abufletcher

Just as a point of comparison, one of my balsa capped ply ribs weighs about 1.5g while one 3mm balsa rib only weighs 0.5g That would be a total savings of 60g. On the other hand, without the cap strips the 3mm balsa rib is far too fragile to accommodate the large box spar. I'd have to go back to the idea of the two smaller spars. And this would eventually have a knock-on effect on other scale structures.

abufletcher

The finished prototype of the top box spar weighs 76.5g. I'm not sure what to think of that. I wonder how much a laminated all balsa box spar would weigh...and how rigid it would be by comparison.

Instead of the four (balsa) sub-spars per box of the current spar, I wonder about using two full length balsa spars (spliced and laminated as in the diagram above) with a 2mm balsa top and bottom sheeting. BTW, this is an example of where I'd happily deviate from the scale design of the box spar (and of course I already have). To me it's more important to keep with the spirit of the original. The original used a single cantilever box spar. That's all I want to accomplish.

vertical grimmace

My Feedback: (1)

Abu, we have this ww1 aviation museum here in CO and they finished the construction of a Fokker DVIII a couple of years ago. I actually was there while a guy was doping on rib tapes. Well, I was talking to him and notice a rib hanging on the wall. He let me hold it and look at it. I was absolutely stunned how flexible and fragile it was. I asked if it was a template and he said "no, it is an actual wing rib". I fully realized at that moment how overbuilt most of our RC aircraft are compared to the full scale counterpart. Once they are built into the structure and covered, everything gains it's strength.
Also I would not worry at all about whether your airfoil will work or not either. So long as you have a well rounded leading edge, just about any shape would actually work. But we know that the scale section performs fine.
As for rigging, I would definately look at having positive incidence in your stabalizer. If not, you will have a ton of down elevater trim to keep it flying level. This was well known and documented on a lot of full scale WW1 birds. In particular the Camel. If you notice, the stab incidence was actually adjustable on the full scale.
On My BUSA DR1, I am pretty sure that the 2 bottom wings are set at zero to the datum. The top wing has 1 degree of positive. The Stab is set to zero and the engine has a degree or 2 of down thrust. I do carry down elevator with this set up, but you are using higher lift sections so I think you would see more.

abufletcher

I've been trying to keep this in mind throughout my build. It's the whole that has to be sturdy not the parts. So on my wing, for example, it's little things like the criss-crossing tape between the ribs that matters and even the covering is part of the strength. I suppose I could have gotten away with the 1/64" ply rib cores. This might have saved 30g overall.

The original seems to have had several degrees of positive decalage to the stab.

That's good to know. I haven't looked carefully enough at the Nieto drawings yet to see whether he gives wing incidences. I would assume so. As far as engine thrust I was hoping to get away with zero-zero but if necessary I can add a couple of washers under the mount bolts.

abufletcher

The Nieto drawings give the following general setup data:

Upper wing: 2Ëš positive at center 3Ëš 45' positive at aileron

Middle and Lower wings: 1Ëš 30' positive

Tail plane: 5Ëš positive

So, none of that sounds too far off from the setup on the BUSA DrI, in essence, a degree or so more incidence to the top wing and around three degrees of decalage relative to the tail plane. I don't quite understand the greater incidence on the ailerons, however, Wouldn't this be like having wash-IN which is normal considered a bad thing?

abufletcher

I'm having a hard time making myself do anything substantial. Those other two spars aren't going to make themselves.

vertical grimmace

My Feedback: (1)

Abu, It is just my preference, but I am not a fan of positive incidence by and large. You see it with trainer type designs so the aircraft will trim out level at middle power settings and then climb when adding power. Especially with highly cambered sections (which create more lift generally) too much incidence can be "uncomfortable". I am sure the rigging numbers you have will work fine though. Just to ease your mind, I would try to check with someone who has built a GTM triplane. I am sure he used the scale rigging with his design and is using the scale airfoil from what I understand. The BUSA airfoil as we all know, is basically a clark Y.

Another note, My triplane has a very powerful engine (Saito 180) and turns a large diameter prop. One of the trim issues I had initially was at low power settings (landing approach) My tripe would drift to the left, a lot. So, I would be lined up perfectly with the runway on my initial final, but once I was over the runway I had drifted over the grass to the left of the runway. I added anywhere from 3-4 degrees of right thrust to cure this. It works great now. I am not sure if this is an issue with less power, but I just thought I would mention this to you to aid in fly ability.

TFF

Sorry for the eyeball measurements but I cant find the protractor. The stab seems to have +2 degrees on the wings or +5 to the centerline. The wings drawn on the plans are the same incidence and are +3 to the centerline and the engine -1. The VK plans go through a pretty convoluted way to align the wing incidence that probably wont work for your plane.

CrateCruncher

My Feedback: (1)

Here are some points about incidence I've had to learn the hard way:
1) Incidence is the trim setting which makes the plane cruise level with the fuselage/tail aligned at the least amount of frontal area.
2) The easiest way to set incidence is to align the horiz. stab with the fuselage then dial in enough wing incidence (relative to stab) to support the weight of the plane at cruise speed.
3) Wing incidence angle is measured from a line drawn through the extreme outer points of the airfoil cross-section, not the bottom of the airfoil.
4) Only symmetric airfoils generate 0 lift at zero angle of attack(aoa). Cambered airfoils have 0 lift at some negative number (Clark Y is - 4.5 deg!).
5) Model airplanes generate slightly less lift at a given velocity and aoa than their full scale versions because of scale effect (ok, difference in chord length in this case). Engineers use the Reynolds number to neutralize the scale effect when they compare data. If you look at a typical lift curve plot there are often a series of nested curves reflecting different Reynolds numbers. Reynolds for a fast .60 sport plane is about 500k, for a full scale Cessna 150 about 1M. You can probably skip worrying about Reynolds, I just mentioned it because I know your curious about it.

If it were my plane I would set the horizontal stab level with the top longerons of the fuselage. I would try to determine the lift plot of the airfoil used in your wings. I would email Glenn Torrance and ask him how he set the incidences of his wings relative to one another to get the best flight performance on a model Dr.1. Then, given the estimated weight of the plane and lift curve of your airfoil I would apportion the lift to the wings in the arrangement Glenn recommends. Frankly, it would not surprise me if he says (-4,-3,-3) or something. The Dr.1 has a huge wing area relative to it's weight. If you set the wings with a lot of positive incidence it will still fly but the tail will stick up in the air and you will have to fly with down trimmed elevator. The relative incidence between the wings is important for good performance. Often, model biplanes fly best with the top wing at a slightly lesser angle than the rest to compensate for it's higher efficiency. I think the reason for this is that the lower wings lose lift due to high pressure disturbance of the lower surface of the wing above it as well as fuselage interference.

abufletcher

Thanks, Cratecruncher! It'll take me a while to digest all of that but I will check on the incidences used on the GTM DrI. As for setting the tail "level with the longerons" I suppose that's what I have at the moment which amounts to 5Ëš positive relative to the datum line.

US185Damiani

Abu,
The +5 incidence of the Horizontal is almost identical to the real bird.  reason is with the undercambered airfoils, and the inherent Tail heaviness of the design will help make it fly on an even keel. If you look at the pics of our full scale bird you'll get a good idea of the incidence and the Horiz Stab positive incidence.  This bird is different than anything else in the air.

abufletcher

So far I've followed the Joseph Nieto technical drawings rigorously so unless he was wrong about the original (see incidences above), my model would be the same. But this is one of the areas in which I"m willing to compromise on a small model.

vertical grimmace

My Feedback: (1)

Normally I would agree with setting the stab to zero, but it just does not work that way with this model. I like the way the 5 degrees sound for the Stab.
That is true about the camber, even at zero degrees, the aircraft will climb with power. If the stab is not set positive(down elevator) with the wings, it will truly "climb like a monkey".

Nieuport nut

My Feedback: (10)

I usually set my wings to the scale incidence as this is usually important to the "look" or "sit" of a plane. Then, set the tail to the same incidence. Relative to each other, this results in a 0-0 set up. You also get effective downthrust. For example, an SE5A NEEDS 5 degrees incidence in the wing to look right. If the stab was set to 0 degrees, the model would "zoom" badly with any change in throttle. Add 5 degrees to the stab and the zoom is tamed by the effective 5 degrees downthrust. The plane flies with the stab level, and the tail high, like an SE5A should.
This is how Dave Platt designed the TopFlite SE5A 40(?) years ago. I have used the same approach with a variety of biplanes. Sometimes there is still a little trim needed but rarely much.
With a high lift wing like the Fokkers, a little more stab incidence is probably needed, hence the incidences Nieto listed. The aileron section is flat, and if aligned with the top curvature, will appear to have greater incidence. It may have been rigged that way by mechanics - there are many accounts of incidences and dihedral angles being changed at pilot's requests. Just because we wouldn't rig wash in because it makes for a snappy wing, doesn't mean it wasn't done in 1918. We use wash-out because we like gentler fliers, just like we don't rig models tail heavy like the originals often were.

Martin

CrateCruncher

My Feedback: (1)

I think we're all saying the same thing. The choice of datum line often makes this topic more confusing than it should be. Abu, the first thing is to determine how you want the fuselage to fly at cruise throttle setting. In my earlier post I assumed having the top longerons horizontal at cruise was scale but you've seen a lot more Dr1's than I have and its your plane.

I'm curious what the datum line is that you are using. I'll assume it is an imaginary centerline through the fuselage side view with the top longerons about +5 relative to it. So setting the horiz. stab at +5 and the wings at +3 relative to the centerline datum would be exactly the same as setting the stab at 0 relative to the upper longerons and the wings at -2 relative to the stab. Not having seen the lift curve for your airfoil this rigging seems reasonable.

In the air a stab wants to weather vane to a 0 AOA so I try to set my fuselage angle first, then zero the stab angle to it, then set just enough wing incidence (lift) to hold the plane in the air. This (hopefully) results in the lowest possible drag at cruise. On a racer you set everything to top speed, not cruise.

vertical grimmace

My Feedback: (1)

It seems to have become habit to use the top of an aircraft to level it and use as the "datum". This may work with Aerobatic aircraft and sport models, but not so much with scale. The Datumn is not an optional component. It was used in the initial drawing of the plane and depending on the design, this could vary. If you look at the side view of the triplane, the rear deck tapers considerably and the Stab is parallel to this.

abufletcher

That's right. The datum line is roughly the same as the support stay on the triangular side fairings. So relative to this line the stab would sit at a definite positive angle. The wings would be at 1-2 degrees to this line.

CrateCruncher

My Feedback: (1)

Ok, after looking over my Dr1 plans I see the point Vertical Grimmace is referring to. I had forgotten that the portion of the fuse frame that the h. stab rests on is not parallel with the upper longerons. It has obvious positive slope "with respect to" the upper longerons (about +3.7 deg on my plan). Assuming that surface is supposed to be horizontal in level flight with a neutral elevator then the stab is likely +3.7 with respect to the upper longerons. The lower wing is +1.3 with respect to the upper longerons (or -2.4 with respect to the h. stab) on my plan. (We still don't know what the actual wing incidence should be without a lift curve). I hope this underlines the point I and many others at RCU have discussed ad nauseam, that choice of datum line really doesn't matter. It's just an imaginary reference. It's the angles "with respect to" that are all-important.

I propped up the tail of a Hasegawa Dr1 I have so that the stab is horizontal. The tail looked kinda high and the fuse looked like it was pointed down in a shallow dive. Is this a scale attitude? It looks pretty draggy. Perhaps it was done to improve pilot visibility? Fokker being a pragmatic sort did they change it because it was easier than redesigning all the wing mounts?

I also discovered something I didn't realize I had. In the back of Paul Leaman's book Dr1 Triplane, I discovered a technical paper from 1918 discussing the relative contribution of the wings on page 216. In a nutshell, they found the upper wing most effective, the lower wing next most effective and the middle wing contributing less still! This may afford some insight into how best to tune the incidence angles so the lift center is close to the center of mass. For example, adjusting the angles at -3 upper, -1 middle, -2 lower, may balance them so they all do roughly equal work.

US185Damiani

With a Triplane Drag was not a big consideration. The airplane was designed, as were all of Fokkerrs Designs, Tail heavy so at full tilt they would climb like the proverbial monkey.
The wing incidence of an undercambered airfoil by necessity is positive. thus the positive in the Stabilizerwill help with level flight.

For example the leveling point of the triplane fuselage is the top longeron. thus the incindence of the tailplane is still positive and willbein level flight as well. The fuselage tends to staylevel as youy see in thisvideo



http://www.youtube.com/watch?v=yMBZgmiYIiY

CrateCruncher

My Feedback: (1)

Happy new year everyone! This officially ends the holidays for me so I'll have to leave the world of Fokker Dr1's soon and get back to work. But I have done a little more digging and discovered the airfoil used on the Dr1 and it's lift curve. It was the Gottingen 298, named after the university wind tunnel that first published the research on it. In John Anderson's book The Airplane - A history of it's technology there is an interesting excerpt I thought Dr.1 enthusiasts might enjoy:

[link=http://books.google.com/books?id=FrvrkXYDCL8C&pg=PA144&lpg=PA144&dq=gottingen+298&source=bl&ots=bAkb6VT2b2&sig=VDKk5WLumCBM_ro-zM2sJoA7GWY&hl=en&ei=yQA-S9rAJ4GCMeKixQY&sa=X&oi=book_result&ct=result&resnum=4&ved=0CBIQ6AEwAw#v=onepage&q=gottingen%20298&f=false]The Airplane Excerpt Link[/link]

Looking at the graph in Figure 5.11, page 150, you can clearly see that the 298 airfoil begins to generate positive lift at -7 degree angle of attack (AOA). In addition, the airfoil makes a huge amount of lift even at shallow angles. This combined with the enormous wing area is not only the secret to the Dr1 maneuverability but also to it's reputation as a good climber despite anemic Oberursel power.

After looking at several in-flight photo's from the period (as well as the excellent video US185Damiani linked) I've come to the opinion that the tail does indeed ride high at cruise attitude. Looking at low fly-by's at 3/4-full throttle the turtle deck line is almost horizontal. (Landings are misleading. When approaching to land the tail sinks to add wing AOA to maintain lift at lower speed). Ok, I am speculating here, but I suspect the Dr1 was designed with the wings at -2 like most planes from the period on the assumption the airfoil would perform similar to a thin one. Fokker had an inch taken out of the tailpost (+3.5 stab) after flying the prototype with constant forward stick pressure to maintain level flight. This made the wings effectively -5.5 and was much easier than redesigning all the wing mounts, fairings, etc.

Abu, I have no idea what airfoil you used to make your ribs so this may be immaterial. I hope someone has found it interesting and good luck with the build.

US185Damiani

That is a cool resource on airfoils!

Thanks for posting that.

abufletcher

CrateCruncher, thanks for taking the time to look all of that stuff up. I'm sure it will be of use on my model or at least historically interesting.

I actually attended this university (Georg-August Universität Göttingen) for a year in the 1970's. Too bad I wasn't into scale modeling at that point or I might have tried to see if there were any relics from this area sitting around. Today the university is best known for its world famous Max Planck Institute doing particle physics.

Again, I don't like second-guessing the original designers. The Nieto technical drawings (not model plans) provide one set of data (+2 top wing, +1.5 middle and bottom, +5 on the tail plane, all relative to a datum line level with the side fairing. I'll confirm that with some other source. But if those are correct, I'll just go with those unless someone can provide a very convincing argument that these incidence just absolutely would not work on a small model.

To the extent that the Nieto drawings provide an accurate rendition of the original airfoil, my model airfoil is identical (to the best of my modeling abilities). I certainly don't see any reason to alter what looks like a pretty typical model airfoil.

abufletcher

CrateCruncher, thanks for taking the time to look all of that stuff up. I'm sure it will be of use on my model or at least historically interesting.

I actually attended this university (Georg-August Universität Göttingen) for a year in the 1970's. Too bad I wasn't into scale modeling at that point or I might have tried to see if there were any relics from that era sitting around. Today the university is best known for its world famous Max Planck Institute doing particle physics.

Again, I don't like second-guessing the original designers. The Nieto technical drawings (not model plans) provide one set of data (+2 top wing, +1.5 middle and bottom, +5 on the tail plane, all relative to a datum line level with the side fairing. I'll confirm that with some other source. But if those are correct, I'll just go with those unless someone can provide a very convincing argument that these incidence just absolutely would not work on a small model.

To the extent that the Nieto drawings provide an accurate rendition of the original airfoil, my model airfoil is identical (to the best of my modeling abilities). I certainly don't see any reason to alter what looks like a pretty typical model airfoil.

*****

Did a little New Year's shop cleaning and that ought to help me get back to building!

abufletcher

I've never completely understood this use of "tail heavy." To me tail-heavy just means "unstable" and results in a aircraft that is "hyper-sensitive" on the elevator. This is typically a bad thing in a model. But the way the term is used here suggests that it has more to do with the "neutral flying attitude" of the aircraft, or maybe with the need to "keep pressure on the stick." In which case, it's like setting the aircraft to climb at neutral, which might not be a bad idea on a fighter.

US185Damiani

That's very close to the accurate description
With these old Birds, there is usually no trim, so the heavier the Pilot behind the CG the more forward stick required in level flight. a mild aft CG makes the model Hunt in pitch as well as the Full scale.


The Waco Straightwing exhibits this characteristic.

Have a Happy new year Abu