Hangar 9 20cc Tiger Moth
01-09-2014, 02:27 AM
#101
Balloning + Decalage check...
To try to re-adjust the climbing tendency of the plane, a first action was to give a bit more downthrust to the firwall, however, this may not be sufficient to give a good all-round solution - decalage modifications should also be combined; for sure, more downthrust will only cause a side-effect of ballooning on landing upon throttle cut - been there done that. The solution is to check what's going on with the decalage as well.
So, the plane was false-assembled for a quick measurement with the trusty Robart incidence gauge:
- Fuz was set at 0 degrees, using the top engine box plank as a datum (here, I assume this is parallel to the main "stringer" of the plane, which would be used as a datum line if this was a kit instead of an ARF - hope I'm right!).
- Engine was @ -2 degrees (as modified - see previous post).
- Tailplane seat was @ +1.5 degrees.
- Bottom wing was measured at +4.0 degrees.
- Top tank/wing was measured at + 5.5 to 6.0 degrees.
Now, this means that there is a positive decalage of 5.5/6.0-4.0 = 1.5 to 2.0 degrees.
If, at flying attitute, we assume the stab to fly at 0 degrees, then
- The fuz will fly at a -1.5degree nose down stance (which "sounds" about right for a TM)
- The effective downthrust will be -1.5 +(-2.0) = -3.5degrees (a generally accepted angle for lightly loaded scale planes)
- The bottom wing will fly at 4.0-1.5 = 2.5 degrees
and
- The top wing will fly at 6.0-1.5 = 4.5 degrees.
- Decalage is then +2.0 degrees.
(Have a look at the attached sketch).
Question now is, at what flying speed will the plane trim out with such decalage. To me, after seeing these values, the ballooning effect is a certainty - plane will climb at idle! Also, take into account that the effective AoA of an undercambered airfoil actually is positive at zero degrees. This further exacerbates the effect.
A fair bit of Googling around (i.e. literature survey) for decalage suggestions from both full-size and scale builders has indicated the following facts:
- Positive decalage is used to incur a benign stall. This has to do with the downdraft of the top wing affecting the actual AoA of the bottom wing (increases it). This pushes the bottom wing to stall first, "hanging" the bipe only from the top wing which is still flying. In a positive stagger design, this will induce a pitch-up (i.e. unstable post-stall behavior). By increasing the incidence of the top wing, the designer makes sure this stalls first (and makes for a "mushy" stall).
- Negative decalage is used to promote sharp stalling, usually used in aerobatics.
- Full size Tiger Moths are rigged in different ways for different purposes. For aerobatics displays or cross-country flights, pilots rig the plane with zero/negative decalage to increase the neutral-trim-cruising-speed and/or to allow for nice clean break upon spinning. For pleasure/training flights, the plane is rigged with positive decalage for increased safety margins in the stall.
- Many modellers have radically improved the behavior of their planes by using negative decalage.
Clearly, the H9 designer has decided to make the plane benign and compromised it by allowing a very small cruising speed (i.e. idle). Anything else, its a homesick angel.
One final snack for thought.
We have all seen the error in the H9 manual regarding the CG values - a pretty basic piece of data for the plane and one that can destroy a new plane. Pretty bummer from H9.
Now, consider the piece of info regarding the mounting of the top wing tank from the same manual (page 16, see attached pdf). If somebody made a mistake on the CG, then it is wildly possible that also this little step has been mixed-up. Just as a check - if one swaps the 235mm strut with the 230mm strut (front-to-back), the top tank sits nicely (without any stress) at +2.5 degrees, instead of +6.0. This changes the decalage from +2 to -2.5 degrees.
With this modification, at a flying attitude (i.e. stab @ zero, fuz @ -1.5), the plane will "sit" as follows:
- Total effective Downthrust -3.5 degrees
- The bottom wing +2.5 degrees
- The top wing will fly at +1 degree.
- Decalage -2.5 degrees.
I am willing to make a bet that this solves most of what we perceive as ballooning (i.e. down-trim at a slow cruising speed) while allowing for crisper aerobatics...
My plane will take months to complete - Easter maybe, so we will no know until then.
Is anybody willing to quickly try this strut swap and see what happens to the behavior of the plane? Worse case, it will need to hold a bit of up on finals and landing and possibly snap a bit easier if pushed!
So, the plane was false-assembled for a quick measurement with the trusty Robart incidence gauge:
- Fuz was set at 0 degrees, using the top engine box plank as a datum (here, I assume this is parallel to the main "stringer" of the plane, which would be used as a datum line if this was a kit instead of an ARF - hope I'm right!).
- Engine was @ -2 degrees (as modified - see previous post).
- Tailplane seat was @ +1.5 degrees.
- Bottom wing was measured at +4.0 degrees.
- Top tank/wing was measured at + 5.5 to 6.0 degrees.
Now, this means that there is a positive decalage of 5.5/6.0-4.0 = 1.5 to 2.0 degrees.
If, at flying attitute, we assume the stab to fly at 0 degrees, then
- The fuz will fly at a -1.5degree nose down stance (which "sounds" about right for a TM)
- The effective downthrust will be -1.5 +(-2.0) = -3.5degrees (a generally accepted angle for lightly loaded scale planes)
- The bottom wing will fly at 4.0-1.5 = 2.5 degrees
and
- The top wing will fly at 6.0-1.5 = 4.5 degrees.
- Decalage is then +2.0 degrees.
(Have a look at the attached sketch).
Question now is, at what flying speed will the plane trim out with such decalage. To me, after seeing these values, the ballooning effect is a certainty - plane will climb at idle! Also, take into account that the effective AoA of an undercambered airfoil actually is positive at zero degrees. This further exacerbates the effect.
A fair bit of Googling around (i.e. literature survey) for decalage suggestions from both full-size and scale builders has indicated the following facts:
- Positive decalage is used to incur a benign stall. This has to do with the downdraft of the top wing affecting the actual AoA of the bottom wing (increases it). This pushes the bottom wing to stall first, "hanging" the bipe only from the top wing which is still flying. In a positive stagger design, this will induce a pitch-up (i.e. unstable post-stall behavior). By increasing the incidence of the top wing, the designer makes sure this stalls first (and makes for a "mushy" stall).
- Negative decalage is used to promote sharp stalling, usually used in aerobatics.
- Full size Tiger Moths are rigged in different ways for different purposes. For aerobatics displays or cross-country flights, pilots rig the plane with zero/negative decalage to increase the neutral-trim-cruising-speed and/or to allow for nice clean break upon spinning. For pleasure/training flights, the plane is rigged with positive decalage for increased safety margins in the stall.
- Many modellers have radically improved the behavior of their planes by using negative decalage.
Clearly, the H9 designer has decided to make the plane benign and compromised it by allowing a very small cruising speed (i.e. idle). Anything else, its a homesick angel.
One final snack for thought.
We have all seen the error in the H9 manual regarding the CG values - a pretty basic piece of data for the plane and one that can destroy a new plane. Pretty bummer from H9.
Now, consider the piece of info regarding the mounting of the top wing tank from the same manual (page 16, see attached pdf). If somebody made a mistake on the CG, then it is wildly possible that also this little step has been mixed-up. Just as a check - if one swaps the 235mm strut with the 230mm strut (front-to-back), the top tank sits nicely (without any stress) at +2.5 degrees, instead of +6.0. This changes the decalage from +2 to -2.5 degrees.
With this modification, at a flying attitude (i.e. stab @ zero, fuz @ -1.5), the plane will "sit" as follows:
- Total effective Downthrust -3.5 degrees
- The bottom wing +2.5 degrees
- The top wing will fly at +1 degree.
- Decalage -2.5 degrees.
I am willing to make a bet that this solves most of what we perceive as ballooning (i.e. down-trim at a slow cruising speed) while allowing for crisper aerobatics...
My plane will take months to complete - Easter maybe, so we will no know until then.
Is anybody willing to quickly try this strut swap and see what happens to the behavior of the plane? Worse case, it will need to hold a bit of up on finals and landing and possibly snap a bit easier if pushed!
Last edited by hpergm; 01-09-2014 at 02:35 AM.
01-20-2014, 11:28 AM
#103
Futaba servo reversing
I am using Futaba 6ch 2.4 with the fs617 receiver.
Due to the space alloted to the elevator servos I need to have both servos turn opposite to each other. They currently both turn in the same direction.
The servos are connected to a "Y" harness. How can I achieve that?. There doesnt seem to be a way to do it with the radio.
Due to the space alloted to the elevator servos I need to have both servos turn opposite to each other. They currently both turn in the same direction.
The servos are connected to a "Y" harness. How can I achieve that?. There doesnt seem to be a way to do it with the radio.
01-20-2014, 12:34 PM
#104
Join Date: Dec 2001
Location: Snohomish, WA
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rafeeki,
If your programmable transmitter does not have provision for two elevator servos with independent servo reversing (as my DX-7 does, fortunately), then you can always use a reversible y-jack.
The best solution, if you are transmitter-limited or receiver-limited, is to use a JR Matchbox. The Matchbox is an independent servo mixer. I use Matchboxes on a lot of my airplanes, like my four-aileron-servo Pitts Special. In that case, I was able to plug the four servos into the four ports on the Matchbox and adjust the servo direction and throws for each servo individually. It will work with your Futaba FS617 just fine, and since the Matchbox plugs into only one port on the receiver, this allows the aux ports to be open and available for other functions if needed. It makes it possible to run as many as seven servos (ala my Pitts), on a four-channel receiver. Nice.
If your programmable transmitter does not have provision for two elevator servos with independent servo reversing (as my DX-7 does, fortunately), then you can always use a reversible y-jack.
The best solution, if you are transmitter-limited or receiver-limited, is to use a JR Matchbox. The Matchbox is an independent servo mixer. I use Matchboxes on a lot of my airplanes, like my four-aileron-servo Pitts Special. In that case, I was able to plug the four servos into the four ports on the Matchbox and adjust the servo direction and throws for each servo individually. It will work with your Futaba FS617 just fine, and since the Matchbox plugs into only one port on the receiver, this allows the aux ports to be open and available for other functions if needed. It makes it possible to run as many as seven servos (ala my Pitts), on a four-channel receiver. Nice.
01-20-2014, 12:35 PM
#105
Join Date: Dec 2001
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The fin bolts down to the stabilizer with 3mm metric bolts, from underneath the stab and into blind nuts in the fin base. Then the stab gets bolted down to the aft fuselage. The instructions don't show this very well, but a close examination of the tail feathers will reveal the secret of how this is done.
01-21-2014, 06:27 AM
#108
There used to be a few old WWI planes in an outdoor mueseum in Athens that I could see from my balcony at the Hilton hotel way back in late 1970's and one was a Tiger Moth. I remember it well as there was a tourist within eardrop explaining to his young son how its mouth looked like a Tiger!!! I never forgot that. So here I am assembling one.
01-30-2014, 07:06 AM
#112
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I am at the same stage. Just finished fixing the bad design of the wing carry fixture. I am using a Laser 150 in mine. I will do some engine running before making any additional cuts in the cowl
Ric
Ric
01-31-2014, 09:25 AM
#117
Dick,
Did u prop up the LE of the stab? If yes did it make a difference?
Not sure where I read that the aileron deflection should be different between up & down. How does one do that? Did u stick with the suggested deflections in the instruction booklet?
Thanks
Rafeek
Did u prop up the LE of the stab? If yes did it make a difference?
Not sure where I read that the aileron deflection should be different between up & down. How does one do that? Did u stick with the suggested deflections in the instruction booklet?
Thanks
Rafeek
02-01-2014, 06:35 AM
#118
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I used the throws as stated in the instructions. I will also program in dual rates at 75% and 50%. The aileron differential was put in with the radio, each servo is on a separate channel and the throws are set individually and the two channels slaved together. I will start with the elevator at the design position and adjust from there. I did not get to balance yet Maiden flight will come when the weather breaks, maybe a month away
Ric
Ric
02-03-2014, 04:44 AM
#119
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In terms of a pilot, I put in the 1/4 scale "Captain Eddy" figure from Ace. I also stripped all of the trim/letters that came with the airplane and replaced it with British markings. I also cut my own "roundels" out of Monokote (sorry, no good photos of the bottom/top of the wings). O.S. GT-22 for power.
