Legacy 40 - Should I move the stab and elevator back slightly?

[Teodorico Terry]

Hello,

I am currently building a Legacy 40 based on the Brodak kit; I ordered the rib set and plans and building the rest of the model from scratch.  It is going to be electric powered so I have made a few changes to keep the model light:  built stab and elevator, lighter construction on the fuse and possibly built up flaps.

I decided on the Legacy based on photos I had seen but once I received the plans I realized that the model is fairly short coupled and that the stab and elevator are rather large.  I was thinking about moving the stab 1" rearwards and I was wondering if anyone had any thoughts.  Last winter I built a heavily modified SIG Fazer and the most significant change was moving the stab back about 2.5" (the stab and elevator are larger than original in span, cord roughly the same).  The model tracks nicely but will do a sharp corner when asked to.  Although not perfect I like the way it flies and I was wondering if moving back the stab on the Legacy would improve its flying characteristics.

Thank you and Merry Christmas,

Teo

[Perry Rose]

It works for me. Build a Legacy stab/elev. for it. The thicker wood helps a lot. Shoot for 28% of the wing area.

[Ken Culbertson]

I have yet had a design that was not improved by lengthening the tail or making it bigger.  I was using 30% stabs back in the 60's but the longer tails are rather new to me.  I am at 19.5" and 35% now.  How much is too much?  Haven't gotten there yet but at 51% it becomes a Canard...   From what I have seen of the several Legacy 40's around here, it could easily take another inch and 5% without destroying the ship's lines.  If it were me the entire 5% would go to the stab.

[Joe Gilbert]

A stock Legacy 40 kit with LA46 built a a good weight ,trimmed well ,will fly better than 80 percent of us flying today.   Start moving things around and  weigh goes up to get balance correct.

Joe Gilbert

[Brett Buck]

A stock Legacy 40 kit with LA46 built a a good weight ,trimmed well ,will fly better than 80 percent of us flying today.   Start moving things around and  weigh goes up to get balance correct.

    +1!!

    While someone might be able to make it better (or more likely, worse) by modifying it, the way it flies has vastly more to how well it is built, powered, and trimmed, than how it is designed, as long as the design is reasonable - which this one is.

     Brett

[Crist Rigotti]

I built one of the 2 prototypes of the Legacy 40.  What Joe said and Brett agreed are true.  Build it like the plans.  However, I would lengthen the nose an inch for electric.  Because electric like the CG a little further forward.  The Legacy 40 is a great flyer!  You'll really like it.

[Alan Buck]

I have the second prototype it flies very well as stock per plan
and at 47oz  with a la 46 max

[Teodorico Terry]

Thank you for the comments; it seems that the consensus is to build according to plans.  The stab and elevators are to plans, the only difference being that the elevator is built up rather than a solid sheet of 1/4 balsa.  I might do the same with the flaps if I feel that they will be sufficiently stiff in torsion.

I weighed a Brodak 40 today while at the hobby shop and the motor comes in at about 8.5 oz with the muffler.  This is the motor shown on the plans.  Mine will be electric. Power will be an Eflite 25, Castle ESC, timer and a 2600 mah 4S battery.  The motor and ESC come in at 9 oz, adding the battery and timer brings the total up to 19 oz.  Even if I add 4.5 oz for the fuel and tank in case of the Brodak, the electric system is about 7 oz heavier.  I do not think that a forward CG will be a problem.  This is what got me thinking about moving the stab and elevator 1" back.  I do not think that I can go further than that simply because the fuselage becomes too narrow to be able to package the linkage. 

I might be able to use a slightly smaller motor such as a Power 15 but the weight saved is only about 1.2 oz.  I will be cutting the wires to size so that everything is short and neat but I do not think that I will be able to save much weight that way, maybe 1/2 oz.

Given that context should I still build to plans?  I should have added this detail in the original post.

Thank you,

Teo

[Crist Rigotti]

I have the second prototype it flies very well as stock per plan
and at 47oz  with a la 46 max

Did you get it from Walt?

[Crist Rigotti]

Thank you for the comments; it seems that the consensus is to build according to plans.  The stab and elevators are to plans, the only difference being that the elevator is built up rather than a solid sheet of 1/4 balsa.  I might do the same with the flaps if I feel that they will be sufficiently stiff in torsion.

I weighed a Brodak 40 today while at the hobby shop and the motor comes in at about 8.5 oz with the muffler.  This is the motor shown on the plans.  Mine will be electric. Power will be an Eflite 25, Castle ESC, timer and a 2600 mah 4S battery.  The motor and ESC come in at 9 oz, adding the battery and timer brings the total up to 19 oz.  Even if I add 4.5 oz for the fuel and tank in case of the Brodak, the electric system is about 7 oz heavier.  I do not think that a forward CG will be a problem.  This is what got me thinking about moving the stab and elevator 1" back.  I do not think that I can go further than that simply because the fuselage becomes too narrow to be able to package the linkage. 

I might be able to use a slightly smaller motor such as a Power 15 but the weight saved is only about 1.2 oz.  I will be cutting the wires to size so that everything is short and neat but I do not think that I will be able to save much weight that way, maybe 1/2 oz.

Given that context should I still build to plans?  I should have added this detail in the original post.

Thank you,

Teo

Sure, build it to the plans.  Stick with the Eflite 25 though.

[Alan Buck]

Crist. yes i did and it still flies well.
 Watt is doing well and still building planes for beginers 
  a southern gentleman and a great freind

[Crist Rigotti]

Crist. yes i did and it still flies well.
 Watt is doing well and still building planes for beginers 
  a southern gentleman and a great freind

Alan, That's good to hear.  Please send him my best.

[Dan McEntee]

   I have a Legacy .40 that was built by the late Ken Nash from Joplin,. MO. It is scratch built with foam wings. A bit portly at 60 ounces but had a LA.46 in the nose and I thought it was a good way to learn that engine.  I think it's just fine with stock moments and even at the weight mine is has a very good corner which I attribute to the stab/elevator area and tail moment. I have a kit stashed away and plan to build it or one from scratch from Watt Moore plans that I have the Allen Brickhaus gave me.  The one I have has an untold number of flights on it and the nose is starting to fall apart. I swapped out the engine for another LA.46 with Brian Gardner ABC guts in it. At first I was disappointed in that set up, as after breaking in the engine, it would not pull an APC 12.25-3.75 in this airplane at first, and I had to bounce around experiment with other props. It wasn't giving me bad runs, just didn't seem to want to make any power. I had used this prop like a lot of other people on the stock engine with good results. I let the model sit over that winter and when spring came around again, I thought I would give it one more try with the 12.25-3.75 before dropping the stock engine back in, and on the first flight it was like old times! I guess the engine just needed more running in. It never did over heat, or run in what i would call a bad way, just didn't have any poop!  But now, all is well. So in short, I'll cast another vote in favor of building it as designed.
    And I will add I would question making the nose longer for the electric conversion. I think that may mess with the turn rate. If you want a more forward balance point, put a bigger motor or battery in the nose, in my humble opinion. Even if it took an ounce of lead, I think that is a better way to go.
  Type and you later and HAPPY HOLIDAYS!!
    Dan McEntee

[Teodorico Terry]

Hello guys,

Once again thank you for the replies and I will build to plans.  We will see how it turns out.  I am hoping for about 50 oz ready to fly which leaves me about 30 oz for the airframe.

Thank you and Happy Holidays!

Teo

P.D.  Crist, I have read through a couple of your threads and found them really useful.  Thanks.

[Crist Rigotti]

You are welcome!

[Perry Rose]

Before I cause any more damage to my scratch built Legacy 40 can anyone tell me the dimension from the wing leading edge to the spinner back plate?

[Teodorico Terry]

Hi - I just measured the drawings:  9.5"

[Dwayne Donnelly]

Eflite 25 is definitely over kill, a 15 will pull this plane very well. 

[Perry Rose]

Hi - I just measured the drawings:  9.5"

Thanks I needed that.

[Brent Williams]

Assuming Crist Rigotti used the stock dimensions for his very nice 2015 Legacy 40 electric drawings, I measured the stock hinge to hinge at 16".

Moving the tail back one inch to 17" certainly is not outlandishly long.   

[Brett Buck]

Assuming Crist Rigotti used the stock dimensions for his very nice 2015 Legacy 40 electric drawings, I measured the stock hinge to hinge at 16".

Moving the tail back one inch to 17" certainly is not outlandishly long.  I would move it back and have no heartburn about it.

  Yes, you could do that and it probably wouldn't hurt anything. One of the potential knocks on the design was that it strictly adhered to the pre-Imitation sacred 5:8 ratio. However, everything else was also optimized around that. Allen built many of these models so one presumes that it is the best he could make it with his considerable skills. So, I wouldn't presume off the top that I knew better without building a few of them, very carefully to eliminate slight construction difference that would swamp the relatively minor effects of changing from 16" to 17", and then deciding where the weakness were, then deciding if changing the tail moment would fix the perceived problem.

     Obviously, this is just advice, so no one has to follow it.  and it is very unlikely that the project would fail just because the tail moment were changed.

      Brett

     

[Brent Williams]

Hi Brett, Forgive my ignorance, what is the old 5:8 ratio you mentioned? 
I've read the Imitation article nearly to the point of memorization.  Ted was certainly ahead of the curve!

[Brett Buck]

Hi Brett, Forgive my ignorance, what is the old 5:8 ratio you mentioned? 
I've read the Imitation article nearly to the point of memorization.  Ted was certainly ahead of the curve!

    It was the sacred, never-to-be-deviated-from, ratio of "nose moment" to "tail moment". Like the 9ish to14.5ish from the original Nobler - .620xxx, or the classic ST46 model ratios 10"/16" - exactly .625. The Imitation was the first common design to deviate from it with at least some engineering reasoning. Part of the experiment was to check out different "nose moment" - which, no surprise at all, turned out to have *no effect at all* on the performance as long as the CG remained in the right place..

    Brett

[Teodorico Terry]

Interesting discussion regarding the moments.  Not to be a heretic but it seems to me that the moments should be measured from the C.G. location which is the point about which, in principle, the model will pivot about.  The nose moment measurement is simple enough but I think that in the case of the tail plane in principle should be to its center of pressure (where the lift appears to be concentrated at).  Maybe picking the mid point on the cord would be sufficiently accurate.

For the nose moment I think that there might be a significant difference in how the model responds depending on whether it is electric or glow/diesel.  In essence, the prop and the electric motor's outer case act as a gyroscope; the faster they spin the stronger the effect.  Assuming similar prop and spinner moments of inertia between glow and electric, the electric powered model will have the additional effect of the spinning motor case.  In principle, it should be slower to respond to a sudden pitch up (i.e. cornering). 

I ran some simple experiments with the Fazer when I first started flying it.  The initial set-up used a 13x10 which is huge by C/L standards which spun at a relatively slow 6000 rpm.  The biggest problem I noticed was that the ESC had a difficult time governing the RPM; you would see significant current spikes as the motor loaded up.  In flight, you could hear the motor load and unload as you maneuvered or when up wind or down wind.  The model seemed to corner quite easily but it was really light on line tension.  I then started playing around with reverse rotation props: 12x6 and 13x8.  To keep the same lap times (5.8 sec, I know, it is slow but being older it works better for me) RPM went up accordingly.  Of the 2 props I liked the 13x8 the best so that is what I settled on.  Without reading the data out of the ESC I think that RPM is now in the 8000 range.  The plane still corners well when asked to but I think that it takes a more aggressive input.  The differences are subtle so this is just an observation.  I have noticed that the trend in C/L is towards high RPM and low pitched props and I think that it is driven in part by history (i.e. piped engines) and also by limitations on the ESC (prefer RPM of 9000 or higher to govern properly).  As a result, the motors tend to be long and skinny to get to the desired Kv and torque (for a given Kv and motor diameter making the motor longer improves torque).  It would have been interesting to experiment with short and "fat" motors to see if they offered any advantages; basically spinning larger diameter props of higher pitch at lower RPM. 

Merry Christmas!