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All Forums >> RC Airplanes >> Aerodynamics >> RE: Having engine, wing and horiz. stab on a common centerline as opposed to having it otherwise.
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RE: Having engine, wing and horiz. stab on a common cen... - 11/1/2012 6:19 PM   
karolh


 

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Status: offline 		Here is a picture of my profile in it's framed stage. Rudder and elevator will be open bays while vert and horiz. stabs will be sheeted with 1/16" balsa. The fuse will be sheeted with 3/16" door skin lite ply with appropriate lightning cutouts in the rear.

Karol

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RE: Having engine, wing and horiz. stab on a common cen... - 11/1/2012 9:38 PM   
rmh



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quote:

ORIGINAL: BMatthews


quote:

ORIGINAL: AA5BY


quote:

ORIGINAL: mithrandir

if you send me drawings, I can model it up in CAD and run a CFD case to determine the theoretical null position....

but on the ''Typical'' plane, located about halfway down the fuselage, (Vertically speaking, or Mid-location) is about right....
high mounted stabs pitch downand bottom mounted pitches up with yaw


I'm failing to grasp why that is true. A high mounted stab will add a drag couple above the thrust line that pitches up to my way of thinking.



The key is the ''with yaw'' part. In level flight it's not an issue as any slight drag couple is masked within the overall pitch trim. But when a pattern model yaws such as in wing overs or during knife edge flight the vertical location of the stabilizer has some odd coupling that promotes some pitching. I suspect that it's related to the ''low aspect ratio vortices'' that flow around the fin when it's lifting strongly to the side with full or nearly full rudder application hitting the stabilizer more strongly one way than the other. But that is pretty much a WAG on my part. I do know from reading the articles found in magazines from the 60's and 70's on pattern designs that the location of the stabilizer vertically was set specificaly to correct such yaw related issues back in the days before it was simpler to adjust in a mix at the Tx.

In one notable case, and Richard can likely remember the design, the article discussed how the stabilizer was set too high and the model had a massive amount of pitch during knife edge. A big meet was coming up quickly and the designer had no time to chop and rebuild the whole tail to lower the stabilizer. So what he did was to simply cut off the stab halves and glue them back on with a fairly gross amount of anhedral. That fixed the issue well enough that he went on to win or place well in the Nats that year and the design was published in MAN or RCM with the drooped stabilizer as a ''feature'' rather than a ''fix''... It was even kitted with the anhedral stabilizer if I recall correctly. And likely it spawned a few copy cat designs since I seem to remember seeing lots of anhedral stabilizer pattern models for a few years after that.

Richard, do you recall the name of that original design?

Never heard that story.
but saw lots of "facts" about stab position importance - of which I believe very little
The anhedral comes into play ONLY when the plane is skidding - -any flight direction.
Our Zlins had arrangement of stab/wing which were all wrong according to these experts . strangely enough, the models had little or no coupling.
When a plane is travelling at little AOA -the wing-thrust line stab -can be practically anywhere.
Makes no difference
When one accelerates - the effective thrust to drag (vertically) makes a difference - so thrust angles count.
when the plane skids - the side area of fuselage AND the angle of the stab shows a difference .
All one big compromise - NO absolutes .
I copied the setup on Prettners' Curare - and found unless the CG was correct -it tucked one way or the other . I also tried various angles and CGs- various arrangements all worked. Out TIPOS had aprox same angles as Prettnre's models -gradually I reduced these angles to zero with very goodresults
The origina lsetups were on the models flown in 1984 Nats - these won the majority of the pattern events (good pilots!)

The aft cg always causes a down pitch when in knife edge because the AOA on the fuselage reduces trim effect on the stab -The fuselage blanks some of the stab.
Witha Huge fuselage such as new pattern stuff - the knife edge is accomplished with very low AOA of the fuselage - the fuselage on some have effective lifting area of the wings in some designs!
Fuselages are not as pitch sensitive (unless the 'designer goofs up) as are wings because they have a lower fineness ratio.just like a extremely thick wing - they are dull in pitch response .

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RE: Having engine, wing and horiz. stab on a common cen... - 11/4/2012 1:42 AM   
AA5BY


 

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Status: online 		It seems to me... that a fore or aft CG point would influence pitch while in knife edge for the reason that gravity is removed from the pitch balance/trim setup. ie, if forward CG has up trim to counter, when in knife edge when gravity is removed, the countering pitch trim will then produce a canopy tuck and just the opposite for aft CG producing a gear tuck.

The plane that is well balanced and requiring no pitch trim to counter off balance might still have pitch issues during KE due to drag changes with rudder and as you state, the skidding effects and stab blanking.

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RE: Having engine, wing and horiz. stab on a common cen... - 11/4/2012 3:45 PM   
rmh



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RE: Having engine, wing and horiz. stab on a common cen... - 11/5/2012 12:07 AM   
WEDJ



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quote:

ORIGINAL: RCPAUL

An old pattern plane - the Eyeball - had no dihedral and the engine, wing and stab all on the same line. It flew well.

Paul

As I recall, the Eyeball had a tendency to "hunt" in pitch during level flight. Many mid-wing designs of that era did drop the wing and raise the stab a tad to avoid this.

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RE: Having engine, wing and horiz. stab on a common cen... - 11/25/2012 6:51 PM   
jnhiller


 

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Status: offline 		This got quite interesting however I haven't noticed any comments regarding the stab flying in downwash coming off the wing in trimmed upright horizontal flight. The flying height of the stab relative to the wing is, I believe less important in modern long tail pattern airplanes and if the stab is close to optimum, KE pitch trim is easily adjusted by changing the CG forcing resetting trimmed upright horizontal flight. That said and likely more important in modern pattern sequences is vertical pitch trim and what works for one doesn't necessarily work for the other without resetting down thrust or possibly repositioning the effective wing - stab vertical offset.
I've found that using more than a few percent mix (5-6%) to correct either pitch or roll couple to be unacceptable and I incorporated a flying stab and adjustable dihedral into my first 2M design to try to find an acceptable wing - stab offset and dihedral. In conclusion there are no hard numbers that work for all designs.
Jim Hiller

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RE: Having engine, wing and horiz. stab on a common cen... - 12/23/2012 9:34 PM   
karolh


 

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Status: offline 		Here is an update on my current profile build. Today I did a complete model mockup to check the incidences and I was pleasantly surprised as all the lines were almost spot on, as the wings registered 0 deg. and the engine and horiz. stab measured +0.5 deg on my incident meter. With some minor adjustment I should be able to have them all at the required 0-0-0 deg.

Karol


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RE: Having engine, wing and horiz. stab on a common cen... - 12/24/2012 12:22 AM   
charlie111


 

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Status: offline 		 Having everything inline is probably the most streemlined airflow you can get look at aerobatic Designs they are also inline.Most highspeed aircraft have the same inline horiz. surfaces?

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RE: Having engine, wing and horiz. stab on a common cen... - 12/24/2012 11:38 AM   
AA5BY


 

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Status: online 		It seems to me that the big issue is positioning the wing at a point where there is fairly good balance of drag forces that tend to pitch a low wing plane down and a high wing plane up. A wing somewhere between high and low can more closely obtain drag balance. The value derived is a neutral elevator not having to compensate for the drag disparity and the benefits of the neutral elevator are greater probability that power on/off trims will be closer, that a trimmed elevator won't be affecting knife edge pitch and less disparity between upright/inverted elevator.

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RE: Having engine, wing and horiz. stab on a common cen... - 1/3/2013 7:10 AM   
mithrandir



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Status: offline 		it would b e interesting if some of you would do some simple flight tests to verify that a CG that is close to being right is NOT a predominant force
in KE Yaw-Pitch coupling.....

Simple to do.....

fly straight and level, pull the nose up 45 degrees at 50 to 100% power.... roll 90 degrees to KE and let go of the sticks.....
I bet it only pulls to the canopy a very small amount as it flies a trajectory....

next repeat that and apply rudder..... I bet it prolly pitches to the gear unless the stab has been carefully located....

If I remember the Zlin correctly, the stab is off the fuse completely no... located on the lower area of the vertical?
Funny.. the real one pitches to the gear...

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RE: Having engine, wing and horiz. stab on a common cen... - 1/3/2013 7:30 PM   
fredsedno


 

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Status: offline 		Gentlemen, Seems that many[most]WW2 fighters had hor.stabs.located higher than the wing.
Are we missing something? Those pilots did some wild &crazy flying to keep their body parts intact.
fredsedno

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RE: Having engine, wing and horiz. stab on a common cen... - 1/3/2013 11:39 PM   
da Rock



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quote:

ORIGINAL: fredsedno

Gentlemen, Seems that many[most]WW2 fighters had hor.stabs.located higher than the wing.
Are we missing something? Those pilots did some wild &crazy flying to keep their body parts intact.
fredsedno


They were designed with many more things in mind than aerobatics. In fact, aerobatics often were injurious to their body parts. Boom and zoom........

Most were tail draggers. It was assumed that most would operate out of barely improved fields. Lots of things were considered in those designs.

You got a point about how they performed however. The designers seemed to have absolutely no reservations moving the stab up. And they did perform.

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RE: Having engine, wing and horiz. stab on a common cen... - 1/7/2013 4:28 AM   
Leroy Gardner


 

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quote:

ORIGINAL: da Rock

In full scale use, those tabs lessen the stick loads for those surfaces. They also provide a convenient place for counterweights. The weights help balance the mass of those surfaces. When the surface is balanced it is less apt to flutter.

For models, they're not as important as we have zero stick load feedback. Our systems are driven by servos and there is no free movement possible.


If there is no feed back as you say then what is it that is stripping the gears in our servos.?.

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RE: Having engine, wing and horiz. stab on a common cen... - 1/7/2013 7:38 PM   
mithrandir



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quote:

ORIGINAL: Leroy Gardner


quote:

ORIGINAL: da Rock

In full scale use, those tabs lessen the stick loads for those surfaces. They also provide a convenient place for counterweights. The weights help balance the mass of those surfaces. When the surface is balanced it is less apt to flutter.

For models, they're not as important as we have zero stick load feedback. Our systems are driven by servos and there is no free movement possible.


If there is no feed back as you say then what is it that is stripping the gears in our servos.?.


the pilot feels the tension of the springs in the sticks... not the airloads.... that is the "feedback" mentioned...

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RE: Having engine, wing and horiz. stab on a common cen... - 1/7/2013 9:51 PM   
da Rock



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Status: online 		In the real world, many designers wished to provide the pilot with less effort required to move the stick. Sometimes they got that result by including those tabs. Sometimes they did what Chance Vought did with the Corsair and added boost tabs. Sometimes they put some leverage in the system.

Our TX's two sticks can't transfer 'stick load' at all. So we really don't need models with more easily moved surfaces.

The load on surfaces is what strips servo gears. With models, the difference in load from those tabs is insignificant compared to what our servo choices can handle.

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RE: Having engine, wing and horiz. stab on a common cen... - 1/8/2013 5:29 AM   
Leroy Gardner


 

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Status: offline 		Thanks for that explaination, now I know what you mean. Terminology used in relationship to aerodynamics can get real confusing when your just begining to learn all about it and there is sooooo much to learn. I get a bit lost in it all and at the same time I understand air foils much better just reading the threads but have a long way's to go. This is information alley for me,Leroy

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RE: Having engine, wing and horiz. stab on a common cen... - 1/9/2013 12:14 AM   
karolh


 

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Status: offline 		Here is the profile all assembled and being tested for balance. Had some concerns about it being tail heavy but was given some very sound advise as to how to proceed and things seems to be working out quite nicely.

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RE: Having engine, wing and horiz. stab on a common cen... - 1/9/2013 3:15 AM   
rmh



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Status: offline 		Nice work - that is a very good basic layout - as long as you don't have any twisting in the fuselage or tail group -it should work very well.


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RE: Having engine, wing and horiz. stab on a common cen... - 1/9/2013 5:17 PM   
karolh


 

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Status: offline 		Thanks rmh, having hopefully covered most if not all of the bases I am being very optimistic about how it will behave in flight.

Karol

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RE: Having engine, wing and horiz. stab on a common cen... - 1/9/2013 7:32 PM   
AA5BY


 

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Status: online 		There is a thrill of designing something that expresses ones interest. That thrill comes with responsibility to do diligence to better the odds for good flight manners and I'm betting you have done yours. We await the finishing and flight reports.

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RE: Having engine, wing and horiz. stab on a common cen... - 1/10/2013 2:07 AM   
cyclops2



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I am sure most every speed plane in the nitro days had everything on centerline. They were very stabile at + 100.
Even the Redhead Dyna Jets. The Jets had all surfaces below the engine.

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RE: Having engine, wing and horiz. stab on a common cen... - 1/10/2013 2:36 PM   
rmh



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quote:

ORIGINAL: cyclops2


I am sure most every speed plane in the nitro days had everything on centerline. They were very stabile at + 100.
Even the Redhead Dyna Jets. The Jets had all surfaces below the engine.

control line
the rock on a string required that the mass be balanced such that lines /airframe, stayed in a straight line .
everything on one lines works fine - as does a setup with thrust line higher or lower than the wing - you simply setup the rest of the airframe and angles to compensate
The CAP232 is a shining example -

Fly more - cogitate less.

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RE: Having engine, wing and horiz. stab on a common cen... - 1/13/2013 3:41 AM   
chuckk2


 

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Status: offline 		Back in the 70's Cessna had a similar problem with one of it's then new 4 place High Wing planes. Seems that on landing, if the attitude was not right, the horizontal stabilizer lost effectivity.


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RE: Having engine, wing and horiz. stab on a common cen... - 1/13/2013 11:23 PM   
rmh



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Status: offline 		Ah yes - the limited travel stabilator and too much flap - resulted in a sudden pitch over - bad news.

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RE: Having engine, wing and horiz. stab on a common cen... - 1/16/2013 1:42 AM   
cyclops2



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They all fly.

German " ARROW " tandam inline engines.

Most all ww II seaplanes had tails everywhere.

Engine offsetts & COG can correct almost any problem plane. So that is will glide when tossed off my 125' cliff. Most all make it gliding down.

Forgot the flying Witches, Doghouses, Lawanmowers.

If they can fly.................Anything can fly.

Enjoy the uncertainty once in a while.

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