Torsional stiffness of full size stunt model fuselage

[Matt Piatkowski]

Hello,
The torsional stiffness of the fuselage behind the TE should be maximized.
The stiffest shape is a circular tube but it carries with it the weight penalty.
Consequently, the stunt models fuselage cross-section behind the wings TE is usually somewhere between elliptical and rectangular shape.

In modern, competitive stunt flying, the corners are executed very aggressively.
At the same time, many top stunt/F2B fliers increase the size of the horizontal stabilizer and elongate the H-T-H distance.

In effect, the AFT portion of the model is exposed to higher twisting and bending loads.

When the tail of the model twists in corners, the flight path becomes distorted.

Like with almost everything in stunt flying, the builder must compromise.

Please give me your thoughts on this matter.
Regards,
M

 



 

[Jim Svitko]

Maybe the question is how much flex/twist is too much.  That is, how much can be tolerated without seriously affecting flight capabilities.

I have needed trifocals for quite some time now so maybe my eyes are playing tricks on me.   Maybe someone with better eyes than me can verify this but I have seen the twist in the aft fuselage of profiles as I watched them in flight.  Yet, from what I see, they perform quite well.  Nevertheless, the lack of rigidity in a profile is the main reason I have not built a profile for a long time.  All of the built up fuselages I make now are rectangular in cross section.  That is good enough for me and it is easier and quicker than making a circular section.

Here is something for discussion, for what it is worth:   I have the plans for both versions of Excalibur (I & II).  For Excalibur I, the plans show a very shallow aft fuselage just in front of the stab.  The top to bottom measurement is about 1.75 inches and the material is the usual 1/2 thick balsa.  The plans also show 1/8 thick balsa for the stab, elevators, and flaps.

I built the first version when it first came out but that was so long ago I do not remember if I changed anything.   I think I used thicker balsa for the flaps, stab, and elevators.  If anyone has built one per plan, let us know how it performed.

All of the materials shown on the plans for Excalibur I might work well enough but the plans for Excalibur II show 3/16 sheet for the flaps, stab, and elevators.   Also, the aft fuselage is a bit taller at 2 inches deep in front of the stab.  The 1/2 inch thick stock for the main fuselage is the same as the first version.  I do not see how adding 1/4 inch to the depth of the fuselage can make any difference but maybe it does.

[Kim Doherty]

Hello,
The torsional stiffness of the fuselage behind the TE should be maximized.
The stiffest shape is a circular tube but it carries with it the weight penalty.
Consequently, the stunt models fuselage cross-section behind the wings TE is usually somewhere between elliptical and rectangular shape.

In modern, competitive stunt flying, the corners are executed very aggressively.
At the same time, many top stunt/F2B fliers increase the size of the horizontal stabilizer and elongate the H-T-H distance.

In effect, the AFT portion of the model is exposed to higher twisting and bending loads.


Like with almost everything in stunt flying, the builder must compromise.

Please give me your thoughts on this matter.
Regards,
M

Matt,
I have been building “all molded” fuselages for the last thirty years. If you could induce the rear of the fuselage to move it would have shattered. I suspect most models with molded components would behave somewhat the same. I doubt that the forces have increased much. So here is my take:

1. Build a fully molded fuselage
2. Use the egg as your frame of reference
3. Use “tortured” compound shaping of the wood (bending in the direction it does not want to go)
4. Close all boxes internally
5. Pay homage to the mighty “Triangle” and use them when possible
6. Leave enough width at the rear to provide a solid mount for the stab
7. Learn to use composites properly.
8. Keep everything light

There are NO compromises in a properly engineered and built fuselage.

Kim

[Howard Rush]

Listen to Kim, Matt

The torsional stiffness of the fuselage behind the TE should be maximized.

That's why I machine mine out of tungsten bar stock.

The stiffest shape is a circular tube but it carries with it the weight penalty.

Wrong sign.  Tube is stiffest for a given weight.  Right, stress man?

Like with almost everything in stunt flying, the builder must compromise.

The builder must optimize.

[Chris Wilson]

Twin booms connecting both flying surfaces should be stronger yet it's rarely adopted.
Personally if I was trying to eliminate twist then spiral wrapped carbon thread as used in high performance drive shafts should be the ultimate.
Chris.

[Howard Rush]

Personally if I was trying to eliminate twist then spiral wrapped carbon thread as used in high performance drive shafts should be the ultimate.

Reckon it would work for flaps?

[Air Ministry .]

https://engineeringlibrary.org/reference/beam-torsion-air-force-stress-manual

Grab the nose between your knees , support the wing on things , Grab the tailplane & twist like s..t .

If it doesnt fall off, its on pretty secure ! 

Stiffness / reinforcing At Tailplane centersection ( and out a bit ) .-> AND say ply doublers epoxied into that , CAN make something ( Eliptical ) , As durable & suportive as
a unreinforced contraption twice dimesionally sized .

https://stunthanger.com/smf/index.php?action=dlattach;topic=43651.0;attach=254770;image

Thats the Typhoon . Ditto . Sorta likea Yatsenko . a ' Member from the Wing Rear Spar ( Trailing Edge ) to the taiplane front . Leading Edge . As its widest there thus most supportive .

https://i.pinimg.com/originals/37/d1/a7/37d1a73dc140c047233ca5d2c627e403.jpg

These are quite intresting, with the ' Engine Bit ' , Wing Bit ' , and the ' aft bit ' . But the Tailplane is a ' further aft bit ' . LIKE a SPITFIRE .

https://www.asisbiz.com/il2/Spitfire/RAF-601Sqn-UF/images/Spitfire-tail-section-from-RAF-601Sqn-at-Lentini-West-Sicily-1943-IWM-CNA1329.jpg

And to be diplomatic, we will include a P-51 , which wernt to bad, either .

http://warbirdsnews.com/wp-content/uploads/NEAMs-P51D-N13Y-mustang-racer112.jpg

https://stunthanger.com/smf/index.php?action=dlattach;topic=43652.0;attach=254874;image

[Air Ministry .]

And Another Thing .

[Chris Wilson]

Reckon it would work for flaps?

What, 45 degree angles carbon thread similar to what top fuel drag cars use  in their drive shafts, the epitome of torsional stiffness?
Nah, best to stick with what you recommend mate, billet tungsten seems so much better.
Chris.

[Brett Buck]

The torsional stiffness of the fuselage behind the TE should be maximized.
The stiffest shape is a circular tube but it carries with it the weight penalty.
Consequently, the stunt models fuselage cross-section behind the wings TE is usually somewhere between elliptical and rectangular shape.

In modern, competitive stunt flying, the corners are executed very aggressively.
At the same time, many top stunt/F2B fliers increase the size of the horizontal stabilizer and elongate the H-T-H distance.

In effect, the AFT portion of the model is exposed to higher twisting and bending loads.

   What Kim is telling you is the right answer to your "question (in the form of an assertion}".

     In conjunction with the "tube" concept, you don't need thick fuselage sides (like 1/8"). I use the lightest 3/32 I can find for the aft fuselage and it forms a flattened tube structure. The turtledeck is molded 3/32.  Inside, I have formers every 2", built-up of 1/16x1/4" mediun balsa so the grain runs the right way all the way around it. In between the formers, at the top and bottom of the fuse sides, I have diagonal braces of 1/16x1/4 balsa forming an "X" between each former. After the rear former, right in front of the stab, it is open on the top. I build a box structure between the fuse sides with enough clearance to clear the pushrod.

   But, most of the stab deflection appears to be right around where it mounts to the fuselage, which is where it also typically the least stiff.   I also use a 1/32" medium *doubler* with the grain running vertically from the bottom of the fuse side to the stab mount, from the rear former  to the tail post. This reduces the local flex of the fuse sides where they tend to bend the most (and where there is a pretty big hole for pushrod access).

    A key thing to remember is that, for the same reason a tube is the stiffest for a given mass, *the most efficient place to put mass is around the outside of the structure*. All that stuff inside is just to keep it in the right shape. That is also why the finish makes some difference in the result. Depending on what you use and how stiff it is, it is in the most efficient place possible.


     Brett

[Perry Rose]

The simpliest is the Legacy and Legacy 40 fuselage. Two diagonal formers between the normal formers.

[Matt Colan]

   What Kim is telling you is the right answer to your "question (in the form of an assertion}".

     In conjunction with the "tube" concept, you don't need thick fuselage sides (like 1/8"). I use the lightest 3/32 I can find for the aft fuselage and it forms a flattened tube structure. The turtledeck is molded 3/32.  Inside, I have formers every 2", built-up of 1/16x1/4" mediun balsa so the grain runs the right way all the way around it. In between the formers, at the top and bottom of the fuse sides, I have diagonal braces of 1/16x1/4 balsa forming an "X" between each former. After the rear former, right in front of the stab, it is open on the top. I build a box structure between the fuse sides with enough clearance to clear the pushrod.

Do you build the nose with 1/8” fuselage sides and then butt 3/32” behind the doublers or wing?

[Jim Hoffman]

Speaking only for myself, I’ve had great success using 3/32 balsa fuse sides and 1/64 plywood doublers from spinner to 1” aft of flap hinge line. I also put a 1/64 plywood fuse doubler at the stab and any fuse access holes. 

The fuse external is 100% covered with 0.2 oz. carbon tissue.  00 silkspan is a good substitute for the carbon tissue

I also box in the fuse as described by Kim and Brett.

[Brett Buck]

Do you build the nose with 1/8” fuselage sides and then butt 3/32” behind the doublers or wing?

     1/8" in the front spliced to 3/32 over the wing this time (just to give myself a bit more meat to carve around the nose). Its not butt-glued, it is a 12:1 splice, backed up by the 1/64 ply doubler (that goes around the entire wing cutout).

   Like Jim, my previous airplane was also 3/32 all the way around, it was fine, too. I had much better wood available this time. I am sorely tempted to go to molded 1/16 next time (presumably electric), but it would have to be reinforced around the stab mount.


   That was most of my point to Matt P. above - the majority of the fuselage is relatively stiff, it's the area around the stabilizer mount that most of the flex seems to occur. Unfortunately that is the place where weight hurts you the most.

    Of course, at one point I have had 3/4 ounce of *lead* stuck to the outside right under the stab. 3/4 ounce is *a lot* of balsa.

     Brett

 

[Bruce Shipp]

Traditionally, designs have had fuselages  tapered to a point at the aft end with the fuse sides glued to each other at the tail and the top and bottom blocks or sheeting tapered to a point. I have always added a 1/4” to 1/2” tail post between the fuselage sides. This requires gradually widening the top and bottom blocks and formers between the wing TE and the tail if the plans show the traditional construction. 

 It does add a bit of weight to the tail but I felt that was worth the advantages: a larger cross section at the tail resulting in greater torsional stiffness, a wider mounting platform for the horizontal stab and much more room for the elevator horn and pushrod hardware. 

[Dan McEntee]

     1/8" in the front spliced to 3/32 over the wing this time (just to give myself a bit more meat to carve around the nose). Its not butt-glued, it is a 12:1 splice, backed up by the 1/64 ply doubler (that goes around the entire wing cutout).

 
     Brett

   If I ever get around to building a new airplane, I was going to try splicing the sides as even decent 1/8" by 4 by 48" sheets are very hard to find. I was going to go with thinner, heavier balsa spliced on to the rear for length and even a narrow length spliced  spliced onto the top or bottom (depending on how the fuselage is shaped) for the width if needed. I figure that the neat' clean and well glued joint would add something to the structure.
    Type at you later,
     Dan McEntee

[Brett Buck]

   If I ever get around to building a new airplane, I was going to try splicing the sides as even decent 1/8" by 4 by 48" sheets are very hard to find. I was going to go with thinner, heavier balsa spliced on to the rear for length and even a narrow length spliced  spliced onto the top or bottom (depending on how the fuselage is shaped) for the width if needed. I figure that the neat' clean and well glued joint would add something to the structure.
    Type at you later,
     Dan McEntee

I think even 6 lb 1/16th would be plenty, even for IC engines,  if properly reinforced around the point loads AND you have a real paint job with silkspan/dope or graphite/dope  The problem is that there is nearly no tolerance left for sanding it and the chance of just poking your finger through it.

    Brett

   

[Chris Wilson]

Code: [Select]

Reckon it would work for flaps?

Seriously, if F2C uses 45 degree carbon threads on the wing to achieve ultimate torsional stiffness then I reckon it maybe worth a look at  for flaps as well.
Chris.

[Air Ministry .]

 Durability / lifeing / handling , etc . A one season top line Lt Wt wound neccesarilly be as DURABLE as something built like a tank . Even though it IS as STRONG . At least strenght / stressc/ Wt. Equality .





 

Actually , his 5.000 1/16th sheet bulkheads aint a bad idea. But horrified they stop short of the top , for pushrod clearance .

A sandpaper wrapped dowel for finishing rough cut pushrod ( and other ) holes , bukheads / formers installed , works well .



 

[Matt Colan]

     1/8" in the front spliced to 3/32 over the wing this time (just to give myself a bit more meat to carve around the nose). Its not butt-glued, it is a 12:1 splice, backed up by the 1/64 ply doubler (that goes around the entire wing cutout).

I may sound like an idiot asking this, but what is a 12:1 splice?

[Dan Berry]

I may sound like an idiot asking this, but what is a 12:1 splice?

Shallow angle

[Ken Culbertson]

I may sound like an idiot asking this, but what is a 12:1 splice?

It is a splice you do on your lunch break.

Seriously it is the run:rise of the joint.  A 12:1 would be 1" of rise for every 12" of run.

Ken

[Matt Colan]

It is a splice you do on your lunch break.

Seriously it is the run:rise of the joint.  A 12:1 would be 1" of rise for every 12" of run.

Ken

That’s what my first thought was but I wanted to make sure I wasn’t incorrect 

[Brett Buck]

I may sound like an idiot asking this, but what is a 12:1 splice?

   As noted. It's really not under much load, so you can do it any way you want and it will be OK.

     Brett

[Matt Piatkowski]

Thank you, guys.

I am building another eParrot.
Its fuselage and wings will be longer and the hinge-to-hinge distance (h-t-h) will be 20".
The horizontal stabilizer will be 6% larger than in eParrot XL that has an h-t-h distance of 18".
The fuselage cross-sections will be about 15% smaller than in eParrot XL - this is the main reason why I initiated this thread.
15% cross-sectional reduction comes from rounded corners.
All fuselage cross-sections in eParrot XL are rectangular. The new fuselage cross-sections are inscribed in these rectangles.

The weight penalty for a longer wing, fuselage and 6% larger horizontal stabilizer will be about 3.5 oz.

eParrotXL RTF weight with Himax 3516-1030 kontra, two carbon composite propellers, two Spin44 PRO ESCs, Fiorotti's timer, and 4S 4000 ThunderPower is 54 oz.
The new eParrot (eParrot 2) will weigh 57.5 oz. with wings loading 12.78 oz./ft.^2.

I hope that eParrot 2 will fly well with Himax 3516-1030 and/or one of the TMT contra systems made in Poland.
I am considering also the geared GAU/D contra, powered by any of the available single motors with appropriate Kv.

Regards and Happy Flying,
M

[Brett Buck]

The fuselage cross-sections will be about 15% smaller than in eParrot XL - this is the main reason why I initiated this thread.
15% cross-sectional reduction comes from rounded corners.
All fuselage cross-sections in eParrot XL are rectangular. The new fuselage cross-sections are inscribed in these rectangles.

  So, a box-section, which is fine, and easy to build. Out of curiosity,  what are the wood dimensions you plan to use (thicknesses, width, and height)? 

     Brett

[Howard Rush]

Code: [Select]

Seriously, if F2C uses 45 degree carbon threads on the wing to achieve ultimate torsional stiffness then I reckon it maybe worth a look at  for flaps as well.

I could maybe wrap some prepreg around an aluminum tube, cook it, then dissolve out the aluminum somehow.  Think that would work?

[Howard Rush]

I may sound like an idiot asking this, but what is a 12:1 splice?

12 parts balsa, 1 part glue.

Hope that helps.

Your new stunt plane is really pretty, by the way.

[Brett Buck]

I could maybe wrap some prepreg around an aluminum tube, cook it, then dissolve out the aluminum somehow.  Think that would work?

     Just a note from another spectrum-dweller - I usually reserve my snark for someone who has done something to deserve it. Or Larry.

     Brett

[Howard Rush]

     Just a note from another spectrum-dweller - I usually reserve my snark for someone who has done something to deserve it. Or Larry.

Sorry. It’s the blue fillets talking.

[bob branch]

 I had been concerned with torsional resistance of the fuse a few years back and wanted to study if a profile could be made as stiff as a full built fuse. Turns out you can and with very simple construction methods. Here is a link to the build thread and what i found. ...   https://stunthanger.com/smf/gettin-all-amp'ed-up!/c-14-carbon-fuse-electric-profile/msg523647/#msg523647

It also present a smiple way to quantify the results. No observers of flight tests were able to detect any visual twisting off of the tailplane in maneuvers.


bob branh

[Matt Colan]

12 parts balsa, 1 part glue.

Hope that helps.

Those 4 parts balsa 1 part glue pieces must weight a ton 

Your new stunt plane is really pretty, by the way.

Thank you! It flies some pretty good stunt too

[Howard Rush]

I had been concerned with torsional resistance of the fuse a few years back and wanted to study if a profile could be made as stiff as a full built fuse. Turns out you can and with very simple construction methods. Here is a link to the build thread and what i found. ...   https://stunthanger.com/smf/gettin-all-amp'ed-up!/c-14-carbon-fuse-electric-profile/msg523647/#msg523647

It also present a smiple way to quantify the results. No observers of flight tests were able to detect any visual twisting off of the tailplane in maneuvers.

Cool story, although the title dates you.

We put a camera on Mark Scarborough’s profile Impact awhile back.  Curiously, aft fuselage side-to-side deflection looked greater than twist.

[Larry Fernandez]

     Just a note from another spectrum-dweller - I usually reserve my snark for someone who has done something to deserve it. Or Larry.

     Brett

What?
I'm just sitting here minding my own business.

Larry Buttafucco Stunt Team

[Ken Culbertson]

, aft fuselage side-to-side deflection looked greater than twist.

When you know it is happening it is really hard to tell one from the other, both produce a fu**** up corner.  There is a reason a Twister is called a Twister.  I have slow motioned the one I flew for years and you can see the tail end flopping around.  It flew so well in good air that I had to remind myself that it was a profile and as such it was it's birthright to take off on some unexpected tangent every now and then.  Only thing I know of that reduces both movements is fuselage width and some serious cross bracing.  Without the width there is little you can do in a 3/4" profile to eliminate it.

Ken

[Chris Wilson]

     Just a note from another spectrum-dweller - I usually reserve my snark for someone who has done something to deserve it. Or Larry.

     Brett

Howard, If I have offended you the I am truly sorry and please accept that it was unintentional.
Chris.

[Air Ministry .]

" There is a reason a Twister is called a Twister."

the Bob Hunt lesson / pronouncement R: ' To Narrow fuselages '. par tick u larly at the rear , tailplane L.E. / Hinge line .

At the hinge line, it make fiting pushrods & horns awkward. Thing Foul . Ply doublers carved away for control movement clearance arnt an optimum solution .

It would appear near 1/2 O.A. Width isnt a bad idea , at the tailplane leading edge . 1" to 1 1/4 " , with 3/4 " to 1 " at the hinge / Horn . So things fit .

If ive gottit wright , Bob says His last Genisis Mk III World Chaps plane ( 1981 ) was a bit narrow aft , and on occasion ( of its choosing , could suddenly veer elsewhere , in a stiff breeze )
Point Being , in unruly air , The Wind Load / Side Load / Torsional Force , may abrubtly alter . So a sudden deflection occurs . Like with understrenght horns & pushrods .
But with the overall alignment of the wing & Tailplane . And obviously the flaps & elevators , also the wake / wash , and force generated by their deflections alignment .

Plus maybe slight ' neutral '  control missalignment ( Flap to elevator relationship ) due to mechanical lengths altering due to structure deflection .

His ( Bobs ) Crossfire was A BEEFED UP FUSElAGE Derivative of the flawed Genisis III . If ive got that right. Swept hingeline . *

Read Recently  allusion , a straight ruler dragged through water wavers Laterally . Whereas if you cut the aft edge tapered forward outward , It wont osscilate . Wynn Paul stateing Bob's obsevations .
Seeing as recently swept Fwd. T.E. / Hinge lines wewre being discussed .

[Steve_Pollock]

In mentioning the P-51 in entry #6, it should be noted that the P-51D flight manual specifically discourages snap-rolls which may cause the aft fuselage to separate. 
The following quote is from AAF Manual 51-127-5, (revision published 15, Aug 1945), p78:

"The aerodynamic characteristics of the P-51D are such that snap rolls cannot be satisfactorily performed.  This has been proved by a long series of test flights.  So don't try any snap rolls in an attempt to show that you're the guy who can do them.  You'll invariably wind up in a power spin - and that's bad.  (Caution: Acrobatics must not be attempted unless the fuselage tank contains less than 40 gallons of fuel.)"

The fuselage fuel tank is behind the cockpit.  It's a "rear fuselage torsional stiffness" thing.

[Howard Rush]

Howard, If I have offended you the I am truly sorry and please accept that it was unintentional.
Chris.

On the contrary, I am a nasty person making an inside joke at your expense. That is how I make my flap torque tubes, as Brett knows. 

[Dennis Moritz]

What about carbon fiber lay up in whatever section thought best?

[Dennis Moritz]

What does Orestes do? Or rather, the folks who make his kit. What do they do?

[Matt Piatkowski]

Please see the attached.
One of my colleagues in Poland is finishing the fuselage of this plane that is another variation of Gee Bee R3.
The photo has been published here with his permission.

The balsa strips method used is very time-consuming but the final results are exceptional from the torsional stiffness point of view.

Fly safely,
M

[Matt Piatkowski]

For Brett:

EParrot 2 fuselage sides: 1/8" contest balsa, gradually sanded in the aft fuselage part to 3/32".
Front fuselage part reinforced with thin glass fabric laminate on the outside of the balsa skins. Carbon strips will be also used.
Top and bottom fuse pieces: 3/32" contest balsa semi-circular shells with 1/16" medium balsa ribs.

I will publish the plan of the fuselage with dimensions.

M