abufletcher

Huh? 2.7cm back from the top wing leading edge??? That must be a typo.

jack steward350

My Feedback: (7)

well yes ! Well no! I mean 12.7 cm ! Sorry. I have been making measurements in 2 and 2.5 mm the last few nights and miss spoke. It is at 12.7 cm or 5 inches if you use that cumbersome base 12 English measurement system. That would be at exactly the half mark on the length of the wing chord. Still to me always seemed wrong when I first looked at it.....but I really don't know.

jack steward350

My Feedback: (7)

Here is a photo of his plans so you can see what I am talking about:

abufletcher

Here are a couple of photos I just took with my model. The first shows the model (with the same lead weight it had on it's last flight) balanced at 9cm. In the second photo I'm balancing the model at the 12cm point. This is obviously far too nose-down with the weight I have in there now. But it flew uncontrollably when I had it balanced at the 12cm point for the maiden.

I'm sure Chris was aware of the Dave Platt design (and maybe also a Boddington SE5). I'll ask Teus (again) where he balanced his CDScaleDesigns SE5a.

*****

PS. The Platt design looks great (which is unsurprising). Personally, I don't think CAD design has much to offer WWI modeling.

abufletcher

This diagram shows the calculation method used to come up with the 9cm location. The photo shows this geometry applied to the CDScaleDesigns plan. Notice that the CG location marked on the plan (about 12cm back) is very similar to that on the Dave Platt design. But the incidences/airfoils on the two models are not identical.

jack steward350

My Feedback: (7)

That is fascinating. The best thing you could do would be to contact someone who has one that they have flown. I think I have seen some Dave Platt builds on line that could be a good source. I don't think the wing incidence would have a relationship to the CG location? So they both work out on paper to be correct....maybe. For what it is worth the first photo I would call tell heavy. The second is obvious. I am just wondering if it seemed pitch sensitive or if you needed excessive up trim in the elevator or if it seemed unmanageably unstable? I realize a lot of other things were also going on with alignment issues but on top of the alignment problems a tail heavy condition would act just like that and make you wonder what the heck is going on with this model? I have been there. Some one out there knows the correct CG for that airplane. As SE5A models go that's a great one!

abufletcher

I just sent an email to Teus. There may be one other (electric version) flying in Germany somewhere.

This is where the darn semantics interfere. In that first photo, the centerline of the model is level and you can see the positive incidence on the stabilizer. If "balanced" really means "balanced level" then this is balanced. If "balanced" really means in modeler parlance "angled slightly downward" then this is slightly "tail-heavy." I suppose many modelers commonly talk about "slight nose-down balance." In the second photo, I'd say the model is far too nose-heavy IF we assume the model should be balanced at 12cm back (as per the plane). I would have to remove most of the nose weight to give the model the attitude seen in the first photo.

On the maiden flight, it was porpoising all over the sky, doing unintended loops, diving, rolling...it was a nightmare. Once I moved the CG forward, the pitch seemed more or less under control and the main issue was the strong pull to the left.

abufletcher

And more on the silliness of the semantics of "balancing." I could obviously, make the model adopt a more nose-down attitude just but moving my fingers back a centimeter or so. Nothing about the model would have changed. I'd just be claiming that the "true CG" is at 10cm instead of 9cm.

But the CG point (or rather "range") should really be inherent in the aerodynamics of the model. I can't just move my fingers around and call the model "balanced."

jack steward350

My Feedback: (7)

For what it is worth, looking at the first photograph and if I were about to test fly it, I would want to bring the tail up a small bit and I would do just that. To my eye it doesn't look level but close. However I think it is highly likely that balance at the 9 cm point would be manageable as a starting point. The big question is was Dave Platt wrong? and the plans you have wrong? Maybe the CAD designer used Dave's design and carried the error forward? Who knows? It's either that or they are both wrong and I have to wonder how likely that would be.

jack steward350

My Feedback: (7)

For what this is worth people are balancing the Dave Platt SE5 at 9 to 9.7cm not the 12.7cm mark. The one person who flew his at the 12.7 cm reported that it flew lousy really lousy. This is from a Dave Platt SE5 thread I just read. There you go more confusion but I think 9cm is it. See if any body else knows.

abufletcher

That's really helpful information! Thanks.

To be honest, designers often don't know until they have actually flown the model. They can make best guesses based on experience flying prior models and looking at other designs (and making formal calculations) but in the end, until it's flown it's all guesswork. With such a skilled modelers (and flyer) as Dave Platt maybe he was just a much better pilot than I am and could fly the thing balanced at 12cm.

At 10cm back, my model would definitely be a bit nose down.

abufletcher

I agree. I'll add another small weight in the dummy radiator.

abufletcher

Teus says that his model, with the CG at 10.5cm, adopts a slightly nose down attitude. He further reports that in flight it resists turning with only ailerons and that rudder is required to make a turn. He also said that because his model has a lot of scale weight it flies fast. I believe my model weighs around 3.5kg.

Anyway, I think I'm done. (With the exception of making sure the engine runs properly.)

bbs428

Interesting thread. I enjoyed your journey working through all the issues with your beautiful SE5A. I hope your sorted now and can enjoy a great, uneventful flight soon!

Best of luck

jack steward350

My Feedback: (7)

I think you are right the Se5 is ready. Good flying Abu.

BobH

My Feedback: (2)

Wow. lots of over thinking here..
The thrust line is a datum line parallel to the earth running through the theoretical horizontal center of the plane. So that's a reference point from which every thing else is measured.
The balance point of a plane would be at some optimal place along that line where all the weight/lift etc come into equilibrium.
That point can change a bit fore and aft to reduce or increase pitch performance. Of course only so much with out effecting the safety of the plane. Out side of that and you are in the Danger Zone.
On a staggered wing biplane if you measure from the front of the forward most leading edge (top or bottom) to the rear of the most rear ward leading edge that will give you the total CG Span of the wing. Take say 22-25% of that total and you are in the ball park for most planes.

FireBee

My Feedback: (34)

I agree with your plan Abu. A little safety margin is always good on maidens. I wish I could be your caller and help with any trim settings. Although I bet you have very little trim needed after all the checking.

Flight impact would be a longer takeoff run as you may need slightly more elevator to rotate if nose heavy. Also as you throttle back, nose would drop more than usual. Control is well within the normal throws to bring her in. With that amount of incidence, I see no real effect for you if slightly nose heavy. Later you can remove the small weight and go back to original.

Most designers use a more forward CG range to ensure success by average flyers. You are above average and will love that plan!

abufletcher

You know me! Seven pages obsessing about adjustments! There are guys with shorter build threads.

kochj

looks like the fuse is leaning left ( the bottom wing doesn't look square with the fuse)

This may act like using left rudder and left aileron.... going by picture...

abufletcher

I know what you mean. It's an awkward looking photo. But that's just a visual illusion.

jack steward350

My Feedback: (7)

I think Abu has got it all worked out and anybody who would think he was overthinking the CG location has no mental grasp of the problem. The plans and one of the most successful model designers ever placed the CG in a location that contradicted the math. To do the math and ignore the contradiction without further investigation would have been foolish. In very short order he discovered where it should be and the math was correctly verified by what others had proven with identical models. That is not over thinking that is being very smart. If it is over thinking, I would do that every time given this situation. I do think he should thank the post that defined to all of us what CG is and what it does , and what a centerline is. I still wonder what the theoretical center line of the plane is since we are talking geometry and the center line is the center line. There must be a theoretical center line on the runway too. That line must run to the theoretical end of the runway.

CafeenMan

You're over-thinking it, jack.

jack steward350

My Feedback: (7)

Yes I am I tend to do that and do that often when it gets late and it's the middle of a winter like the one we have had up here. :-}

abufletcher

Maybe we're over-thinking over-thinking it.

*****

There is still one more "upgrade" I could do. The demise of my Puppeteer means I have a Saito-62 just sitting around. The size is identical to the Saito-56 I have in the model now, so I could just swap out one for the other. Shouldn't take more than 15 minutes. This photo shows the 62 (on the scale) and another Saito 56 I have. I was curious about any possible weight difference so I weighed each with its prop (the Saito 56 with a 12/6 and the Saito 62 with a 13/6).

And the amazing thing is that they are both, props included, gram for gram exactly the same weight!!! Each is 469g (without the muffler). The total weight of the model is about 3.7kg (about 8.2lbs).

One reason NOT to swap in the 62 is the smallish tank.

*****

PS. BobH is a fellow scale SE5a builder and his comments are always welcome.

Nieuport nut

My Feedback: (10)

I agree, alignment and CG are two things it is well worth taking time over. I've just spent a week setting up the struts and rigging on a 1/4 scale Nieuport 17 prior to covering. I'll hopefully spend a little less time when I do the final assembly!

Martin