rmh

The trouble with math models is garbage in = garbage out.
Not trying to be difficult but unless one can proove all steps along the way - the result may be meaningless. So on models I just cut n try and try to remember each result .
The popular cheeks are a waste of time -in my book
The flying flounder look has real advantages tho - if side area distribution is well balanced
as far as fuselage adding downline drag- another waste of time .
Going electric is a BIG step forward because constant speed (which yeilds more constant trim setups) is a BIG advantage. On the models you show , the flounder would be easiest to trim but my approach is to reduce decalage to absolute mins FIRST then balance till the model gets wandering tendencies.

Then if tucking is a problem - I would mix as required
the roll couple - add dihedral till it stops -if possible- or add mix.
This all works for me but I no longer compete in Pattern - so the subjective pattern stuff is of no interest to me
I just want a model to be easy and predictable in flight
going for zero input in any atitude - may be possible (tho I think not) .

ramcfarland

My Feedback: (44)

Remember R.C.M. Magazine ? Remember Don Lowe? Remember what he wrote about this phenomenon any body......He wrote about adjusting the (wing) dihedral. [8D]

rmh

Don and I lived in Cincinnati area in the 1970's
Flew models togather and both worked last TOC in 2002
etc..
Both wrote for magazines etc..
Trimming hints and guides were as common as fleas on a dog
some were actually worthwhile.
It's interesting to see the ideas recirculate.
Sawing the wing in half and resetting it were a given

UStik

Thanks for reply. Yes, good old GIGO, but the tool I'm using is made by a professional. Though I may use it unaware I trust in it being better than me. It helps me getting aware of new aspects.

In this case, I was after only one point, the side force (tucking) the fuse might produce in knife-edge. I mentioned the cheeks because they enlarge the top-view area and thus this force, not downline drag. I think I understand that you meant the lateral flounder area producing the lift in KE.

I'm just wondering what's going on. Meseems with this thread and looking at the Z-50 I chanced upon a trick, namely positive incidence of wing and stab. When I tried, the model turned out to be still (nearly) neutral upright and inverted (as trimmed before), but now neutral in KE as well - to my big surprise. So it seems to be possible; now it would be interesting to prove it and find any requirements.

Edit: Forgot to mention that I'm talking about the simulator. It renders only the fuelages drag, side force, and vertical force in a simple manner, but no real airflow, and probably it linearizes all phenomena. So it's easy to get all-around neutral behavior - probably unlike in reality.

rmh

The ZLIN 50 was a design I built for Dave Brown for the 1976(?) TOC. I reoriented fuselage positioning by lowering LE of both wing and stab so that in level flight they were closer (vertical seperation) - engine downthrust was also added.
I never flew the models - Dave did all the flight stuff and won 2nd as I recall.
Years later , I did a scale Zlin 50 L - which used the inline inverted engine in place of the Lycoming. also a larger tailplane was used . It was designed as a club aerobatic plane.
This setup I left "stock" as to angles and placements - I did opt for fully sym wing and setup angles all 0-0-0
It flew well but the structural requirements of the horizontal tailplane was a pain. One more thing -On ZLINS and other low wing craft- the low wing eliminates divergent flow around wing root - which is part of center wing placement designs. Makes for effectively more fuselage side area -BUT requires more dihedral
ZLIN also added huge fillets on this design as well as 256AFS models
This was done to reduce turbulance (hitting the tailplanes)as the craft made abrupt pitching maneuvers .
our rinky dink models operate in a much more forgiving environment.

UStik

Yeah, that wing-fuse crossing makes for a nasty diffusor. The fillets may be necessary due to lower aspect ratio than the Zlin trainers have, so bigger AOAs. Might have to do with spinning as well (blanketing).

The Z-50 has symmetrical, good-natured NACA 0018 tapered to 0012, good for a strong wing. Together with the 0.8 planform taper the airfoil taper makes for a total 0.5 taper ratio - good for snaps. Obviously, the 1 degree dihedral is sufficient for the low wing position. Did you notice that both wing and stab have a decent positive incidence angle?

You mean the Z-50 M with the Czech inline engine (called Minor) and the bigger rudder. As far as i know, the stab is the same as on the L (Lycoming) versions. I wonder why you had trouble with the stab, but maybe it was due to the struts. The original is said to have a weak spot where the tail meets the fuse, kind of a structural bottleneck.

Unfortunately, I never flew one, only Z-142, so my knowledge is second-hand.

rmh

Thanks for the reply- I always confused the L and M- 50's My drawings were supposedly accurate and the M stab was larger. likely an error. In a model - that tail setup is awkward to make as a very light unit.
as for incidence - my drawings showed a lot of wing incidence -but no figures were given
All of those ZLINS, were great full scale aerobats but compared to other ligher designs with more power -they were quickly made obsolete.
years before - the Bucker bipes were considered the std for judging aerobatics and other planes which could do difficult stuffusually power maneuvers scored higher - It wasn't long before everyone went for the most powerful, lightest setup.
I still have one of the 1992TOC Buckers I designed -flown by three entrants that year-and later scaled up and flown by another flier
great fliers but power hogs.

UStik

It was different times, aerobatics changed in the 1980s. In 1985 I saw Strössenreuther in his Z-50 and Extra in his EA-230 one after the other (but in reverse order). Extra showed the modern, dynamic style with slap-bang maneuvers, and Strössenreuther tried to imitate that so he wouldn't look unfavorable. He did well but it wasn't the old elegant style. I didn't like the new style, still don't.

Even the clipped-wing Z-526 AFS has a lower wing loading than a pitts byplane, and even the 300 hp Z-50 LS has a lower wing loading than all more modern aerobats. It has even a quite good power loading but still flies different because it's bigger. The Czech invented the Lomcovak as an extremely gentle maneuver with their Zlins. What has become of it?

Our old chief instructor used to taunt that there's nothing to poking in the air with 300 hp.

P.S.: The original's C/G range is 21.5 to 28% MAC.

ramcfarland

My Feedback: (44)

I got a lot out of you alls articles.....I remember back in the day if it was written by Don Lowe in RCM iI found it was well worth considering for example His "sawing wood" explanation with regard to an article he wrote on doing 3 consecutive axial rolls helped me immensly back in the day. Now to check Marley for fleas

rmh

Did you once-have a Runaround model?

mithrandir

My Feedback: (2)

Upright and inverted can be both made to "Balance"... but making both "Stable" is perhaps more evasive.

rmh, you suggest insensitivity to stab height on the fuselage affecting Yaw-Pitch coupling.... how would you explain every
IMAC plane these days having lowered stabs? How would you explain the centrally located stabs on every pattern plane I have seen in the past 20'ish years?
How would you explain that the yaw-pitch coupling occurs in all flight conditions at all speeds? Well... at least with every plane I have owned.....

I dunno what your flying technique is like... if you flew faster pattern planes back in the day... becuase the beta never got high.. the coupling may not have been pronounced... I enjoy flying 3D and frequently get my GS planes in KE's with a Beta of 45 degrees... and higher.... this is particularly where the coupling rears its ugly head.....


Biplanes??? I can't 'splain bipes.. I had a 42% ultimate that did OK KE until it got to a beta of about 42 degrees.. then it pitched so bad no amount of back stick would rescue it.....

Also... in an earlier post, I suggested that CG-Trim combo didn't affect KE Pitch...well... I meant if all the other factors are small... then the CG-Trim effect is notice'able.... (But it isn't really yaw-pitch coupling as the plane pitches regardless of yaw or rudder)
Essentially.. all these phenomona exist and can occur.... but if everything is done to null the effect EXCEPT locate the stab properly... the coupling is likely to persist.

My Eclipse for example... the stab location is very close to the Null spot.... so the CG-Trim situation is the predominant force... in KE..... irregardless of speed or beta or rudder...

rmh

I can't explain anything I see on models except for "follow the leader" seems to be a common link
I have seen perfectly good designs stretched out to copy "pattern" models - The results being a very mixed bag.
The stab position thing -another idea I can't explain as lots of objective tests say it is NOT critical. A T tail isn't a good layout tho for aerobatics
keeping the "drag and thrust" in fairly close alignment makes trimming at different speeds easier
Ditto for decalage
My flying ?
I flew control line then rc mainly pattern and then IMAC All sizes and speeds .
The biplane thing is very interesting trimwise because the drag (wings) relative to the thrust really kicks in at higher AOA-especially if there is stagger. and the more seperation btween panels the more pronounced it gets. The Bucker is still the best setup I have seen -The Ultimate was horrid.
I am an admitted fuddy duddy - I don't like the "look" favored on current pattern-Also detest stying on current cars which resemble dung beetles (or Japanese beetles)- and never have figured out why a commuter pickup truck has to be as large as a Peterbilt.
Reality seems to have taken a holiday.

mithrandir

My Feedback: (2)

IMHO, A Mach 1 just looks right....



'cept the stab is too high!! lol

rmh

I had one -early kit - pretty plane
They did build up on th heavy side -but th e patterns were all single sequences done in front n center then the plane went to end of field - did a huge split S and came back fo the next maneuver
The momentum carried it thru big loops
these maneuvers wee all done at low angles of attack
stab position really of little consequence at really low angles of attack
Lots of "facts" about stab position for aerobatic models were generated for the last 30 years
most of em - of little value.

mithrandir

My Feedback: (2)

I did some CFD simulations on a 2 meter Yak 55 and found a relationship between tucking and pulling verses tail height...

there was definitely a "sweet spot"....

rmh

Honestly -I don't believe there is a relationship as such
In real life the craft does not have a variable tail height .
HOWEVER
The placement of the center of drag -vertically- does affect trim.
I have proven this using split rudder which was controlled by two servos
as the rudder opened -the model climbed
The center of the rudder was higher than the thrustline by a fair amount (std setup.)
Tho some tend to doubt it - -you can steer a craft by simply shifting drag at the tail left-right-up or down.
it is easily proven.-by actual demonstration.

mithrandir

My Feedback: (2)

It was evident from the CFD, that a high stab had a higher pressure on the lower surface, and a lower stab had a higher pressure on the top surface... it appeared that there is a sort of "Stagnation" line along the side of the fuselage that marks the location that seperates the flow that goes over the fuse from the air that goes under the fuse... if the stab is on that line, there is no yaw coupled pitching...
Curiously, I ran a case of Zero Beta with 45 degrees of rudder... and regardless of the tail location, there was a nose up response... the coupling does not occur until there is some degree of sideslip...
I will see if I can find the simulation and post some pictures.....

As far as an offcenter drag... I remember back in 1993, with my Jekyl Pattern plane, lowering the gear had a definite pitching consequence...even with the long tail....

mithrandir

My Feedback: (2)

these images show what I was tryin' to 'splain!

image 1 shows the model in the flow domain...

images 2-5 just show the model itself

image 6 shows a closeup of the flow vectors over the aft region of the tail....
you can easily imagine a line along the fuse side that marks the "Stagnation" point.

If the stab is high, the induced flow creates an AOA that causes the stab to lift up, thus pushing
the nose down....

if the stab is low, the induced flow creates an AOA that causes the stab to lift down, pulling the nose
upward.....

As a test simulation, I placed the stab on the wing CL, cut the model on that plane, and mirrored it on that plane
to verify the simulation resulted in a net zero coupling... it did.

combatpigg

My Feedback: (3)

Even though you say you have tried various CG locations, you could still have something going on with your plane that is forcing you to fly with "down" trim, or a stab that sits positive.

mithrandir

My Feedback: (2)

the way to determine if KE pulling or pushing is CG related is easy......

1) pull from level flight at intermediate power to 45 degree upline
2) Roll 90 degrees to KE orientation
3) let go of the sticks..... let the plane assume a trajectory NO RUDDER INPUT

did the plane pull or push?

now repeat but apply top rudder.... did it pull or push again? Same as the No-Rudder test?

I bet most pattern/aerobatic planes in the No-Rudder case will very gently pull to the canopy from the positive stability trim condition...

and I bet most will push towards the gear when rudder is applied.....



also try.... apply rudder from level inverted.... most likely the plane will climb

rmh

Don't take it personally -but in actual practice I found that the business of critical stab position simply was not a factor - there is a pretty broad positioning which works just fine
The downforce does not change with position
I as well as others have moved it all over the place and properly set and balanced they all worked .

Jezmo

I like the one in the picture Dick. Looks REAL light, should be a great flier.

tomfiorentino

Hey Dick listen, I know this thread was a while ago (so I hope you see this post!) but I have a quick question for you regarding a CG change and slaving elevator to rudder to fix KE flight.

I understand your quote above and have in fact experienced it with a Tiger 60 I flew the covering off of....twice! . So when the wing is not working against gravity in KE the trims from hands off level flight become apparent. Great. Why then, are the mixes used to fix it in KE flight still relevant in level flight as well...when the wing is working against gravity?

For example, if I remove the down elevator trim through a mix with rudder input to prevent tucking in knife, why might I still need that mix to remove down elevator trim with rudder input in straight and level to prevent a dive with rudder input. The wing forces are very different relative to gravity, but the mix is the same.

Dumb question...what am I missing?

Thank you,

Tom

tomfiorentino

Hey Dick listen, I know this thread was a while ago (so I hope you see this post!) but I have a quick question for you regarding a CG change and slaving elevator to rudder to fix KE flight.

I understand your quote above and have in fact experienced it with a Tiger 60 I flew the covering off of....twice! . So when the wing is not working against gravity in KE the trims from hands off level flight become apparent. Great. Why then, are the mixes used to fix it in KE flight still relevant in level flight as well...when the wing is working against gravity?

For example, if I remove the down elevator trim through a mix with rudder input to prevent tucking in knife, why might I still need that mix to remove down elevator trim with rudder input in straight and level to prevent a dive with rudder input. The wing forces are very different relative to gravity, but the mix is the same.

Dumb question...what am I missing?

Thank you,

Tom

mithrandir

My Feedback: (2)

Great Question!

The answer is because the stab height matters....

Try a rudder input from inverted with the mix off.... try it on a down line.....

I challenge Dick to take his plane out and do the flight test maneuvers I have suggested....
particularly.... the pull to 45 degree test.....

I challenge him to pull his trimmed plane to a 45 degree line, roll to Knife Edge and let go of the sticks....

(If his plane isn't super-nose-heavy).. it should basically follow a trajectory and pull slightly to the
canopy..... now Dick... repeat and apply rudder.... guess what... it pitches! (Unless you have a model that
happens to have the stab in the correct location)

Repeat this on down lines.... on a vertical downline, most nicely trimmed models will slightly pull to canopy.....
apply rudder.... pitch to gear....

This is fairly typical and I challenge yuh'all to try this and see what happens....

(A model that has significant dihedral or a high wing like a cub may be difficult to observe some of this phenomena as there will be
significant roll coupling to the rudder)

But with a Yak, Edge, Extra, Su etc... this can be seen and repeated....