattern model in just the first panel of this calculator and adjust the "desired Stability" box until it achieves the actual CG of your preference, and tell us what thet Desired Stability is??

rmh

I never use calculators.
The parameters for good results are pretty well known and the final cg setup includes the speed of the model .
Speed definitely affects CG selection -for best results
. Current designs are much like the indoor pattern designs -which have much the same effective lateral area as horizontal area.
The idea is not all that new -
Fairly neutral cg is common today but IF we were allowed to use the stabilization setups such as the AS3X -my Cg would actually be dead neutral
years back -Ii opted for a more forward CG-for better stability- the inverted flight stuff - no big deal -you quickly learn to hold in corrective trim.
The patterns were different . This worked just fine.
Looking for a true hands off setup -is a waste of time -
A model which is predictable and locks in, is much better

BMatthews

Alasdair, the CG calculators account for some but by no means all of the factors. The ones that are not included are not all that significant for MOST models. But when you're talking about a precision aerobatics model the small things become large. And that's when there's simply no replacement for test flying and fine tuning based on knowledge and experience. The CG calculators will get us into the ball park. But they can't fine tune to each pilot's tastes when you're talking about being close to the ragged edge.

rmh

Good aerobatic planes all follow the same the recipe
Too much power
Too light
(Ill take two)
most of the fussing around is because the power is too low or the wing loading is too high for a particular maneuver.
It wasn't until I got into little electrics that I got a chance to see what it was like to have enough power for any speed and a low enough wing loading for even the slowest maneuvers.
The fussing with supposedly critical engine offsets - stab placements - CG, all went away
Full size aerobats have exactly the same problems
The latest stuff is spooky. -Just like a really good 3D model.
Can one effectively use math models to improve our models ?
Maybe - beats me -
The whole idea of the models we fly is the chance to try an idea in real time.
Much like the so called perfect, calculated model airfoils -
yet to see one . I can do as well with an old shoe outline

MTK

This calculator gives a CG that is a bit aft of where I place mine. My latest design, Delta, was "calculated" using this same calculator. I used 15% static margin, a reasonable amount for most pattern planes. The answer it gave me was 33% MAC. My plane would have flown okay at 33% MAC but I believe has a better envelope for all maneuvers we currently fly in AMA Masters and FAIF3A with cg at 28%MAC. It needs very little mixing there when yawed. The plane is new and early in its trimming process and I will eventually get it trimmed to where no mixing will be needed. It won't take much since I currently don't mix electronically the little it needs....

BTW, I typically will start and stay with cg at around 28% MAC. That's a matter of preference developed over many years of practice. Some models may fly slightly better at 25% MAC and others at 30% MAC. I don't wander far from the 28% in mine tho, and usually don't use CG as a trimmer variable

Weight is probably the biggest detriment. I chose todesign it arounda piped DLE55. My thoughts at the time were 11 lbs of plane 35 lbs of thrust....juuuuust right. But in practice that's an extra 17 ounces of weight that doesn't need to be there, had I used the OS GT33 (10 lb airplane with 24 lbs of thrust, pretty darned close on power to weight). Alas, there was no OS GT33 available when I was thinking the design through 2 1/2 years ago. The current weight RTF is 4965 grams or 10# 15 ounces so it makes weight. BUT roughly 17 ounces lighter would have it weigha hair under 10 lbs which would be perfect for the size....Maybe I just need to fly 100 flights and get used to it

It's hard to describe a model that flies extremely lightly on its wing. Everything is better. One has to experience the feel for himself and once that's done, the search for lightness will be never ending

I continue to get arguments regarding breezy conditions andothers' preferences for more heavilyloadedplanes, but I prefer the feel of the light model in every condition

da Rock

The CG calculators really aren't meant to provide a perfect trim. They use your choice of Static Margin to return a value. Nothing more.

The idea behind them is to give a safe starting point. Nothing more.

Run them once with a small SM and you get one boundary for a safe CG range. Run them again with a larger SM and you get the other end of the boundary. Balance you model anywhere in between and you've got a safe CG for your maiden. You still have to consider your surface deflections relative to your choice, just as you would if you used any of the accepted rules of thumb, like simply balancing on the spar etc.

CG has almost no significance relative to the stability affect from props, but they both have something to do with stability. Little to do with each other.

alasdair

I totally agree (with you and da Rock) that the CG calculators are just meant to get you a safe CG "in the ballpark" from which we refine the CG by flight testing. But I feel that a tweak, including a mention of the prop, might get us into our preferred BIT of the ballpark.
It's just that, I noticed that pattern models end up twichier than trainers (maybe not a bad thing) and that gliders and jets seem to use a smaller Static Margin (going by the usual formulae). The old CG formula that was invented in the fifties that worked well for the early R/C models with big tails was improved upon. Maybe it is time for another update, with a Prop term included.

rmh

good pattern models -all I have flown -are not twitchy - The key to a GOOD pattern model is that it is extremely easy to fly and simply goes where you want them to go and stay there till you input a change
On teh other hand -the typical "trainers have built in stability which usually is intended to be self righting and may be a constant fight-some trainers are wretched.

The Seniorita- probably the best of all this breed- will, when correctly trimmed - self right and settle into a level flight given enough time and altitude. My old Seniorita is now fitted with a 635 rx - adding a gyro/accel system which tries to maintain last commanded attitude by countering any unintended airframe attitude deviations
Fascinating- I can now command a turn -release input - and the model maintains turn AND altitude- either direction.

The aileron "gyro" -there are three roll -pitch and yaw is providing corrective inputs from the aileron (roll) output in the 635 rx
the elev gyro controls pitch-
rudder gyro is not connected to a servo output- but on the Seniorita a roll command creates the turn

BMatthews

The thing is that for the majority of the sizes of props used on our models the prop effect is one of those minor factors that are not big enough to be really significant. It ranks up there with lift off the fuselage, which the model CG calculators ignore as well. However on something full size with big paddle wheel turboprop blades or airliners with very large fuselages these things cannot be ignored with safety. Just like how on the later Marks of Spitfires with the big contra rotating props and bubble canopies it was required that the fin and rudder grow to almost twice the area as the original prototype Spit.

It's simply that on our models the props are just not as large in proportion to the rest of the model so they can be safely ignored .... unless it's some oddball design with a gear driven over size prop.

MTK

That is simply not true for all prop driven models. Certainly trainers don't really require engine offsets and neither do ducted fan jets

But I'd say the majority of tractor types of models, be they aerobats, scale, even racing, all require engine offsets in an attempt to have them fly most of their envelope with minimal rudder correction by the pilot....

BFoote

Spitfire went from 1000 hp to 1400hp to well over 2300 hp. Weight went from 5000lbs to ~6000lbs to 8000lbs during the war depending on type and then contra went to +10,000lbs.

Lets see, twice the power, twice the weight, twice the moment of inertia to overcome simply to maneuver the skies. Hey!

Gee, I wonder why the tail grew...

PS. I will leave it to the novice to do simple research on the P51 and its tail growth as well. Give ya a giant hint. It scales with power/weight increase of the planes. Shocking I know.

BMatthews

I'd have to disagree with you on the trainers part. Folks may be more tolerant of this stuff on a trainer but a well trimmed trainer would most certainly involve some amount of right thrust.

Take the case of a simple cabin style sport free flight model. It's very similar in many ways to a cabin style RC trainer that can't move its control surfaces other than on the ground. It would be near impossible to attain a good friendly flying trim without a touch of right thrust to cancel out the engine torque.

MTK

Hmmmmm!! I'm not too sure you are disagreeing with me. I think you are disagreeing with you! LOL

So you are saying that prop effects do need some offsets on models to allow hands off SNL....If small props hadonly minoreffects (other than pure thrust), no offsets would be required....Which brings us full circle to the current contras commercially available and the YAW destabilization of the second prop (actually both props but we are used to single prop drives and their effects, in general).

BFoote

For neutral stability there has to be a force pushing the plane back into a straight line of flight. This is accomplished by the rudder. A yaw force/orientation with a trailing rudder will always push the model back onto a straight flight path. If it doesn't, you have some serious fuselage issues to work out. If fuselage volume/shape is predetermined, then growing the rudder is the only solution.

On a single rotational propeller some of said rudder authority is continuously being used to counter said propellers effects. Effectively decreasing its size when looked at yaw stability. On a contra, this effect does not need to be countered. Well, its effect is diminished anyways. Therefore, the rudder will have a larger stabilizing effect.

A rudder is nothing but an airfoil. It has a straight lift AoA/Cl lift curve like every other airfoil going from some negative angle of attack to some positive angle of attack and rounds over at both ends.

Stating that putting said rudder at one point of said straight lift curve compared to another is stabilizing is ludicrous.

Gets back to your refusal to contemplate that the discussion over on the other thread concluded that their problems were occurring ONLY at neutral where it seemed to wallow a bit back and forth. Not the plane refusing to go back into line from the rudder. Wallowing at neutral(flutter) can only be created via a dynamic control problem. Whereas stability is effectively a static problem(Its not, but can be treated as such).

This is a classic flutter problem. Ailerons get away with very poor balancing on RC planes as there is always a LIFT force vector up, but any high speed pilot in RC land quickly understands that they too must balance their ailerons to eliminate flutter. Or, they quickly learn after their wing explodes in mid flight, or at a minimum their ailerons rip off/strip out servo etc. Generally complete loss of aircraft is the result. This aileron flutter is caused when effectively the eddy currents are greater than the lift force from the airfoil, allowing the aileron to now be in a "neutral" force state where the slop in the control linkages on said aileron will now oscillate at its natural frequency(stiffness of aileron). Combine this slope oscillation with the dynamic instability of an unbalanced aileron and the flutter forces are now great enough to move the servo, eventually stripping it creating WILD oscillation of the aileron itself and KABOOM!

SO: How does this apply to the rudder? Before, under single propeller conditions, the rudder was ALWAYS, LOADED in a single direction to counter the propeller effects, just like in the aileron case. This load is fairly high. Therefore the rudder does not need to be balanced(for the most part) and servo slop is taken out of the equation due to this continuous load, though eddy currents can still cause rudder failure, but generally the stiffness of a rudder is vastly higher than that of ailerons and this never happens. You will see this problem manifest itself in high speed RC airplanes rudders where the rudder rips off in a dive etc, but pilot "saves" the day. This is rather rare though as it is fairly hard as the area to generate said eddy currents on a rudder are vastly smaller than that on a wing/aileron combination. OF course no one actually learns, they just slap on the exact same unbalanced rudder and keep flying.

In contra propeller land, this LOAD is eliminated. Now an unbalanced rudder, like all crude simple RC rudders, "flutters" at neutral. Combine this with servo slop(impossible to eliminate entirely) and VVVVIIIIIIOOOOOLLLLLAAAAA, guess what, your model wallows.

All you have to do is move the pivot point on the rudder rearward and this problem will vanish. Or rearward + counterbalance weights. IT is how I have gotten away with using steel geared micro servos on my rudders for a very long time while everyone else on the flying field are all out buying these super expensive super over powered super torque servos that weight 5 tons. Same goes for my ailerons and elevator.

Yea, makes them harder to build.

PS> When making 3d fun fly, I do not balance. I just grab super cheap servos and use them as I know I will be crashing a lot. Kinda goes with the political sign/duct tape/ broom handle build...

BFoote

Ducted fan jets don't have offset because the momentum created by said jet fan is much smaller than that of a propeller. Momentum is mass dot velocity after all. Likewise in the Jets case, they WOULD be better off if they did add offset, but very few actually know dynamics and fewer yet will bother with the calc and figure things are "good enough". Which of course they are as it makes very little difference in RC land. Of course the end result is many of these jets need increased rudder area to deal with their lack of offset and they likewise scale said jets from twin engined planes that don't have to worry about this dynamic effect as they have it canceled out by opposite turning engines.

rmh

The effect of props is a never ending source of wonderment .
The contra stuf adds more problems than cures-my opinion.
The rudder AND fin size n shape is a moving target.
The entire tail group is part of the TOTAL stabilizing force need -which -is another moving target.
Essentiallyyou need more stabilizing forces than the combined destabilizing forces.l
Double talk?
No
The compromises needed for a stable craft at some given AOA are not the same as those needed for a different AOA
You can easily prove this by flying slowly, inverted -and then upright.
FWIW- when the first of the big fiberglass IMAC planes hit the market - the tail wag was a common issue
adding a chunk of wood to the rudder TE was enough INCREASE in drag, to fix this particular issue . Those turkeys had more drag forward the wing than aft.
The old Quik Fli models -with the canopy placed "just wrong" had a annoying tail wag -
Fun Fly stuf with a cup attached to top of the fuselage -often flew with the same wagging
The turbulence was enough to reduce the stabilizing effect of the stabilizers .
Look at the whole model - each bit affects all the other bits .
- no math needed - just pay attention. -

BMatthews

Matt, I think we're beginning to mix up torque effects with prop disc de-stabilizing effects. The right thrust needed in most models is to indirectly counteract torque, not any de-stabilizing effect of the prop disc. So I don't think I'm disagreeing with myself....

... it wouldn't be the first time though....

There's no doubt that the new contra rotating props on our models TECHNICALLY calls for bigger tail surfaces. But most of our models used for aerobatics already have such large area tails that the designs can accept the small size contra rotating setups in place of a regular two bladed prop with no need for altering the tails.

Going back to the lineage of the Spitfire it was interesting to note that the final Marks with the bigger props and contra-rotating setups also had extended span stabilizers in addition to increased size vertical tail. I'm sure that there were many reasons for this given the build up in weight and power as noted already. But likely as not one of the reasons was to do with the 5 bladed props and contra-rotating setup.

alasdair

"- no math needed - just pay attention. -"

I doubt that mantra has ANY following at Boeing or Airbus, or other aircraft manufacturer.
yes, you are right, it all does depend on everything else BUT, this is a science, not art.

We need to calculate things. That's why we have formulae for picking a CG in the first place. We don't leave it to blind luck or intuition (well some modellers do I admit), we use a simple formula that can be applied on the field. We start by assuming that the AC of the wing is at 25% mean chord and then we try to calculate how much further behind that the Neutral Point will be due to the H-stab. Lots of formulae. They all include tail size and moment arm and a refinement is some reference to the wing shape (Aspect ratio), and the length of nose ahead of the wing.

What I am suggesting is that we include in the formula some reference to the prop as well. Diameter, pitch& RPM, plus distance fore/aft of the wing AC.
I suggest that if we include a prop term, we could have a CG formula that works equally well for low powered model, high powered models, jets, pusher propsand gliders.

MTK

Come on guy.....Certainly small models and full size airliners play in the same medium and are bound similarly. Yet the sizes, forces and power involved make the two types of aircraft totally different animals at opposte extremes of the spectrum. RMH is on the money regardingb toy planes

otrcman

In post #11 of this discussion MTK referred to another thread in the Electric Pattern Forum:

"The current crop of models have adequate yaw stability for single prop drives up front....Interestingly, there has been an ongoing discussion in the Electric Pattern Forum regards to the destabilizing effect of a contra prop drive."


Matt, could you post a link to that discussion ? I'd like to read it. There may be some connection between that discussion and a link provided by UStik in his post #3. UStik's link was NACA Report L-219, which says in part:

"Further conclusions are: A dual-rotating propeller in yaw develops up to one-third more side force than a single-rotating propeller. A yawed single-rotating propeller experiences a pitching moment in addition to the side force."


So yes, the people using contra-rotating props on their aerobatic models may well be seeing some increased destabilizing effects due to the prop. Basically, all that means is that the stability (static margin) of the model changes slightly as power is increased or decreased. If your stability is marginal, this effect may become noticeable.

Dick

rmh

Food for thought:
Flying IS an art- in the truest sense
if you don't understand that, all the math in the world is of no value
The math is simply a method of quantifying this art-
It may make things easier for some to better understand the art.

UStik

I think he meant [link=http://www.rcuniverse.com/forum/m_9833118/tm.htm]this thread[/link] which is quite lengthy (3 years old). But searching the thread for the words "tail" and "fin" shows some pertinent posts.

otrcman

Thanks for the link, UStik. I skimmed the entire link (whew !) and read everything I could find regarding the directional stability issue due to contra props.

Very impressive read. These guys are very careful observers and there are some excellent minds at work. I think they'll get it, but they are hampered by the extremely complex mission that they are flying.

From what I read in the thread, the F3A airplanes are by nature very low in directional stability. When they add in the destabilizing effects of the contra props, the small loss of stability takes them over the edge to where they begin to notice some directional oscillation. The simple minded fix is to increase vertical stabilizer area. But everything they change has ramifications in some other part of the flight envelope. That doesn't mean the increased vertical area didn't fix the prop destabilization issue; it just means that they have created some undesirable side effects. So you either try some other fix with fewer side effects or find a way to keep the bigger vertical and correct for the side effects.

On the art-versus-engineering debate, I believe that there are elements of both in competent aerodynamic design. An artless engineer is limited indeed. But an artist without the engineering is condemned to a much longer cycle of experimentation before arriving at a successful conclusion. The engineering insight permits one to interpret a result and postulate what to do next with fewer iterations than the artist would require to achieve the same end product.

Dick

rmh

A bit more
having seen the math trotted out over and over as an explanation - to the NON math audience, I suggest the better approach is to work on the basic concept of an issue as the first approach.
As an example :
Painting and musicianship are art forms.-
However both follow rules of math,often unknown to the casual observer.
without the concept -which is the art- the math is just numbers
When someone asks 'why does the prop destabilize?- the concept as an explanation comes first - that is you must first qualify -then you can use math -if necessary - to quantify the relative forces which really influence the craft.
My own experience says the advances in power ,construction and material- are the most meaningful in advancing the art of aerodynamics
Just my perspective -