While reading the Tipo construction article Dick mentions that if not using a bottom center mounted pipe, less "droop" should be used on the horizontal stab. IIRC the droop with pipe was 4.5" and without 2". When building the Curare is this cathedral angle constant or determined based upon certain factors? If this hasn't been addressed before, should we consider Dick's suggestion when building a Curare for SPA?

That's interesting. Just to be sure I'm tracking with you, by droop you mean the anhedral in the tail, correct? Was there any info on the build as to the reasons they thought less anhedral with the requirements you mentioned was preferred? I'm curious to hear some thoughts, as I will be building a new Curare over the winter.

You got it. He wrote that removing the pipe from the underside drastically alters the drag profile when the plane is yawed, and goes on to state that yawing is present to some degree in all rolling and point manuevers (that seems obvious). He says this is why different angles are required on different planes, and he had flown planes that were "absolutely weird" because the angle was not correct. He doesn't go into detail as to why though.

Though, when looking at the Magic, which has an internal pipe tunnel, the anhedral looks to be the same as the Curare. So, who knows??

I modified the Blue Angel model in the Ikarus Aerofly simulator by introducing anhedral to the tailplane a la Curare, to see what it would do.

The effects were most noticeable in knife edge flight.

There were two effects:

- The anhedral reduced the tendency to pitch towards the belly in knife edge.
- The anhedral reduced the tendency to roll out of knife edge.

To get both effects to "zero" it was necessary to also increase wing dihedral. That is, when enough anhedral was introduced to remove the tendency to pitch towards the belly, the anhedral caused the aircraft to roll _into_ knife edge. To remove that tendency, it was necessary to increase wing dihedral.

This agrees with the Curare design, which does use a bit more wing dihedral than most other pattern designs that don't use any tailplane anhedral.

Now the pitching effect in knife edge is caused by the change in how the downwash from the wing hits the tailplane in knife edge vis a vis straight and level flight.

This shouldn't really be affected by the presence or not of a pipe under the wing centre section. So the amount of anhedral needed to remove the tendency to pitch towards the belly in knife edge should not change whether a pipe is hung under the fuselage or not. This agrees with Prettner's Magic design which used as much tailplane anhedral as the Curare, with the pipe hidden in the upper fuselage rather than hung below the wing centre section.

What probably will change to a greater or lesser degree, with hanging the pipe under the wing or not, is the value of wing dihedral required to cancel any rolling tendency in knife edge, given a value of tailplane anhedral already chosen to cancel the pitching tendency.

So omitting the pipe, I think you should be looking at changing wing dihedral, not tailplane anhedral.

But by how much?

For my simulator tests, I use my Multiplex transmitter and have all the mixers and so on set up as I would for a real pattern model.

I found that by using the mixers it is possible to trim the plane to stay true in knife edge in both pitch and roll, with or without tailplane anhedral, for a range of reasonable values of wing dihedral.

So with a modern transmitter you will be able to "zero" the model in for true knife edge flight, whether you use a pipe or not, and whether you stay with Prettner's recommended anhedral or not - or even without anhedral (Curare modified back to Super Sicroly... Super Curaricroly? Super Sicrolare? ).

My recommendation therefore would be to keep Prettner's suggested tailplane anhedral and wing dihedral, use your mixers to zero in the model, and look for evidence requiring a change in wing dihedral (wings rocking at low speeds, aircraft rolling into or out of turns etc) when flying the model.

My guess is you won't find much reason to change wing dihedral, and in the end building a Curare as per Prettner's plans will do you nicely, pipe hung under the fuselage or not.

With best regards,

George

Good post, thank you!

Gentlemen

I've built about a dozen Curares for SPA. I have never changed the anhedral in the stab because I don't know what it will do. I have messed with dihedral in the wing. DON'T. Hans and Hanno worked out the dihedral. Reducing it creates mix issues that will not be present if the wing is built according to plans.

For SPA, balance can become an issue when using a 4-stroke. You'll need to plan for some servos in the rear. Moving the wing just a little, an inch or a bit less, forward, can allow the radio gear to go in the wing saddle.

Use any heavier wood you have to build the tail. You'll get a little more strength which is not critical and you can move weight rearward.

If you are using foam cores and don't cut your own, Don Turnock at Eureka does great work and can make a foam turtledeck for you, too. The TD saves about 5 ozs in total weight and a lot of carving and sanding.

Hi Steve,

I can't picture what you mean by moving the wing forward an inch to let the radio gear go in the wing saddle. Where exactly does the radio gear go and why do you need to move the wing forward to do it? I'd be very happy if you explained, or even better, if you provided a photo.

Best regards,

George

George,

I think Steve is talking about changing the moments of the plane. Moving the wing forward increases the tail moment thereby eliminating the need to place weight in the tail with a 4 stroke. This facilitates the installation of the gear where it usually goes. Steve, correct me if I'm wrong.

Blake,

the topic of stab anhedral (referred to as cathedral by Hanson) and its relation to wing dihedral was extensively tested by Hanson and Brown during that time. As George points out above, the wing cannot be flat topped as many pattern wings were (e.g., Deception) if the stab has anhedral - the model will just not fly well. Such interactions eventually materialized in the form of the Tipo's evolution into what became known as Dave Brown's design - the Illusion. The Illusion revised the anhedral to decrease it along with corresponding wing dihedral reduction and tucked the pipe into the belly of the fuse by raising the wing closer to the thrust line. This was no doubt an exploration of the statements made by Hanson in his article.

The introduction of stab anhedral into a design is to effectively lower its draft in the fuse and result in equal and opposite lift vectors on it when the rudder is deflected. Differential lift occurs because the usual negative lift produced by the stab is altered the moment the rudder is deflected. If one deflects left rudder, the negative lift on the left side of the stab is decreased and might even become positive lift. This causes the nose to pitch down or to the belly of the plane. Getting the anhedral angle right results in stab lift vectors that prevent pitching tendencies. However, there is one more problem. Since vectors are now equal and opposite on the stab with deflected rudder, the model is subject to adverse roll. When yawed it might be pitch neutral but not roll neutral and in level flight the model will seem unstable. In knife edge flight the model will tend to roll out to level flight. This can be taken care of by increasing the wing dihedral resulting in 3 axis neutral flight... or so one hopes!

These ideas are echoed by George in his simulator tests and evidently Steve has found that decreasing the wing dihedral while retaining the same stab anhedral does not work out too well. Tony Frackowiak recently had some comments to the effect of how Phoenix 7's flew with either side mounted exposed pipe engines vs vertically mounted engine and concealed pipe installations. Here is the thread in question:

http://www.rcgroups.com/forums/showthread.php?t=1509844

Jim Kimbro makes some related comments in his Deception article. As you likely know, the Deception is a design with a flat top wing and a symmetric, 0 degree incidence, 0 anhedral stab. It also has a rudder which is cut short of the bottom of the vertical. The rudder's deflection occurs entirely above the stab thereby eliminating the "blanketing" effect of the rudder on the stab as with the Curare/Tiporare design. Jim is emphatic about two things: 1) Don't build the Deception with stab anhedral and 2) Don't build it with a full length rudder. Both factors were explored and experimented with by him and he found out that the model flew best with the tail setup as recommended. An interesting aspect of the Deception design is also that the wing and stab are both set with chord lines equidistant from the thrust line. That was the main principle Jim wanted to follow in designing that model.

In short, if you build a Curare, I'd stick to what Hanno designed and what George also suggests. Then, set it up for SPA but stick a pipe on it for practice and see if the addition of the pipe vs a muffler has any effect on coupling tendencies of the model. It is possible that if a muffler is used instead of a pipe, that reducing stab anhedral and wing dihedral will result in an equally neutral aircraft. But as George says, it's a matter of how much... only experimentation can tell as well as the experience of people like Dick and Dave themselves. You might want to drop Dick a PM and see what he has to say.

David.

My dad has come up with a design for the Curare fuselage that opens up enough room for all the servos without needing to adjust the tail moment.  I will be building a new fuselage in the next couple of months and will do a build thread to show the way we changed the fuse to lighten and simplify the build.  Thanks for the answers guys, very educational.  I had built my Curare's to Hans and Hanno's plans and they have flown great, but I was also very curious.  Now I understand the concepts of the anhedral and dihedral of the wing even more.  Good stuff!<div>Lindy</div>

Blake

Thanks for the great explanation. My early models were built with reduced wing dihedral per the suggestion of a much more experienced local "expert" with the described pitch and roll results. Building these according to plans saves lots of grief.

Correct on moving the wing forward. I found the need to bury elevator and rudder servos as far back in the fuse as I could get them to save adding gobs of lead on early models. As I learned to build lighter, moving the wing forward allowed placing these servos at the rear of the wing saddle.

For 4 stroke installation, I moved the firewall back to maintain fuselage profile.

Steve,

I believe your thanks for the explanations might be due to George or myself. Blake is the fellow with the good thumbs who started the thread with some questions...

David.

That is true, Dave and George deserve mucho thanks! I just have a lot of questions right now, and I know that all the answers are here!