Flight Times

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Greg Doe
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Re: Flight Times

Post by Greg Doe » March 21st, 2010, 12:03 am

Illusionist,
You actually are getting a fairly good performance for your airplane, and Jeff has already given
you some excellent suggestions, but first let me show you why one airplane out performs
another. Wing loading plays a huge roll in duration flying. Your airplane has a combined wing
and stab area of 547.5 sq. cm. The guy who is going to beat you has an airplane with a combined
wing and stab are of 880 sq.cm. For the sake of this arguement both airplanes weigh the same.
(7 grams we hope!) Your airplane has a heavier wing loading, so it has to fly faster to generate
the same 8.5 grams of lift. Remember we have to include the weight of the motor. You will use
up your fuel faster, so your motor won't run as long, so your flight time is less. If you have time
to build another airplane consider these dimensions. Wingspan 39.9 cm; wing chord 15cm;
stab span 27.9 cm; stab chord 10 cm. Interestingly your airplane's combined wing and stab area
is only slightely more than a "bonus" airplane's combined area would be (ie:smaller wing/larger
stab).

If you have relatively good rubber you are only winding to about 55% to 75% of what it
should accept. You should be able to get 1200 to 1500 winds. The extra winds should up your
time a minute or so, if everything else is right. You need to wind a motor until it bursts, so
you know how many winds it is safe to put on. You need to lube your motors, and of course
stretch wind. Jeff Anderson highly recomends a torque meter, which can be helpfull in getting
maximum winds, without exploding motors. Good advice if you have time to make one, or
the resources to buy one. Here is something to ponder; if your airplane dimensions were the maximum allowable in the rules for this year, it would probably be competitive. What you need
is more winds. Even if you had the larger wing and stab that I've recommended, you still need
more winds. Good luck.
Greg Doe
Smyrna, TN

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Re: Flight Times

Post by Littleboy » March 21st, 2010, 10:24 am

What would be a top flight time for state in MI?

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Re: Flight Times

Post by illusionist » March 21st, 2010, 12:52 pm

Greg Doe wrote:Illusionist,
You actually are getting a fairly good performance for your airplane, and Jeff has already given
you some excellent suggestions, but first let me show you why one airplane out performs
another. Wing loading plays a huge roll in duration flying. Your airplane has a combined wing
and stab area of 547.5 sq. cm. The guy who is going to beat you has an airplane with a combined
wing and stab are of 880 sq.cm. For the sake of this arguement both airplanes weigh the same.
(7 grams we hope!) Your airplane has a heavier wing loading, so it has to fly faster to generate
the same 8.5 grams of lift. Remember we have to include the weight of the motor. You will use
up your fuel faster, so your motor won't run as long, so your flight time is less. If you have time
to build another airplane consider these dimensions. Wingspan 39.9 cm; wing chord 15cm;
stab span 27.9 cm; stab chord 10 cm. Interestingly your airplane's combined wing and stab area
is only slightely more than a "bonus" airplane's combined area would be (ie:smaller wing/larger
stab).

If you have relatively good rubber you are only winding to about 55% to 75% of what it
should accept. You should be able to get 1200 to 1500 winds. The extra winds should up your
time a minute or so, if everything else is right. You need to wind a motor until it bursts, so
you know how many winds it is safe to put on. You need to lube your motors, and of course
stretch wind. Jeff Anderson highly recomends a torque meter, which can be helpfull in getting
maximum winds, without exploding motors. Good advice if you have time to make one, or
the resources to buy one. Here is something to ponder; if your airplane dimensions were the maximum allowable in the rules for this year, it would probably be competitive. What you need
is more winds. Even if you had the larger wing and stab that I've recommended, you still need
more winds. Good luck.
Greg Doe
Smyrna, TN
thank you so much, this should help, but also, my airplane does keep hitting the ceiling, and i don't have a larger place to do some test flights... yes, my plane weighs 7.1 grams
2012-2013 Building Event Captain
Rule 7d. "Event Supervisors are allowed to break any competitors' devices" -bearasauras

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Re: Flight Times

Post by illusionist » March 21st, 2010, 12:54 pm

jander14indoor wrote:
illusionist wrote:I am getting around 1:30 with my plane. it has a 38.5 cm wingspan and 11 cm chord, 19 cm tailspan, 6 cm tailchord, 3/32 inch rubber motor, and a 20 cm ikra prop. Any suggestions on improving my times? the flight path is very stable and it flies in smooth circles. I am putting in 800 turns. Please help!
You didn't say what your plane weighs? If you aren't close to 7.0 gms, that's the first thing, lose weight.

If weights OK, try more turns, so long as the motors don't break, you may be near the limit, but I don't think so. If you are banging the ceiling, wind more and then back off till you stop hitting the ceiling.

If no turns left on landing, thinner rubber or more turns on this rubber.

Try raising the leading edge of the wing or the trailing edge of the tail till you start stalling on descent, then back off a little. Then rematch rubber and prop.

Where's your cg, if not at the rear wing post, or behind, move it back, readjust the wing and stab to stop stalling, and then rematch rubber and prop.

Take and review data every step of the way. Try to repeat each setting twice to make sure some other anomaly didn't mess up the result. Go with what the data says. Be careful about changing too many things at once unless you have someone who can guide you in Design of Experiments (DOE). Note, DOE is NOT experimental method, its a mathematically efficient and correct way of varying multiple factors at once and sorting out the true effects. The basic math to execute DOEs are within a middle school student's capability, theory and correct application will be more of a stretch.

Jeff Anderson
Livonia, MI
thank you very much for the suggestions. my cg is right in front of the rear wing post, and my plane weighs 7.1 grams.Could you further explain DOE? My regional competitions in a week, and I'm a little worried about my times...
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Re: Flight Times

Post by jander14indoor » March 21st, 2010, 4:29 pm

illusionist wrote:<SNIP> my airplane does keep hitting the ceiling, and i don't have a larger place to do some test flights... yes, my plane weighs 7.1 grams
If you are banging your plane off the ceiling with that few winds on that motor, it is WAY to fat. You really need thinner rubber and to wind harder.

And the problem isn't really with the ceiling height, you are climbing to fast due to the motor being too strong. While low ceilings limit flight time, there are ways to compensate so its not as bad as most people think. Last years planes would do 3 minutes in a 25 ft gym (expertly flown). This years planes will beat that.

As to explaining DOEs, dang, not easy. Probably best to get a local expert to help, but I'll see if I can write up something coherent in less than a page and post it later. I've used them lots at work to solve difficult technical problems. I've set them up for others to execute. But teaching concisely is SO much harder. OK, enough complaining, I brought it up, I'll try to explain it in a follow on note.

In the mean time, you may not need such a powerful technique. Seriously, your motor is too fat. I've been flying on motors less than 0.080 inch wide 1500-1800 turns with last years plane and its smaller tail.

Jeff Anderson
Livonia, MI

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Re: Flight Times

Post by illusionist » March 21st, 2010, 7:16 pm

jander14indoor wrote:
illusionist wrote:<SNIP> my airplane does keep hitting the ceiling, and i don't have a larger place to do some test flights... yes, my plane weighs 7.1 grams
If you are banging your plane off the ceiling with that few winds on that motor, it is WAY to fat. You really need thinner rubber and to wind harder.

And the problem isn't really with the ceiling height, you are climbing to fast due to the motor being too strong. While low ceilings limit flight time, there are ways to compensate so its not as bad as most people think. Last years planes would do 3 minutes in a 25 ft gym (expertly flown). This years planes will beat that.

As to explaining DOEs, dang, not easy. Probably best to get a local expert to help, but I'll see if I can write up something coherent in less than a page and post it later. I've used them lots at work to solve difficult technical problems. I've set them up for others to execute. But teaching concisely is SO much harder. OK, enough complaining, I brought it up, I'll try to explain it in a follow on note.

In the mean time, you may not need such a powerful technique. Seriously, your motor is too fat. I've been flying on motors less than 0.080 inch wide 1500-1800 turns with last years plane and its smaller tail.

Jeff Anderson
Livonia, MI
It's alright about DOE's, I only have a week till competition, so I won't have time to learn it anyways. I won't be able to get a smaller motor in time, but I'll keep that in mind if my team makes it to States. It doesn't rise too quickly though, it gradually rises, but once it does, it keeps hitting the ceiling. Thanks-Illusionist
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Rule 7d. "Event Supervisors are allowed to break any competitors' devices" -bearasauras

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Re: Flight Times

Post by Greg Doe » March 21st, 2010, 7:59 pm

Illusionist,
You and Jeff Anderson are both in Michigan arn't you? Ask (beg) Jeff to bring some extra rubber
to the state event. You might be suprised to see how willing people are to help others? Good luck.
Greg Doe
Smyrna, TN

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Re: Flight Times

Post by jander14indoor » March 22nd, 2010, 3:27 am

Actually, due to my role in SO, the day of a competition is the only day I can't help. Wouldn't be appropriate.

Before or after the contest, gladly. You can tell by my sig I'm in SE Michigan. And if you aren't near Livonia, I may be able to find someone closer who can strip rubber for you.

Jeff Anderson
Livonia, MI

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Re: Flight Times

Post by illusionist » March 22nd, 2010, 4:11 am

Alright thanks, I'll let you know after the competition. :) Right now, I get my rubber from aeroracers.com. I'm using the ".093" inch rubber, should I get the ".062" inch?
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Re: Flight Times

Post by jcollier » March 22nd, 2010, 6:10 am

IME, .093-.062 would be too drastic of a change. Our planes fly well with .080 to .085 rubber. If you are pressed for time and can't get thinner rubber, you could always decrease the wing incidence (angle of attack) a mm or so and see what that does.

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