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May 21, 2013, 03:35 AM
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vil64's Avatar
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Discussion

tail heavy, nose heavy or just on the mark?


This is a question for pylon racers. I used to fly IMAC and a slight nose heaviness was always more welcome as the plane tracks and snaps better. Now, I read somewhere (here) that for pylons a slight tail heaviness makes it go faster. Is that true? I just don't get it. I know for fact that tail heavy plane will be sensitive on elevator. So perhaps quicker in turns? How about that snap effect, you can get in tight turn, overloading the wing? Can somebody enlighten me?
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May 21, 2013, 05:30 AM
Aka: Tom Jenkins
ApexAero's Avatar
Rocket Ray Brown taught me the center of pressure changes and the airfoil becomes faster back in the early nineties. If the CG was to be 2.5 then we went to 2.75 Nose heavy for windy days.
May 21, 2013, 05:50 AM
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An aft C.G. will cause the plane to be less stable (more sensitive to elevator inputs from the pilot) thus causing the plane to turn faster when in a banked turn. The throws are set accordingly to avoid stalling the wing (unwanted snaprolling). Throws are minimal anyway.

Tim
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May 21, 2013, 06:22 AM
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vil64's Avatar
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Thanks guys, seems like that's pretty much what it is. With such a small throws, you still need bucket loads of expo to de-sensitise the centre of the stick on your trany ..I imagine. For sport pylon flying, I probably wouldn't be too keen to experiment with rearward CG
May 21, 2013, 02:38 PM
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Generally speaking, moving the cg back will unload the stab, thus causing less drag more speed. The downside is less stability.
May 21, 2013, 02:53 PM
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Another perspective by Dave Norman an experienced glow pylon racer posted at another site that I've used as a guide:

"I thought I would talk a little bit about setting up the center of gravity on your racers. There are a few different schools of thought on this, but Iíll just describe what I like in a proper cg. I primarily use the cg to adjust for drift, what I mean by this, is that if you put your plane on knife edge and fly it directly at yourself, does it drift slightly to the canopy or landing gear as its coming towards you? That is what I call drift, and Iíll explain one reason why it happens. If your plane is slightly nose heavy, in level flight you have to compensate for that nose heaviness with up elevator trim. This is great when you are flying level, but as soon as you roll to vertical that slight amount of up trim will cause your plane to drift slightly to the canopy. To me, there is nothing worse than making a great turn on #1 where your lined up perfect coming to #2 and to have your plane start to float in on you. When this happens, you are stuck with pushing the plane out with down elevator, which many racers are not all that comfortable doing. Conversely, if you are slightly tail heavy, you need to compensate for that with down elevator trim. When you roll vertical, your plane will drift away from you. This is frustrating, because you will be giving up the inside track, and flying a longer course. Drift can often be controlled by simply moving the cg forward or backward; depending on which way the plane is floating on you. I know many people that use the cg to adjust for level flight in knife-edge, but to me, controlling drift is more important than the speed loss due to a couple clicks of rudder. When you consider that you will be negating elevator trim, in trade for rudder trim, it is really a wash anyway. Not that my way is the best, but it seems to work best for me. I absolutely hate adjusting my #1 turn to compensate for a plane that drifts. I like them all to be the same, and eliminating drift is the easiest way for me to make consistent #1 turns.
"
May 21, 2013, 03:09 PM
DHG
DHG
Kinetic Sculptor
Quote:
Originally Posted by vil64
With such a small throws, you still need bucket loads of expo ... For sport pylon flying, I probably wouldn't be too keen to experiment with rearward CG
Nose-heavier is safer, up to a point, but don't be afraid to tape a quarter to the tail and see how it flies. The sweet spot is probably around 28% or 29% of M.A.C. (mean aerodynamic chord). I use 25% expo; some use more but I like to be able to see small stick movements around neutral. If you're overcontrolling, it's not an expo issue but a total throw issue.

The most common mistake new racing pilots make is to set up too much control throw. The roll rate should be 120 to 180 degrees per second (2 to 3 seconds to make one complete aileron roll). Elevator throw should be no more than necessary to make about an 80-foot loop at full throttle. Don't use any more rudder throw than you need to counteract torque on takeoff; double Brownie points if the airplane is incapable of performing a snap-roll even with full rudder. That means you can be sure it won't snap-roll by accident on the race course.

Dub Jett (www dot jettengineering dot com) has what he calls the "rule of 3/16". A racing model that is properly balanced will need about 3/16" of up elevator throw to make a smooth, tight turn at Pylon #1. If you need more than that to get a tight turn, the airplane is nose-heavy. If you need less, it's tail-heavy. An airplane that is nose-heavy will tend to drift inside the course (toward you) when banked for a turn, and a tail-heavy airplane will tend to drift out (away from you). Dub has more tips on his site, under his occasional series of articles entitled "The Crap Trap."

Hope this helps!

Cheers,

Duane
May 21, 2013, 03:41 PM
Registered User
I've found it's easier to fly a better course with less expo - you don't get that changing elevator rate when you're trying to make corrections around a turn.

I used to run about 30% expo, until a fellow racer told me he could tell by the way I was flying that there was too much expo... since then I don't use more than 15%, and it does make it easier to fly the course.
May 22, 2013, 01:16 AM
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Hmm, very interesting indeed Changing the tracking in knife edge by shifting the CG makes sense to me. In pylon racing, you probably spent most of the time in knife edge anyway. Another interesting idea is not too much expo. I guess that gives you better or more consistent feel in turns rather than fighting with different rates of movement in different positions of the stick. I see it very similar in setting of an IMAC plane. You basically try to minimize your workload by having the plane set up as neutral as practically possible. In pylons, I imagine you don't want to spend time correcting the plane in the race course during or after the turn either.

Also another question, what if your plane tends to climb or dip in turns? Do you play with the motor side thrust?
Thanks guys really appreciate your inputs.
May 22, 2013, 05:40 AM
turn, turn, turn.
like others here, I set my elevator throws so that it will not snap on the turn.
May 22, 2013, 05:48 AM
Aka: Tom Jenkins
ApexAero's Avatar
Set up the elevator for a 100 foot radius turn on full pull, set ailerons for one roll per second
May 23, 2013, 09:23 PM
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vic welland's Avatar
I do the same as DaveN and Fizz. I doubt any of mine have more than 5% expo in full throttle rates.
As far as roll rate goes I have ailerons set for one roll per straight away on the long course.
Inside the fuse my battery is on a velcro slide till I get the CG just right.


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