W&B, angle of incidence

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W&B, angle of incidence

Postby mr157ifhz » Mon Sep 06, 2004 12:12 am

What is the better route to go if one has a nose heavy plane. My plane seems to have quite a large cg range. My problem is that it seems to fly nose heavy. The cg is set so as to fall at around 30% chord, which seems to me to be too far aft, 'cept it doesn't fly like it. I am reluctant to add weight to the tail, as I think perhaps the tail is not producing enough 'down' load. Cruising around, I need a substantial amount of up trim, probably creating lots of drag. So, what would be the better way. Lowering the leading edge of the tail, thus creating more down force; or just adding weight to the tail 'till the plane flies level. Which way would produce the least drag?
-Matt
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Postby Sean_Caranna » Mon Sep 06, 2004 1:34 am

Before I try to recommend anything could you post your CG data?

You stated in your intro post that you started with a Rotax 582 and now have a Subaru EA81. Did this just become an issue or get worse after the engine install?


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Postby mr157ifhz » Mon Sep 06, 2004 8:47 pm

The cg location is the same now with the subaru as it was with the previous rotax engines, except the plane is 122lbs heavier. The battery was relocated midway to the tail, and a small 4lbs tool kit at the farthest point back in the tail. The nose heavyness was there with the rotax as well. My cg calc papers are at the hanger, I will post 'em next I am up. The previous owners had raised the incidence of the wing, and claimed it flew much better. I did not like how this was done and when the plane was reassembled, the wing was mounted in the position it was designed for. It would be much safer, from the structural side of things, to lower the leading edge of the tail. If need be of course. I sold an earlier version of this plane, that was basically an airframe project. The new owner sent me some pics of the project, and I see he has lowered the leading edge of the tail. Awaiting his reply as to why he felt this was neccessary.
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Postby mr157ifhz » Tue Sep 07, 2004 10:52 pm

Ok. rechecked my figures, as a % of chord. My cg now is at 22.5% chord. Definitly not too far aft. Seeing as I can likely move the cg back some with no adverse affects, the orig. question still stands. More weight, or more downforce on the tail? Which is most efficiant?
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Postby Sean_Caranna » Tue Sep 07, 2004 11:13 pm

Without knowing the area of the tail and the angle that will be required it's hard to tell how much drag will be induced. However! Adding weight is the last thing I'd usually recommend. I'd see what you could relocate aft. Look at this topic about keeping weight aft under the cowl http://www.wingsforum.com/viewtopic.php?t=12

See if you can just move things back but don't go nuts and make your CG too far aft. You'll want to breakout the scales for this. Once you're done, take her for a test flight. If everything feels good run a high alt stall test series. If you need any info on a stall test profile let me know.

P.S. I studied Aerospace Engineering at Embry-Riddle University but have not completed the degree. Ran out of money. I have about a year left when I go back.
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Postby mr157ifhz » Tue Sep 07, 2004 11:33 pm

Without adding weight, there is not much that can be shifted back. The engine is right at the firewall. The battery is limited by it's size from going further back (is quite small, an odyssey pc680). The radiator is mounted back behind the gear p-51 style. The seats (side by side) are adjustable. They move about 8", with the current cg falling right about in the middle. I've tried different positions in flight, but because of the location so close to cg not much difference is felt. The newer versions of my plane have a jack screw that raises or lowers the leading edge of the tail for trim (like a cub) Perhaps a winter project...
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Postby Altoq » Fri Dec 31, 2004 11:30 pm

I'm new to the forum and just getting tuned in, so I've got a couple questions to bring me up to steam with your problem.

1. When you do a major modification, like replacing the engine with one that is 122lbs heavier, you should weigh the aircraft and establish the new basic weight and moment. Did you do that ???

2. When You say that the aircraft is "nose heavy", do you mean the Aircraft cruises with the nose low, or that you don't have enough elevator to control the aircraft

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Problems caused by CG too far forward or too far aft

Postby Kevin Horton » Sat Jan 01, 2005 9:14 am

If the CG is too far forward, you will have symptoms such as difficulty holding the nose up during the flare at landing, inability to trim at low speeds, and high stick forces during manoeuvring. Note: an inability to trim at low speed may also be caused by problems in the trim system, so this symptom alone does not necessarily mean the CG is too far forward.

If the CG is too far aft, the static longitudinal stability will be low, or maybe even negative. This means that you won't get normal changes in stick force as the speed changes. I.e. if you are trimmed at a particular speed, and you slow down, you should have to hold an aft stick force until you retrim. If the CG is too far aft, you may find that you don't have to hold a stick force at all at the new lower speed. This is bad as it means that the aircraft won't hold a trimmed speed, and you might get dangerously slow (or fast) without realizing it. If the CG is far enough aft to produce negative static longitudinal stability, the aircraft's nose will want to pitch up as you slow down, and you will need to push harder and harder on the stick the slower you go - obviously a very dangerous situation.

There is more info on stability, definitions, etc at:

http://www.seqair.com/FlightTest/Stability/Stability.html and http://selair.selkirk.bc.ca/aerodynamics1/Stability/default.htm

A too aft CG will also lead to degraded stall and spin characteristics, possibly leading to an inability to recover from a stall or spin.

Performance aspects - the farther forward the CG is, the more down force the tail needs to create to hold the nose up. The wing must generate an amount of lift equal to the weight of the aircraft, plus the amount of down force that the tail is generating. So, moving the CG forward increases drag in two ways: from the increased elevator deflection (or stab if we have a trimmable stab) required to hold the nose up, and more importantly, an increase in drag as the wing must generate more lift. Moving the CG aft will improve performance, but aft CG movements must be done with caution, due to the possibly dangerous adverse effects on stability, stall, spin etc mentioned above.

In any case, every builder should do flight testing at the forward and aft CG limits that he declares, and any change in these limits should lead to a new flight test program. It is far better to find a problem during a flight test program when you are at a safe altitude and mentally prepared to deal with it, rather than find the problem in service, where it might catch you by surprise and lead to an accident.
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Postby mr157ifhz » Mon Jan 10, 2005 11:08 pm

Ok. Finally back on line. My problem may already be solved. But to fill you guys in: Yes, a new weight & balance was done. Cg was found to be around 22~23% chord. Probably not ideal for performance, but safe and within the range for my plane. I do not have a problem with lack of elevator authority. Even at the low speeds (30mph, thanks to VG's and booster tips) that I land I can plant the tailwheel well before the mains. Seems to be plenty there for flare. I suppose a more accurate description of my problem would be to say the plane seems to want to be flown 'tail down'. There was an article in the most recent UPAC! News (Ultralight Pilot's Assotiation of Canada, newsletter), in which the fellow wrote about his 'lil Buzzard and the modifications he made. He had increased the wing's angle of incidence. My plane had this done before I bought it, and as I said before I did not like how it was done. This fellow had done the mod neatly, and safely. I called and discussed the modification with him. The way he described the way his plane flew before the mod is exactly the way my plane flies now. His cruise went to 80mph (from 70), and his takeoff roll is greatly reduced. Tough to explain but it seems the angle between the thrust line and the wing needed to be greater, so the engines's power is pulling the plane foreward; not foreward and up. It took a weekend to do the modification. The weather is like a yoyo here in southern Ontario, freezing cold then very mild but always blustery. Might have to wait 'till spring to try it out.
-Matt
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Postby Kevin Horton » Tue Jan 11, 2005 12:44 am

Ok. It sounds like you should leave the CG where it is. If the nose is pointing too far up at normal cruise speeds, then perhaps the wing's angle of incidence should be increased a bit, if there is a structurally sound way to do this. If you just change the wing's incidence angle, without also changing the tail's incidence angle, you will want to do some careful flight testing afterwards. This will change the angle of attack of the horizontal stablizer (assuming you have a fixed stab, with a trim tab on the elevator), and would increase the risk of a tailplane stall. Tailplane stall is most likely to occur with flaps extended (if you have flaps) at high speed. A sudden nose down push by the pilot may trigger the tailplane stall. The symptom of a tailplane stall is a sudden nose down pitch. Full aft stick may stop the nose down pitch, or you might not regain control until you retract the flaps.

If your aircraft uses a moveable horizontal stab for pitch trim, then a change in wing incidence will have no effect on the risk of tailplane stall, as you will set the trim as required, and the stab will end up at the same angle to the wing as before. You can ignore my concerns about tailplane stall.

If your aircraft has a fixed stab, then you can either change its angle by the same amount as you change the wing's incidence, or you can do a careful series of flight tests to look for a tendency for the tail to stall. The flight test program would consist of a series of test points, using a build-up technique. I.e. you start at the lowest risk test point, and gradually move to more risky conditions in small steps, looking for the early signs of a problem as you do each test. Start at low speed, with flaps up. The risk of tailplane stall increases as you deploy flaps, and as you increase the speed. At each test point, you would make forward stick inputs, looking for buffeting of the controls, or for a tendency of the stick force to lighten as you push. These are signs of airflow separation on the horizontal tail, and are early signs of an impending tailplane stall. Do small stick inputs first, followed by ever larger stick inputs, until you either hit a low g (no need to go less than zero g), or full forward stick. If a particular airspeed and flap angle is OK, then repeat at a slightly higher airspeed, or with more flaps deployed. If you see signs of impending tailplane stall (e.g. buffeting of the elevator, or stick force lightening as you push forward), then stop the testing and either increase the angle of incidence of the tailplane, or decrease the max speed with flaps deployed, or decrease the angle of incidence of the wing.

If you get an uncontrollable nose down pitch - pull full aft stick, and retract the flaps. This will hopefully reattach the airflow over the tail, and allow a recovery.

On type certificated aircraft, the aircraft design normally is such that tailplane stall is not a problem. But, there have been problems with tailplane stall if the aircraft has collected some ice, as the presence of the ice decreases the angle of attack at which the tailplane will stall. On homebuilt aircraft, especially ones that have been modded by the owners, there are no guarantees, so you must be careful.

http://www.avweb.com/news/safety/183056-1.html
http://www.aopa.org/pilot/never_again/2002/na0201.html
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Postby mr157ifhz » Tue Jan 11, 2005 2:56 pm

Good advice, thanks Kevin. The wing was repositioned by relocating the wing mounting tabs on the spars. The angle was increased 2 degrees. Slightly down at the back, and up at the front. I had to make new jury struts. The aileron pushrods had more than enough adjustment available to compensate. Don't have flaps. Who needs 'em? My plane falls out of the sky with some sideslip. My horiz. stab. is fixed. With the amount of up trim I had to fly with though to remain level in cruise, I don't think I will have to move it any. I have made a bracket that will move the leading edge of the stab. to compensate though if need be.
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Rigging

Postby Altoq » Wed Jan 12, 2005 9:33 pm

Two degrees is a pretty big move and thats not only relative to the fuselage,but the entire aircraft. That includes the horizontal stabalizer and thrust line.
You should consider at least moving the angle of insodence of the Horizontal Stabalizer down the same two degrees. With the CG somewhat forward of design and having moved the thrust line angle down, that elevator has realy been doing a job. Now the aircraft aft of the center of aerodynamic presure has been moved down out of the slip stream. That eliminated any assisting down force it was providing.
Make some moderate speed taxie runs to insure elevator athority and slow down the process. Make smaller adjustments and take time to evaluate them.
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Postby mr157ifhz » Thu Jan 13, 2005 1:22 am

Moving the wing +2 degrees incidence puts the wing where it should have been from the very begining. It puts the wing at the correct angle in relation to thrust line and fuselage (I understand newer versions of my plane have increased wing incidence, along with adj. stab.). So now the angle between the wing and the tail has increased, +2 degrees. The tail does not have to have to produce as much downforce, to stay level. Moving the incidence of the tail down (increasing tail downforce) would increase the angle between it and the wing. So now I would have a plane that flies as if it is tail heavy, because moving the tail adresses a problem already looked after by moving the wing. Am I missing something here?Sometimes I think I have it, then it slips away when another variable is pointed out. Aerodynamic pressure. What did you mean when you said that it has moved 'down out of the slipstream'? Pressure on what, the tail? What was producing the 'assisting' downforce? I understand that with the center of lift behind the cg, downforce on the tail is required. With the wing angle changed, lift is moved back slightly, requiring less down load on the tail. It makes sense to me now, maybe not in the morning. Could you explain further?
-Matt
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Postby mr157ifhz » Thu Jan 13, 2005 1:28 am

Aha! no sooner had I posted that I realized what I think you were trying to say. With the lift moved back, and with no cg changes, the front end is effectivly 'heavier', requiring more downforce at the tail to counteract. Now I can sleep.
-Matt
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Postby Altoq » Thu Jan 13, 2005 1:58 am

PLEASE disreguard My earlies posts. I was in error in that I thought you had decreased the wing angle of insodence. I am still not sure what you mean by nose heavy. :?
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