acceptable weights for gliders.

[beefa]

Hey all,

Im just after some feedback on weights.

I have just completed a Spirit, first plane I have ever built, and it comes in at 35oz, or just on 1 kilo for us metric folk....

Does anyone else have one of these planes and if so how close am i to a "good" weight.

I did make a modification during the build, as per a website and sheeted the front of the wings with balsa all the way to the end of it, so no doubt this would add some weight.

Also i have used some lead under the spoilers to get the flaps to lower back down, but i am toying with the idea of using a spring out of a ballpoint pen to save some weight.. any thoughts on this ?


TIA
Beefa

[CloudyIFR]

To really determine if it's too heavy or not you're really concerned about wing loading. This is how much each square inch will support.

Take the Weight divided by wing area and multiply by 144 to get wing loading.

Example: 600 square inch wing and 35 ounce model weight.

8.4 ounce wing loading. This means each square inch will support 8.4ounces. For the gliders the lighter the better with no wind, once the wind picks up a higher wing loading is better to penetrate the wind. My 110" contest ship has a wing loading around 10. My 2 Meter electric has a wing loading around 12. Both are great flyers. Airfoil makes a big difference as well, but that and many other factors are for another thread in what makes a good thermal plane. Most importantly, really, is to fly, fly and fly some more.

Curtis

[Ollie]

As the wing loading goes up so does the minimum sinking speed, the ability to penetrate and the turning diameter for a given angle of bank.

For example, a hand launched glider which must be able to work small, weak, low level thermals might have a wing loading around four ounces per square foot. Typical wing loadings for 2- meter thermal duration gliders are in the range of six to nine ounces per square foot. The increase in sinking speed is proportional to the square root of the increase in wing loading. A 10% increase in wing loading only results in a 4.9% increase in sinking speed. A 20% increase in wing loading results in a 9.5% increase in sinking speed. A 100% increase in wing loading results in a 41% increase in sinking speed but a doubling of the circle diameter for a given angle of bank. Increasing wing loading has its most serious detriment on the ability to work small, weak, low level thermals. There is a big penalty in sinking speed for having to bank higher to stray in a small thermal.

A number of years ago I built a beefed up 2-meter Prodigy with a wing loading of 14 ounces per square foot. I had several thermal flights with it that were over an hour but I never was able to work lift at less than about 1/2 launch height with it.

As Don Bailey points out in a article in the current Quiet Flier magazine, you can have the best of both worlds by building light and strong and only adding ballast as the wind picks up. This is exactly the design objective that has been so exceptionally realized in Dr. Mark Drela's designs.

[Gliderguy]

Weight of the plane will make a big difference in performance (or whether you can fly at all that day) as far as wind is concerned. We can get quite a variety of wind conditions here.
A very light plane with a strong wing that can be ballasted up is ideal as long as the airfoil isn't too draggy. Dr. Drela designed his new airfoils to be able to work a wider range of conditions with good lift and penetration.
There are 2 meter planes a lot lighter than 35oz but for the airfoil you have with the Spirit you'll probably not notice much increase in performance if it were much lighter. Wind verses plane weight will make the biggest difference in how it flies or handles (remember, you still have to land it). That's something you'll need to get a feel for.
Also, beware of the wing joiner. They are notoriously weak. Beaf it up if you can or just be very carefull on a winch or strong high start.

[beefa]

Some fantastic info guys.. thanks for taking the time.

Will get to fly it tomorrow, weather permitting, so will either come home happy or come home with a garbage bag of kindling ; )

I have left the wingloading at a calculated 7.5 and set it up so I can balast it up as needed in stronger winds.

Cheers

[ejett]

I did most of my learning to fly on a GP Spectra which is the same as the Spirit except that it is electric powered. Flying weight was about 50 oz. and it flew well. You should be fine at the flying weight you have and you can balast it up a good bit if you need to.

Regards,
Ed Jett

[Hodgepodge]

Cool, I have a Spectra also and it is right at 50.1 oz (This is after a repair from a low-speed encounter with a baseball backstop)

BTW it has a very nice d-box construction and no the center spar is not a solid 3/8" in sheet :rolleyes: .

You should see the neat cuts chainlink makes in soft LE balsa!!:(

Oh and the wing loading is 11 oz/sq ft.

Vs my nice homebuilt glider/parkflier at 4.2.:D

[philami]

If you want your balsa glider to go oos and fly away use this formula:

1. 10" or less wingspan: 7-9 grams.

2. 15" wingspan: 12-16 grams.

3. 20" wingspan: 15-28 grams.

I never defy my gram scale and have been rewarded by too many fly aways! lol.

[John Gallagher]

What size servos did you use? Did you need to put ballast in the nose to get the CG right?

As far as lead weights to keep the spoilers closed. There are times when the weights might actually make the spoilers open. I would not trust the weights, and go with a spring to keep them closed. A ball point pen spring is low quality and would tend to stretch/deform.

[dwells]

The lead weights for the Spirit spoiler just add more to the plane. A simple piece of rubber band works well. Just CA it in the right spot with the right tension. This is also easier on the servo. I have used this and the movement is very good. They open easy and slam shut.

[atjurhs]

If you want your balsa glider to go oos and fly away use this formula:

1. 10" or less wingspan: 7-9 grams.

2. 15" wingspan: 12-16 grams.

3. 20" wingspan: 15-28 grams.

I never defy my gram scale and have been rewarded by too many fly aways! lol.

Phil, by your "formula" you'd have a 4oz. 2meter plane :eek: I'd sure like to see that!

[philami]

Phil, by your "formula" you'd have a 4oz. 2meter plane :eek: I'd sure like to see that!

No, check the wingspans in my list. None went over 20". Like you I'd marvel at a 2meter weighing 7-9grams (bad example for sure)! I'd also worry about it's survival! lol.

[lincoln]

35 oz. is fine for a Spirit, although of course the less weight in the tips and the more in the middle, the better for handling. The airfoil on the Spirit probably isn't going to to really well if you get it really light. I used to do well in competition with a Sagitta 600 (same size as your glider) which weighed 42 or 43 oz. But of course if you use a thinner airfoil going lighter pays off.

The rubber band trick works well. Another is to make a torque wire. I'm not sure the exact size, maybe 1/32 music wire. It runs along the front of the spoiler bay and has two legs bent off at right angles from either end of the straight part. One attaches to the spar, maybe by a little socket of plastic tubing, such as that inside a ball point pen. The other goes into something similar in the spoiler. You need to adjust the angle between the two legs to get it to pull the spoiler down. I've also used the long spoiler spring that came with my Cumic kit. That was like a much higher quality and longer version of the ball point pen spring, with less space between coils.

The Spirit is a bit short tailed, so you might want to enlarge the fin and or rudder a bit. Especially since you may have somewhat heavier tips on your wings. Try to use light weight balsa for this, maybe 6 lbs per cubic foot if you've got it.

If you liked building it, you might try an Olympic II. When it comes to advising beginners what to fly, I admit that I sound like a broken record, but it's a great design. The Spirit isn't, although many people have learned to fly with it and have enjoyed it.

I'm not sure I understand how a Spectra or whatever it's called can weigh 50 oz. We have brushless and lipos now. I had an electric glider around that weight, with very primitive equipment, many years ago. Not a lot of fun to fly it. It's not necessary to go straight up. Two or three hundred feet per minute is probably enough, particularly if it means the glider is lighter.

[aeajr]

35 ounces for a spirit is very good.