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Question- Nose weight in sailplanes - 4/1/2002 10:43:13 PM   
ejbjh


 

Posts: 36
Joined: 3/12/2002
From: Archbold, OH, USA
Status: offline 		Since weight is always an issue in sailplanes, I've been trying to figure out why
designers don't just increase the length of the fuselage ahead of the wing, thus
increasing the moment arm of the nose weight, whereby less weight would be
more effective in balancing tail weight. What am I missing?
       Post #: 1

Question- Nose weight in sailplanes - 4/1/2002 11:14:15 PM   
R. Carver



Posts: 154
Joined: 12/9/2001
From: Oviedo, FL,
Status: offline 		It affects the flying characteristics of the plane....I'm sure Ollie can explain the details better than I could. Ollie?

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       Post #: 2

Longer Nose ? - 4/2/2002 12:46:23 AM   
MTT



Posts: 561
Joined: 1/18/2002
From: West Chester, OH, USA
Status: offline 		As Ron already pointed out, it would affect the flying characteristics. Also, the extended nose means more structure&more weight, so at the end you probably will arrive at pretty much the same gross weight.

Michael

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Question- Nose weight in sailplanes - 4/2/2002 4:30:07 AM   
dhrider


 

Posts: 22
Joined: 2/25/2002
From: Northglenn, CO, USA
Status: offline 		It was explained to me once like a weight on the end of a stick. the longer the stick the harder it is to control.

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       Post #: 4

Question- Nose weight in sailplanes - 4/2/2002 5:24:53 AM   
seafury_fb11



Posts: 236
Joined: 2/22/2002
From: Los Angeles
Status: offline 		I'm not so sure that I buy this entirely. I agree that if you were to take it to an extreme and do something silly like make the nose moment twice as long as the tail moment, that might make the plane fly a little wierd. But at the same time, that nose had better by pretty darn light in order to balance properly - you still need the CG in the same place.

I don't think that adding an inch or two to the nose of a plane that is 50 or 60 inches long in order to save a little weight is going to hurt the aerodynamics much. I have done this lots of times with my own planes - moving the wing back, effectively making the nose longer and the tail shorter - and they fly fine.

Russ.

< Message edited by seafury_fb11 -- Apr 2 2002 2:50AM >


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Nose Moment Arm - 4/2/2002 7:52:42 AM   
Ollie


 

Posts: 958
Joined: 12/8/2001
From: Punta Gorda, FL
Status: offline 		The effect of nose moment arm length on balance is linear but the affect on polar moment of inertia is squared. What you gain by lengthening the nose to balance is off set by the larger effect on the polar moment of inertia. The control forces required to maneuver in pitch and yaw increase faster than the gain in balance by lengthening the nose.

It is far better to reduce the weight in the tail to balance than it is to lengthen the nose moment arm. Reducing the weight in the tail achieves balance while reducing the polar moment of inertia a lot so it is a win-win solution. It is worth the effort to use light weight materials in the tail. Every gram saved there reduces the weight of the model by about 4 grams and reduces the polar moment of inertia by an even larger factor. Any control force reductions translate into maneuvering drag reductions making the lighter tail a tripple win situation.

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Question- Nose weight in sailplanes - 4/2/2002 8:48:16 AM   
seafury_fb11



Posts: 236
Joined: 2/22/2002
From: Los Angeles
Status: offline 		I hear what you are saying Ollie, but there is a happy medium in there too. You could make the nose so short that it would take almost no movement of the control surfaces to maneuver, but you would have to add so much nose weight to balance it, that the plane would only fly arcs (like a rock).

By the same token, going to extremes to lighten the tail so that you can balance a plane with such a short nose, and making the the plane too fragile in the process is also not practical.

I'm no physicist, but as I said, I've played around with this quite a bit on different planes and I've never had one just be a totally unflyable dog. I generally setup my elevator control to have minimal throw - only about 6-8 degrees max in each direction. When flying, I rarely yank the stick all the way back. The amount of elevator I use is truly minimul. Hardly enough to stop the plane in its tracks due to excessive drag.

I agree that it is better to reduce the tail weight in order to help balance the plane. All I am saying is that in cases where it is not possible (or maybe not practical) to reduce the tail weight anymore, you won't kill the performance of the plane by lengthening the nose a little.

Russ.


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Question- Nose weight in sailplanes - 4/2/2002 10:19:03 AM   
R. Carver



Posts: 154
Joined: 12/9/2001
From: Oviedo, FL,
Status: offline 		Ollie,
This is actually a topic that's been in the back of my mind for a while... How would you best descibe the reasoning behind the design of two planes that are on opposite ends of the nose moment spectum, the Mantis and Drela's Allegro?

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Question- Nose weight in sailplanes - 4/2/2002 5:37:13 PM   
ejbjh


 

Posts: 36
Joined: 3/12/2002
From: Archbold, OH, USA
Status: offline 		I appreciate all the imput. Sounds like there may be a lot of different theories
on this, but I especially appreciate Russ's imput as someone who has actually
done some experimenting with it. I agree with you Ollie in theory-and you have to
stay within reason, but like Russ, I have not flown anything yet that I couldn't
get plenty of elevator authority. Keep the respnses coming- this is fun !!

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Question- Nose weight in sailplanes - 4/2/2002 8:45:53 PM   
dgliderguy



Posts: 334
Joined: 12/13/2001
From: Snohomish, WA, USA
Status: offline 		There are two things that happen when you lengthen the nose, more frontal area forward of the CG and more mass moment arm.

Frontal area: When you install EDO floats on an airplane like a Cessna, you have to add vertical fin area at the back end to compensate. This is done with a large ventral fin in the case of the 206, or small finlets out on the horizontal stab in the case of the 172. The idea is that the floats present more side area forward of the CG, which has an adverse effect on the yaw stability provided by the vertical fin, so area must be added at the aft end. Model sailplanes like the Mantis get the nose weight way forward with a long pencil-nose that presents little projected area, and hence minimal adverse effect on stability.

Mass moment arm: I once hung an OS Max 10 engine on the front of my tired old Goldberg Junior Skylark, and had to add mucho lead in the tail to compensate for the heavy engine. The weight was now distributed way forward and aft of the CG, and combined with a too-small vertical fin/rudder, I ended up with an airplane that would flat spin hopelessly all the way into the tall grass. Nothing I tried would get it out of the flat spin once it got into one. The weight forward and aft acted like the weights on the ends of a dumbbell, and once the angular momentum was imparted, the poor little rudder couldn't muster the forces needed to counter it. I went back to my trusty Baby Bee 049, took out the lead in the tail, and no more flat spins.

Ollie is right, remove all the weight you can from the tail before adding weight to the nose. You can save all-up-weight by lengthening the nose and reducing nose ballast, but you end up with the same mass moment arm as with a shorter nose and more nose weight. Conservation of angular momentum at work here. The ideal model has all the weight in the middle, near the Center of Mass, and very little weight at the extremes.

By the way, I think the new Great Planes Spirit Elite has a very loooong probiscus, no doubt for the very reasons we are discussing here. The old Airtronics Legend was a good example of a model with a too-short nose, that needed gobs of nose lead no matter how well you built it.

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Question- Nose weight in sailplanes - 4/2/2002 9:55:53 PM   
seafury_fb11



Posts: 236
Joined: 2/22/2002
From: Los Angeles
Status: offline 		[QUOTE]Originally posted by dgliderguy
Ollie is right, remove all the weight you can from the tail before adding weight to the nose. You can save all-up-weight by lengthening the nose and reducing nose ballast, but you end up with the same mass moment arm as with a shorter nose and more nose weight.[/QUOTE]

dgliderguy,
I don't dispute that Ollie is right in that respect. Your second statement is a point that I did want to bring up.

If you can save half-a-pound of lead in the nose by lengthening it a little, that might make the difference in overall weight (for example), between being able to fly a sloper on a ridge in a 10 MPH wind as opposed to a 15 MPH wind. Granted, it may take a little more elevator throw to maneuver, but the key word is a little more. The additional drag caused by that extra throw is so small it is almost insignificant, *unless* as I mentioned, you take it to an extreme.

I have seen planes where this does actually come into play, namely, the F-20's that seem to be so popular down at our flying site. Guys love these planes because they look like streamlined little darts. But they have a very short tail coupling and a very long nose. If you look closely at the configuration you will see that the distance between the CG of the wing and the aerodynamic center of the horizontal is only about 75% of the distance from the CG of the wing to the center of mass of that big chunk of lead they have in the nose. As Ollie has stated, because the polar moment of inertia is squared, the plane does not rotate around the CG. Instead the point of rotation moves to a point somewhere out in front of the wing. Thus, when applied in large amounts, the elevator will push the entire wing down causing it to plow through the air. This is why a warbird is actually a much better configuration for a slope glider than a jet.

But I haven't seen any sailplanes that are anywhere near having this kind of problem. A typical sailplane has a relatively short nose moment by comparison to the tail moment. I believe that somewhere in the happy medium, you can benefit by lengthening the nose *a little* in order to save overall weight. The extra control throw required is not going to hurt you that much.

Russ.


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Mantis vs. Allegro Lite - 4/2/2002 10:17:08 PM   
Ollie


 

Posts: 958
Joined: 12/8/2001
From: Punta Gorda, FL
Status: offline 		The Mantis seems to be designed to minimize wing loading and sinking speed without resorting to custom tail boom and innovative weight reduction of pushrods, tail structure, ETC. The result is the need for large control surfaces to handle maneuvering forces.

The Allegro Lite is a far more sophisticated and thoroughly engineered design where the contribution of each sub unit to the polar moment of inertia has been calculated and taken into account. In designing the Allegro Lite, Dr. Drela took an uncompromizing approach to saving weight. The result is a very maneuverable design that also has the highest strength to weight ratio that I know of. In addition, it achieves that result with small tail surfaces and minimum parasitic drag. It has a much lower sinking speed than the Mantis in spite of the Mantis' size advantage. It beats the Mantis at its own game.

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Regards, Ollie

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Question- Nose weight in sailplanes - 4/3/2002 12:36:33 AM   
seafury_fb11



Posts: 236
Joined: 2/22/2002
From: Los Angeles
Status: offline 		One thing I guess I should clarify - the planes I fly are all slope gliders that weigh between 100 and 130 oz. These planes are heavy and carry their momentum quite well. When doing stall turns and making 90 MPH passes, a little additional drag due to some extra elevator throw is virtually unnoticable. However, for a thermal plane weighing maybe 20 oz, and flying at very slow speeds, the same amount of extra drag on the elevator is likely more significant.

So I guess what it boils down to is you have to consider your application and then decide if you can get away with a longer nose. Then if you still don't know for sure, just try it and see what happens. That's how we learn right?

Russ.


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       Post #: 13

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