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Author Topic: Christen Eagle - Correct thrust angles  (Read 403 times)
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permalozo
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« on: January 17, 2017, 12:58:33 AM »

Hello Fellow Modelers. I'm experiencing a little trouble setting up an ARF Christen Eagle from Hangar 9 to fly right.
I'm already an experienced builder and a decent pilot and I started working on this model a while ago.
Despite my effort to follow the directions included with the kit I found a certain dissonance between my personal impression of the design of this plane with the numerous reviews selling this model as an excellent flyer, smooth, round capable of very soft landings.
The bird frankly behaves aerodynamically like a musk covered boulder. It has the tendency to change direction jerking left and right, and has no stability on the rolling or pitch axis. It is very difficult to land and has the tendency to pick down and loose lift especially during landing.
I thought it might be related to weight balance first and then I blamed on the motor's thrust angles.
Despite playing for a long time with these, the initial, stock setup seemed to be the best (the least problematic) setup.
Then I swapped my nitro setup for an equivalent electric, lowering the all up weight, perfect CG, the worst flight characteristics.
Then I noticed something strange. The lower wing seems to have a neutral to slightly positive thrust angle, which I found normal.
The upper wing though seems to have a neutral to slightly negative thrust angle and I don't think this sounds right.
I have the impression that I was simply unlucky and I got a defective plane with mismatching thrust rods and wings not sitting right.
I looked on several build plans for this model noticing the most different thrust angles. On the Pilot plan the upper wing is set at +1 degree, which sounds reasonable enough and the low wing has a neutral 0 degrees thrust. On another plan published on a magazine from some years ago though the wings are thrusted in the inverse way, with a neutral thrust angle on the top wing and a 0.5 degree at the bottom (weird?). The plans are consistent on the motor thrust angles (0 down, 1.5 side ), ok. So I decided to do some research to find out what the original thrust angles from the original Christen Industries plans are with no success. HAs anyone built a similar model or have any idea what the correct thrust angles should look like on this thing?
Thank you for understanding. Tongue
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Christen Eagle - Correct thrust angles
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Konrad
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« Reply #1 on: January 17, 2017, 09:14:04 AM »

I've flown a few and thought it was a fun bipe.  The Incidence set up you describe is much like the way I like to set up my biplanes. That is have aft wing stall first (set with the leading edge 0.5 to 1 degree positive, up, higher) I find this is more efficient. But will allow the plane to get into a deep stall easier. When the aft bottom wing stalls the plane acts a bit tail heavier. I like this for aerobatic maneuvers. But I have to approach my landing with a little bit more speed until she is in ground effect.

What are you using as your Zero datum, the stab?

I'd look at the radio (servo and linkage, throws) installation for issues with your particular model.

As this is a wood model I'd look for wash in on the wings due to the shrinkage of the covering film.
(Wash in is where the wing tips are twisted so they are at a higher angle of attract to the air flow than the wing root.  This causes tip stalling. )
« Last Edit: January 17, 2017, 09:31:41 AM by Konrad » Logged

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« Reply #2 on: January 17, 2017, 03:17:15 PM »

I just measured the force arrangement on my only flyable “Performance” bipe, an old E-flite Ultimate 20-300 (the wooden one).
Using the stab as my datum and set at “Zero”. I find that my top wing is showing negative 1 degree (down in the LE). The bottom wing is rigged at Zero degrees. The motor thrust line is down 0.5 degrees and I’m showing 3/32 of an inch total of right thrust  between the 12 inch prop tips.

I can see a bit of wash out in all 4 tips, but I can’t measure it.

I’m using an old Robart incidence meter. My repeatability is in the range of 0.5 or more degrees

Note; I try to fly this as a 3D plane as a result the CofG is rather aft, yet she land better than 90% of the other sport ship at my fields.

I have to ask what does perfect CoG mean, per the manual?

Full size force arrangement rarely have any relevance to our models. I like to fly my models with a lot less margin of stability than the full size
« Last Edit: January 17, 2017, 03:32:59 PM by Konrad » Logged

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permalozo
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« Reply #3 on: January 19, 2017, 02:44:35 AM »

Yes Smiley 0 stab and I think that I got the thrust of the motor aligned. What I noticed is that the model has an embedded thrust angle in the frame and the side part is visible. I have like the impression that the motor points slightly upwards. It might be an impression though. I think it is normal for a pipe to ballon up a little when you push on the throttle but this one has the tendency to stall very quickly and at low throttle wants to come down like a meteor and it looses lift quickly during landing. Could it be the aft wing,  maybe sitting with just an unnoticeable negative angle because of a defect in the kit and so at slight attack angles might interfere with the total lift generated? I think this model has the tendency to climb a lot when you have give a very gentle up elevator and it comes down like a rock when it is supposed to be level. And it's lightweight, which doesn't make any sense.
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permalozo
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« Reply #4 on: January 19, 2017, 02:57:28 AM »

Thanks Konrad I think I measured the angles the same way as you did, using the elevator as reference point. Great info by the way! Thank you for taking your time to help me with this. the actual thrust line should be according to the drawings just an 1/8 inch below the elevator, so the elevator should seat a little higher than the spindle centerline, and I think I got that right too. It actually comforts me to know that the upper wing might not be an anomaly after all.
I'll take a closer look at the wing covering film to look for deformation, who knows.
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Konrad
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« Reply #5 on: January 19, 2017, 09:52:19 AM »

Yep, most bipes drop like a meteor when the power is pulled back. That is part of the bipe charm! Roll Eyes Actually it is as a result of the drag from all the struts wires and wing tips.  Also Bipes have a large yaw to roll coupling.  Again part of their charm!  Wink I often have to give opposite aileron when making a rudder correction.

To test for up or down engine thrust line I trim the aircraft for full power level flight. When the aircraft is crossing in front of me I pull the power to idle. At the very moment that I cut power I watch which way the aircraft jumps. If she pitches UP a have too much DOWN thrust. If she pitches DOWN I have too much UP thrust. Think of level flight as having all the forces in balance. When you remove one (like the power) you are left with the only the other (like the elevator trim). For example if you have too much down thrust the elevator is trying to balance this with some up trim. So that is what the jump up is telling you when you pull back on power.

"Embedded engine thrust angles" have little to do with actual trim. They are where the designer thinks the trim will end up. He uses them to help the spinner and cowl align. In my initial set up I make sure I have some down and right thrust. (For my first flights I don't want up or left thrust! Shocked ) I them let how the aircraft responds during test flight dictate which way and how much I should move it.

Having the aircraft nose up when you apply power is an indication of being nose heavy. That is carrying up elevator trim.

Your concerns about where the elevator and thrust line intersect really has no effect on basic trim. Yes, this shows up in knife edge flight and post stall maneuvers.

In the past have you used a chart like this in your trimming effort?
http://www.wtp.net/DBEST/trimchrt.html

BTW; It takes me about 10 test/trim flights to get a model in trim.

All the best,
Konrad
« Last Edit: January 19, 2017, 10:50:19 AM by Konrad » Logged

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Konrad
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« Reply #6 on: January 19, 2017, 11:40:31 AM »

For what it's worth.
I looked at all of my bipes and they all have ether the elevator rigged with a bit of down or the stab is high in the front. This is to align the stab/elevator with the relative wind coming off the two wings.

I hope this helps.

Konrad
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permalozo
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« Reply #7 on: January 20, 2017, 12:34:45 AM »

Yes thank you. This is very good information to base my work on. I've flown only low wings and I wanted something to practice aerobatics with. I understand there is a lot more going on in biplanes because of the interaction of the two wings, but at least I'm relieved to know that probably part of the problem is that I'm not used to their behavior. Thanks again.
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« Reply #8 on: March 27, 2017, 09:34:42 PM »

I have not flown that particular biplane due to great experiences with the Goldberg Ultimate bipe, Midwest Super Stinker and the current .40 sized Pitts I scratch built. Can't bring myself to buy one. I have heard that some of the new bipes are not that great. If I may make a suggestion, take a look at the plans for an Eagle that is as close in size to yours as possible. Two of my favorite Bipes had symmetrical airfoils, pattern ship like setups with zero incidence stab to wings. The key is to figure out the airfoil and then set up the incidence accordingly. I also have had a Byron Pitts and a Sig Smith Miniplane which required more work to get set up correctly. The Miniplane was the worst, it had no downthrust, no side thrust, and a flat bottomed airfoil. The Byron Pitts was similar in that it had flat bottomed airfoils but already had the side and downthrust built in. I would also suggest that very minor changes can have a very large effect. one change at a time is the key. The Smith Miniplane required right thrust, down thrust, top wing washout, and an incidence angle change by raising the rear of the top wing to create negative incidence of 1 degree. My current Pitts was designed in the early 70's and when built with down and right thrust flew amazing. Semi symmetrical airfoils and near zero-zero setup on the wings. You are obviously experienced so can fly these airplanes well, but keep in mind Hanger 9 copied a full sized aerobatic airframe and as such will fly that way. Make sure the balance point is correct and perhaps watch the full size on youtube, it can lend insight as to what the airplane should fly like. After flying many bipes I will always have one as I like them but they are different to fly than, say an Extra 300. Hope this helps.
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