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Sukhoi build

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Sukhoi build

Old 07-14-2019, 03:12 PM
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rj lemay
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Hi Everyone,
I've started a Sukhoi SU26 scratch build from an AeroFred plan. I have many years of experience building from scratch, and I've usually had good success,with a few exceptions! Anyway, can someone help me with an exact incidence on the wing and stab for a plane this type? The plan is showing 0 degrees for both, but I don't think that's correct. It'll have a 90" wing, and the fuse is around 63". Power will be from a Saito FG R3 radial. I'm going for around 20 lbs, rtf.
Thanks much,
Bob Lemay
Old 07-14-2019, 04:25 PM
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speedracerntrixie
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Bob, you are exactly right that 0-0-0 is not what you want although up until recently it was pretty much the standard. I think at some point guys started to figure out that it is much easier to fly precision aerobatics if the airplane were set up to fly more true. I think we have all read/heard guys state that certain aerobatic airplanes are less stable, snap happy, a handful etc. Over the past 20 years I have learned that those comments simply come from the old school that didn't really take the time to learn why their airplanes behaved the way they did.

Lets start with the wing. A symmetrical airfoil needs positive angle of attack to generate lift. If the wing incidence is set to zero then the entire airplane needs to be flown at a positive angle or as we would visualize, tail low. To achieve this positive attitude the airplane would require up trim. Other then not looking right the consequence of this up trim would be pulling to the canopy on an up line, down line and during Knife edge. So with all that said we want to put in some positive wing incidence. Usually 3/4 to 1 degree works for starters.

Engine thrust. Because we put in some positive wing incidence, the airplane will pitch trim well at half throttle but will want to slightly climb at full power ( CG plays a part in this as well. ) To help eliminate this we will want to throw in about 1/2 degree of down thrust. All reciprocating engines produce torque. When it comes to airplanes, the larger your propeller is the more the effect of the engine torque, P factor or spiral slip stream ( Whichever guys want to call the phenomenon) will want to make the airplane yaw to the left. The fix is a few degrees of right thrust.

Stab, I tend to use the stab as a reference because it really is the only thing that should always remain at zero. You never want to carry any elevator trim. This comes from the combination of wing incidence, down thrust and CG. Any elevator trim that you may have when flying upright and horizontal will be highlighted when flying vertical or knife edge when the wing is not under load.

Maiden flight. With the settings described above and your CG set at 30% MAC, take note if you need to add any elevator trim. You will need to determine if it is actually trim or that the elevator was not perfectly level to begin with. Let's say that it is indeed required up trim. Two causes can be possible, one being that additional positive incidence is required or that the airplane is nose heavy. To check for CG, put the airplane into an inverted 45degree up line. Nose heave will have the airplane pulling to level quickly. What I set mine to is a slight pull to the canopy in that attitude. Get the CG locked down as step 1 as it influences everything else. If you are happy with the CG, the stab is set to zero and it still requires some up trim then more positive incidence is needed. Once you get rid of the elevator trim you can see what happens on up lines. Pull to the vertical and go hands off. If the incidence is now correct and you have no elevator trim, it should go straight up without pulling or tucking. Pulling to the canopy means more positive incidence, tucking means less. Next upline should be done looking at the top of the airplane. If it pulls left, it needs more right thrust. It should be able to go vertical about 200' and then slowly start drifting left. Once the airplane will not change pitch trim from 1/2 power to full power and will do straight up lines, it should do a very nice knife edge. If anything it will have a very slight tuck. Most times you can use a rudder to up elevator mix. It usually only takes a little bit. Down lines will want to pull slightly as well. That gets corrected with a throttle to elevator mix. A touch of down elevator at idle.

Sorry for the very lengthy response but aerobatic trimming is fairly complex. It usually takes me around 50 flights before an airplane is really dialed in.
Old 07-14-2019, 04:52 PM
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rj lemay
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Thanks a lot SRnT!
I certainly appreciate the great info! Enjoyed reading your response, as my aerobatic set up skills have been shelved for a few years, due to my walk through the world of turbines... I had to put them down due to the anxiety factor!.... I'm about to cut the fuse sides, and will certainly cut the saddle to + .75 degrees or so, and leave the stab saddle at 0. As far as engine thrust goes, I'll use the shim spacers down the road, during engine install. Let me ask another question, about how much washout on the wing tips would you or anyone recommend? The plan has the attached tabs to the ribs to set the trailing edge heights, but it would be nice to double check their measurement against whatever you all might have to say.
Thanks again,
Bob
Old 07-14-2019, 05:52 PM
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speedracerntrixie
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If you intend to fly aerobatics no washout. IMO washout is a band aid for a poor wing design. With a built up wing that all the design work is already done. When I design a wing I will usually employ a couple tricks. One is moving the high point of the airfoil forward a little at the tip and increase the thickness a few percent. With something that is already designed you could radius the tip LE a bit more while making the root LE a bit sharper. The goal is to make the inboard wing section stall before the tips. Of course this takes away from snap rolls and spin entries. That being said, the way current IMAC and pattern is flown the airplanes don't stall during those maneuvers anyways. In most cases of tip stall the airplane is set up too nose heavy and has too much elevator throw to compensate.
Old 07-15-2019, 03:41 PM
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rj lemay
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I'll keep the radiusing of the tip L.E and sharpening of the root L.E in mind, that's one technique I've never used. Thanks again for you time and input, SrnT, I've been away from the forum for a couple of years, and I forgot how helpful it is here. I recently restored a 68 Beetle, and attempted to use a forum dedicated to that subject, and was driven away by very rude people, snide comments, and just general put downs and disrespect if you were not a 1000x + poster. I've never experienced anything like that here, so it's great to be back, and I'll update the build from time to time, and not be afraid to ask anything!
Bob
Old 07-16-2019, 03:27 PM
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rj lemay
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Hi Again Everyone,
Well as the painstakingly slow progress of hand cutting the wing ribs is underway, I was studying the plan and looking at the dihedral detail. It's showing 1.5"
with one panel flat on the building board. What's your opinion? I always thought a Sukhoi had a flat wing. What is the advantage or disadvantage of the dihedral?
I probably should know the answer to that, but my strong point is the building of a model not always why certain things are done.
Thanks,
Bob
Old 07-16-2019, 06:02 PM
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Bob, when it comes to an aerobatic airplane the dihedral will affect two things. A yaw/roll couple and a pitch/tuck in knife edge. These are things that are associated with both having too little dihedral or not enough. Not knowing which plans you are building from it is difficult to say where to go concerning this matter. The difference really boils down to where the horizontal stab is placed on the fuselage. If it is in the scale location then I would go with the plans dihedral. If the stab has been lowered ( very common on most large aerobatic models ) like the pictured H9 Sukhoi then I would suggest making the top of the wing perfectly flat and the dihedral effect is created by the taper of the wing thickness root to tip. Even if your stab is in the scale location and you want better flying performance at the cost of scale appearance you may want to consider the modification. Hope this answers your question.

Last edited by speedracerntrixie; 07-16-2019 at 06:05 PM.
Old 07-17-2019, 01:24 AM
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rj lemay
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Great info. I will drop the stab, as yes, it is in the scale location now.
Thanks again for a nice clear, concise answer.
Bob

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