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Old Jul 18, 2013, 04:56 AM
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dinamich's Avatar
Slovenia, Ljubljana
Joined Aug 2010
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Would weight reduction improve performance?

Hi, I would like to start a discussion about performance gains or losses as a result of reducing high performance sailplane weight by a small amount (lets say 10 percent).

Of course such weight change and its effects are probably somewhat different for each airframe, but lets try to keep our findings generalized.

We could also talk about my concrete sailplane which is Nan Shadow http://www.nanmodels.com/Shadow.htm with F5J fuse converted to electric drive. Current specs are:
  • wing area 73 dm2 (1132 sq. in.)
  • weight 2300g (81.1 oz)
  • wing loading 31.5 g/dm2 (10.3 oz. / sq. ft. )

This is probably on the heavy side, because I used big 4S 3000mAh battery (I had it from other glider). I had to put in a 40g of lead into the end of tail to balance it.

I am happy with performance, but I keep wondering what would I gain if I try to reduce AUW. I calculated that with smaller battery and lead counterweight removed I could gain a weight loss of around 230g (that is 10% of current AUW).

So, AUW reduced by 10 percent, how would that sailplane fly, better or worse?

My thinking is:
  • I would probably not even notice any change in flight
  • stall and cruise speed would decrease by 5% ( sqrt of 10% change)
  • small reduction of stresses on airframe
  • smaller energy retention (how much?)
  • harder to fly in high winds

All in all I think that such a small change in AUW is not even noticeable. How much would I have to reduce weight to start seeing improvements (if any)?
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Old Jul 18, 2013, 05:22 AM
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Tuomo's Avatar
Jyvaskyla, Finland
Joined Aug 2003
2,421 Posts
10% weight difference is easy to notice. I usually increase ballast in about 150g increments.

2.3 kg Shadow might be just right in more windy weather but most of the time you maybe want to fly in lighter? Why not go to smallel bettery to get away of all tail weight.

Weight is not only important thing, you should be equally interrested in the amount of inertia. Ideally you have plane ballasted right to the conditons with very light wing tips and tail (low inertia).
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Old Jul 18, 2013, 06:27 AM
Marc PUJOL
Joined Feb 2010
124 Posts
What is performance?
If it is speed, then 150g will not make much difference.
If it is sink rate, then it's the same
If it is abbility to go upwind and reach the landing zone from downwind... Make a few calculations and you will see that you will have 10 to 20m improvement...
Look also at the ability to take a thermal. You will see that the minimum circling radius will be "quiet" different and then this may conduct the plane to circle outside the lifting zone or have a very less lifting ability.

Marc
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Old Jul 18, 2013, 12:57 PM
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United States, TX, Dallas
Joined Mar 2009
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Dinamich, the squareroot of 10 is 3.3333, not 5, thus the change in sink rate and stall speed will be minor ( in a first order approximation, neglecting effects of different RE).

I only appreciate light weight in marginal lift, the climb rate of the lighter plane will be higher ( or when the lift is really weak, a light plane might be able to just maintain, while a heavier might not).

F3J pilots appreciate light weights for faster acceleration on tow to minimize time on the line. If you do not fly J, extra light is not really needed IMHO.

Reto
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Old Jul 18, 2013, 03:03 PM
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United States, AL, Madison
Joined May 2002
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If it only costs you the amount of a smaller battery pack, try it and find out!
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Old Jul 18, 2013, 03:22 PM
Detail Freak
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Harbor City, CA
Joined Oct 2003
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I think that in a dynamic sense, lighter airframes are better at making course corrections while in smooth air. The plane is more responsive to the same angular input, and the damping better.
The downside of this I assume would be in bumpy air, where the lighter airframe might require inputs that the heavier one might not....

I agree with nuevo, try it and see. You might find a lighter pack in smooth air is better and maybe if the air is choppy, a heavier pack and wing loading would help.

I think the trend in ALES these days is a light battery and a ballast system where dead weight is added if its windy. That gives you the ability for both easily, but you have to start with a plane that is as light as you ever need it to be. That seems safer to do with a powered glider than one you launch with a winch.

G/L,
Target
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Old Jul 18, 2013, 03:50 PM
In F3J size does matter!
roydor's Avatar
Israel
Joined Nov 2006
839 Posts
I use 850 mAh 4 cell and 1300 mAh 3 cell batteries in my F5J models (two different setups), each battery weighs around 100 grams and has more than enough power and I can do at least 3 full altitude launches (200 meters and I usually launch lower) before having to charge a battery.
I have two batteries of each so while I fly the model I charge the second battery. This is how I can fly nonstop without waiting for a battery (each flight 10 minutes, +3 flights on a battery, between 1C to 2C charge on the charger).

Lighter models are fun when the wind is light. They signal thermals a little better and are more forgiving but once in a thermal heavier models climb very similarly, losing very little to the lighter ones.
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Old Jul 19, 2013, 01:10 PM
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United States, CA, Midway City
Joined Dec 2003
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Quote:
Originally Posted by dinamich View Post
Hi, I would like to start a discussion about performance gains or losses as a result of reducing high performance sailplane weight by a small amount (lets say 10 percent).

Of course such weight change and its effects are probably somewhat different for each airframe, but lets try to keep our findings generalized.


We could also talk about my concrete sailplane which is Nan Shadow http://www.nanmodels.com/Shadow.htm with F5J fuse converted to electric drive. Current specs are:
  • wing area 73 dm2 (1132 sq. in.)
  • weight 2300g (81.1 oz)
  • wing loading 31.5 g/dm2 (10.3 oz. / sq. ft. )
This is probably on the heavy side, because I used big 4S 3000mAh battery (I had it from other glider). I had to put in a 40g of lead into the end of tail to balance it.

I am happy with performance, but I keep wondering what would I gain if I try to reduce AUW. I calculated that with smaller battery and lead counterweight removed I could gain a weight loss of around 230g (that is 10% of current AUW).

So, AUW reduced by 10 percent, how would that sailplane fly, better or worse?


My thinking is:
  • I would probably not even notice any change in flight
  • stall and cruise speed would decrease by 5% ( sqrt of 10% change)
  • small reduction of stresses on airframe
  • smaller energy retention (how much?)
  • harder to fly in high winds
All in all I think that such a small change in AUW is not even noticeable. How much would I have to reduce weight to start seeing improvements (if any)?
As others have written 10% weight change is not such a huge change in flying speed, which changes 5% ( you are correct about the sq rt of the speed formula, not just sq rt of 10%)

How ever 10% weight change also changes your radius of thermaling 10% which is large.

Now also consider you are removing most of that weight at the nose and tail and that alone would make it a no brainer. Your handling will improve noticeably, response will be more crisp.

A Shadow with wing loading in the 9oz/ft?^2 would do fine up to 15mph wind. Ballast, if needed, concentrated near cg; would then make the plane even more responsive. I'd definitely get the weight out of the tail and nose!
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Old Jul 25, 2013, 10:35 AM
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Slovenia, Ljubljana
Joined Aug 2010
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So to summarize, everybody says go for it. You all agree that even such a small weight reduction will be beneficial.

Am am intrigued to found out, therefore I have ordered some new batteries and when they come, I will report on results.
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Old Jul 25, 2013, 01:27 PM
Detail Freak
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Harbor City, CA
Joined Oct 2003
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I think that you will find it to fly better in light lift, if the air is smooth.
If the air is turbulent, you may like it better heavier with the larger battery, to push through the turbulance.
What you won't be able to test, is how the plane flies with lighter wing tips and tails (not by just changing the batter pack, at least. You would have to get lighter tail and tip parts)....
The tail weight being removed will have the biggest effect of all, I believe.

I will be interested in your opinion of what you find.

Good luck,
Target
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Old Jul 25, 2013, 06:28 PM
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Joined Nov 2004
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I have become a total Weight Weenie since I have started flying F3J. Actually… I am obsessed with it. I have spent hours making my Maxa as light as I possibly can. Changing wire harness, servos, clevis, and any other tiny thing I can do. I have noticed a dramatic difference by just taking 3 ounces off my plane. Although it seems the most important weight savings come from the extremities like wingtips and tail.

I always see Daryl talking about light weight models. After all these years I think he knows what's important and what is not. Weight always seems to be a high focus for him. So once I opened my mind to that thinking I have been… Obsessed! I bought a new scale and have been using it constantly. It might just be in my mind… But that's okay! It seems like every gram I take off my model I notice it… Or so I think I do. As long as my feeble mind believes it I think I fly better. :-)

Kyle the Weight Weenie Paulson
On a Serious Learning Curve, lovin it, having a ball!
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Old Jul 25, 2013, 06:39 PM
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BTW..... For an F3J model I would define "light weight" performance as, and the benefits of;

1) Accelerate quicker for lunch.
2) Better min sink/float slower.
3) Slower landing speeds.
4) More responsive.
5) Better air reading indications.
6) Less drag because control surfaces reflect less for the same response.
7) My list and understanding is still growing..... :-) more thoughts welcome
8) I am completely aware that none of this will ensure that I make my times! :-(
Kyle
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Last edited by Kyle Paulson; Jul 25, 2013 at 06:55 PM.
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Old Jul 25, 2013, 09:25 PM
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wakumann's Avatar
Canada
Joined Jul 2003
2,477 Posts
Quote:
6) Less drag because control surfaces reflect less for the same response.
maybe more drag because its need more correction in turbulent air ?

Thomas
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Old Jul 25, 2013, 09:34 PM
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IHAVAWDY's Avatar
United States, MT
Joined Mar 2008
3,262 Posts
Quote:
Originally Posted by Kyle Paulson View Post
BTW..... For an F3J model I would define "light weight" performance as, and the benefits of;

1) Accelerate quicker for lunch.
2) Better min sink/float slower.
3) Slower landing speeds.
4) More responsive.
5) Better air reading indications.
6) Less drag because control surfaces reflect less for the same response.
7) My list and understanding is still growing..... :-) more thoughts welcome
8) I am completely aware that none of this will ensure that I make my times! :-(
Kyle
Is that 1) Accelerate quicker before lunch or after lunch? I thought you were throwing up...
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Old Jul 25, 2013, 09:38 PM
Detail Freak
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Harbor City, CA
Joined Oct 2003
21,550 Posts
during!
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