RCdude13

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I am building a 1/4 scale Boeing F4B-4 biplane and I'm going to need some help. I just got a very detailed set of Peter Westburg drawings from the Smithsonian. The drawings show the airfoil is a "BAC 106". I was going to use a Clarke Y, however, perhaps this Boeing airfoil would be better, and cooler since it would be scale? Another question...the drawing shows both wings with 0 degrees incidence, the stab adjustable, and no down thrust. From all my searches on RCU I had decided to go with 0 bottom wing, -1.5 top wing, +1 stab, and a couple degrees down and right thrust. Any comments would be ever so appreciated. Finally, how am i going to make those scale corrigation ribs on the control surfaces?

RCdude13

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Here is the Boeing 106 airfoil.

Top_Gunn

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It would probably be a good idea to find plans for a model of the F4B that has been flown successfully and take incidences and maybe the airfoil from that. Sometimes the full-scale setup doesn't work well on a model, even a large one. If I remember right, the Vailly Cessna L-19 has a stabilizer incidence that's way different from the full-scale plane, because the model was close to unflyable with the full-scale incidences.

I'm doing corrugated control surfaces for a model by gluing triangular plastic strips at appropriate intervals. If you can find corrugated plastic the right size you will save days of really boring work.

Lnewqban

Could you give us a reference that advises less incidence for the upper wing and more for the stab, both respect to the lower wing?
What I have learned about stability tells me otherwise.

RCdude13

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There is a TON of info and opinion on biplane wing and stab incidence.

I can't give a specific reference wrt the upper wing, however, i have been led to believe that models track better with a bit of negative in the top wing. I originally had planned on going a bit positive wrt the lower wing using the theory that this would make the upper wing stall first. This makes the bipe more stable but less aerobatic. The arguement to this is that the top wing will stalls first anyway even with negative incidence due to the interaction of the top wing on the lower wing (the lower wing acts in the slipstream of the top wing). This is why the seperation of the wings is so critical. Please argue with me if i am dead wrong.

The reason for the + stab is experience. I have a Ziroli Mustang which requires down elevator to make a low, high speed pass. This is with the elevator already trimmed down a few degrees just for level flight. When I built my Ziroli Bearcat a put some + in the stab to prevent this, and it works great! This is not the way a full size plane is set up because they have the luxury of in-flight trim. Again, please argue that I am wrong.

BTW, i am using Mammoth scale plans from Bob Holman (drawn by Morse), and a set of Swietzer 60 " plans as reference. The fuse formers are right from the Peter Westburg drawings I got from the Smithsonian.

Lnewqban

I have no solid reason but those same opinions; hence, I cannot say that you are wrong.

It would be great if you can provide means of adjusting those and the engine's angles before making them final after some tests flights.

otrcman

Well, first off, your model will FLY with either the scale wing incidences, your proposed incidences, or most any other combination plus or minus 5°. The question is, how WELL will it fly and what will it do best ?

It's important to separate out in your mind the terms INCIDENCE and ANGLE OF ATTACK. Incidence is simply the angle at which the wing is mounted to the fuselage. Incidence has nothing to do with lift. Angle of Attack is what makes lift, and it's not a fixed number for your airplane. Angle of Attack can vary all over the place, depending on your pitch control inputs. The horizontal stab controls the angle of attack of the wing. The wing angle of attack can be varied by changing the angle of attack of the horizontal stab or by changing the deflection of the elevator. The fuselage just goes along for the ride and serves as a handy place to suspend the engine, landing gear, and empennage relative to the wing.

The paragraph above is the summation of a basic aerodynamics course in airplane lift and trim. Once you understand it, you will be ready to move on to lesson two.

Lesson Two: Biplanes are a little more complicated than monoplanes. The upper and lower wings both contribute to the overall lift and if they perform differently, they affect the performance of what aerodynamicists call the "equivalent monoplane". To make matters even more complicated, each wing operates under the influence of it's mate. Wings create "downwash" when they move through the air. That is to say, the airflow around and behind a wing is deflected slightly downward by the passage of the wing. In the case of a biplane, the lower wing is flying in the influence of the upper wing, which has preceded it slightly. For that reason, the lower wing is usually given a little more positive incidence than the upper wing. In this configuration, the two wings are actually seeing pretty much the same Angle of Attack. Think in terms of a degree or two more positive for the lower wing. Why would you want the two wings to fly at the same Angle of Attack ? The primary reason is that airfoils have one Angle of Attack at which they are most efficient. Matching the Angle of Attack of the two wings simply matches the highest cruise efficiency point of the two wings.

Remember at the top of the page when I said that INCIDENCE had no effect on ANGLE OF ATTACK ? On a biplane that's not exactly true. By fixing the incidences of the two wings, you are establishing a relationship between the Angles of Attack of the two wings. Getting more complicated, huh ?

But what about when you fly upside down ? What about stall behavior ? Well, aerodynamic design is all a bunch of compromises. That's why I said that you could make the two wing angles most anything you wanted and the airplane would probably still fly.

What I want for you to take away from this is that there is nothing magic about the relative incidences of the two wings unless you want to optimize some particular performance characteristic. Do you want lowest drag at cruise ? Do you want a gentle stall ? Do you want sharp snap rolls ? Every desire calls for a somewhat different incidence relationship between the two wings.

That's enough for now. My fingers are tired. Once you digest the incidence issue, we can talk about horizontal stabilizer angles and why you might not want to use the same angles as the real plane used.

Dick

gerryndennis

Hi Dude,

The reason that many models have down thrust is to reduce the trim changes required from high to low power. In full size flying its totally normal to retrim the elevator every time the speed or power is changed, us modellers prefer not to have to do this.

A pattern model with a large tail plane flies nicely with zero down thrust, but a scale model with a smaller tail plane requires the balance point to be further forward for stability. This forward balance requires a down force from the tail plane to fly level (provided by either the tail plane incidence, or elevator deflection, or both). If power is added to increase speed this 'up trim' causes the nose to rise. If the engine has down thrust then when power is increased there is an extra force tending to hold the nose down. Power down and vice versa, the nose drops as speed reduces but the nose down force from thrust is reduced as well. You can end up with a model that doesn't require too much retrimming around normal cruise speeds and power settings.

It's not perfect but a couple of degrees usually fles a lot nicer in a scale machine.

Same sort of thing with side thrust. A full size aircraft requires rudder trimming for every power and speed change. High power and/or low speed requires right rudder trim and vice versa. Again a couple of degrees of right thrust alleviates this.

If it was me I would use 2 degrees right and down or thereabouts but its not vital.

As for incidence angles I would tend to go as per the full size just so it's scale. There may be some advantages fo a full on aerobatic biplane having different angles, but is that what you are after? You would have to be prepared to modify your angles a lot to see which is best, it might not turn out to be -1.5 on the top wing.

Plus 1 degree on the stabiliser relative to the wing will probably be close, and then trim the elevator from there. A lot of people get upset about the elevator not being lined up with the stab, but this is completely normal for full size aircraft without an adjustable stab. So unless you are are racing or looking for better fuel endurance there's no point worrying about the 1/2% drag increase that might occur.

Nice plane, good luck with your model.

Dave H

RCdude13

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Awesome lessons guys...i am learning a lot which I believe is one of the best parts of this hobby. I will be studying both of the last posts to to get a complete understanding. You guys rock!

RCdude13

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Dave,
I plan to fly the F4B the same scale way I fly all my warbirds. That means no snaps or hammerheads or inverted flight or knifedge, etc. I do immelmans, cuban 8's, loops, rolls and maybe the odd 4 point roll to try to make it look as scale as possible. I want to set wing and stab incidence accordingly.

How about the BAC 106 airfoil vs. A Clarke Y?

Just to throw another curve into this discussion, the fin has 2 degrees offset to the right...should mine? (See photo).

gerryndennis

That aerofoil looks fine to me, I would be surprised if any one could tell any difference between that and a Clark Y while flying.

The offset fin is a full size trick to reduce the need for rudder trim (which it looks like the full size doesn't have). It will be optimised for cruise speed and power so will be wrong for takeoff (low speed/ high power) or descent (low power/high speed).

It will work but the advantage of right thrust is that at high power you have the nose right force and at low power it's gone. Right thrust isn't perfect, it can't be correct for high power low speed and high power high speed, but its better than nothing.

Having said that, it won't hurt your model and will look scale. You could probably reduce the side thrust if you do put the offset in, maybe 1 degree?

When you say offset right do you mean offset so that the nose would go right, or offset so the leading edge of the fin is to the right (which would be the same as left rudder input)? It depends on what direction the engine turns. It looks like the full size is clockwise viewed from the rear (same as a model engine) in which case the fin should be offset with leading edge to the left to give right rudder effect.

If the fin is offset the wrong way for your engine rotation then don't do it.

Dave H

BMatthews

I agree that the BAC 106 looks just fine. It resembles many of the semi symmetrical airfoils used on some of the early pattern RC models.

If you're really in doubt build a Stik style quicky model with the airfoil or build a wing using the BAC106 for an existing model and try it out.

otrcman

+1 on the Boeing 106 vs Clark Y. You probably wouldn't notice much difference.

I took a look at the xfoil predictions for the two airfoils and they appear to have pretty similar aerodynamic characteristics. In addition, the Boeing 106 shape looks a whole lot like the NACA 2412, which is what the Cessna 172 uses. 60,000 Cessnas can't be too far wrong !

Dick

RCdude13

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BMathews,
I'm too lazy to build a stick plane to try things out, although it would be a great learning tool. I am going to use the BAC 106, especially with the research Dick did on it.

Dave,
I fly big warbirds all the time so I am pretty good with the rudder. They all need lot's of right rudder at take-off especially. For this reason I am going to skip the fin offset and just trim as required.

Dick,
I land my warbirds pretty fast so the stall is not a huge concern within reason. I like flying fast although this bipe would look silly going 100 mph. I don't want it to fall sideways out of big loops. You know how and what I fly so the million dollar question is still top wing incidence wrt the bottom wing at zero???

These opinions mean a lot to me...thanks so much guys.

BTW, note the down trim on my Mustang. This bothers me less from what I have learned from Dave.

gerryndennis

Even with right thrust and/or left fin offset you are still going to need to use lots of right rudder for takeoff, it'll just make the model a bit nicer overall. So I would still be quite tempted to offset the fin (assuming its the correct direction) especially if it's noticeable visually. Obviously up to you though, I don't think it will make the model hard to fly either way.

That having the top wing at minus 1 thing isn't really a general rule for all biplanes, it depends on stagger and the wing gap etc. so different designs will have different angles. Some guys have experimented and found minus 1 was best on their model, other guys have tried minus 1 and found it was good but never experimented to see if a different set up was better. There's just as much chance that zero zero will fly nicer than zero minus1, so unless you want to try lots of different set ups, I would just go for scale.

What airfoil does the tail plane have? The Boeing Stearman (similar aircraft) has a positively cambered one.

Dave H

otrcman

Before talking about the incidence angle of the wings, let's be sure we're talking about using the same reference system. Post #2 has a good picture of the Boeing 106 airfoil. When I say, "zero degrees", I'm talking about the black line that goes from tip of L.E. to tip T.E., not the bottom of the airfoil.

Now, two incidence angles need to be considered. First, the relative incidence of the two wings. Second, the incidence angles relative to the fuselage. I'd suggest that the top wing should be set positive relative to the bottom wing. One to two degrees should do it. You suggested 1.5°, so that's fine.

On the second question, wing incidences relative to fuselage reference line, I have a little different viewpoint. I like to place the wing such that the fuselage is level when the wing is flying at the required AOA for straight and level flight. Remember that the wing needs at least a little bit of angle-of-attack to make lift so you can fly. Just how much AOA will depend on the weight and speed of the airplane. I'm guessing that you will need about +1.0° AOA for straight & level flight. So placing the top wing at +1.0° relative to the fuselage reference line should make the fuselage level in cruising flight. The bottom wing also needs an AOA of 1.0°, but since it is flying in the downwash field of the top wing, it needs that extra 1.5° that you planned. So the bottom wing would need to be at 2.5° relative to the fuselage reference line.

Does all the above matter ? Not really. Your suggested wing incidence angles in Post #1 would work fine. It's just that in flight the fuselage might look a little nose-up with your originally proposed incidences.

For the stab, we still need to know what airfoil you will be using. As Dave pointed out, some planes have flat-bottom or even flat-top airfoils for the horizontal stabs. Most are symmetrical, but we need to ask the question. That's especially true since the F4B-4 and the PT-17 came from the same company and at about the same time. They could have used the same design philosophy on both planes.

Dick

RCdude13

My Feedback: (6)

The stab will be a fully symetrical airfoiled shape. I was planning on using the rib patterns from one of my Ziroli warbird stabs, however, I may use Profili which I am just starting to play with for the wing ribs. Is there a fully symetrical airfoil I could use for the stab?

otrcman

If the stabilizer airfoil for the Ziroli warbirds has worked well for you in the past, then it will probably be fine for your next plane as well. A common choice is the NACA 0009 symmetrical airfoil, but I wouldn't necessarily choose it over the one Ziroli uses.

If you go with the wing incidences suggested in Post #16, then a stab incidence of about 0° would be about right.

By the way, I'm with Dave H. on engine downthrust and sidethrust. I've never yet regretted using engine offset. Sometimes you will look back and wonder, "Did that offset really do any good ?" If you don't notice anything out of the ordinary, the offset is doing its job. It certainly won't do any harm. If you place the wings at a slightly positive incidence (as in Post #16), then you will automatically have some downthrust. That's because downthrust isn't measured relative to the fuselage centerline; it's referenced to the wing zero lift line. And downthrust contributes to static longitudinal stability as well as easing trim changes with power.

One last thing: You mentioned not wanting to see the plane roll off at the top of a loop. Are you thinking yet about a little bit of wing washout ? Like downthrust, it's one of those things that can't do any harm and usually does some good. Three degrees or so usually helps with stalls.

Dick

Rodney

Since bipes are my favorite type of plane, I have built and flown many RC bipes, most 1/4 scale. I did a lot of experimentation on the relative angle of the top and lower wing (all with forward stagger) and found that all of them flew best when the upper wing was at about 1.5 degrees less angle of attack than the lower wing was. I think post #8 above was right on track, every thing Dave said agrees with my experience. It is quite startling how just a minor change in relative incidence change on the upper wing will change flight characterists. If you can, in your build, try to make it easy to change that upper wing angle of attack and try some minor changes when you get it flying. You will be amazed at how small changes effect the flight characteristics.

RCdude13

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The thrust line which i am using as reference goes through the prop hub to a spot just a little up from the tailwheel. If it where to fly level with my thrust line, and at 0 degrees AOA with wings (which are 0, 0) the tail would already be sitting nice and high, ...Yes? Maybe I don't need that + incidence in both wings if the "thrust line incidence" is already rather positve to the feasalage?

Nick Ziroli always puts a little washout in his wings (at least the four I have built). I am a strong believer in wing washout, but never really considered it for this bipe. I can do 2 or 3 degrees in both wings if everyone thinks its' a good idea.

RCdude13

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I have read this opinion of yours elsewhere on RCU and I am going to follow this advice. Thanks.

gerryndennis

I'm usually quite an advocate of washout, but in this case it's probably not really necessary.

Washout would be very important on your WW2 models (nice models by the way) because of the plan form taper, higher aspect ratio, and probably higher wing loading. The F4B-4's wings are lower aspect ratio with no taper, effectively they are plain rectangular wings with a rounded wing tip which will naturally stall from the inboard end. Washout is not really necessary to control tip stall in this case.

It won't hurt though, but be aware that if you reduce the incidence at the tips (washout) you may have to increase it at the fuselage to avoid the fuselage flying nose up thing that Dick (otrcman) mentions.

The full size Stearman has similar wing design with zero washout and has impeccable manners at the stall. So again I would be tempted to leave it scale. Over to you though, and as I say it won't hurt anything.

Dave H

Rodney

Wash out only works for upright flight. In aerobatics or stunt flying, washout becomes washin when inverted (a big detriment).