Designing from scratch: nose moment

When reading people's opinions on the subject, I've come across several different statements, such as "nose moment should only be long enough to balance the tail weight" and "nose should only be long enough to squeeze the tank in". I have, however, also heard several people refer to some magic "minimum length", below which the flying characteristics of the airplane will change.
Most of my planes, if built per plans, come out way, way nose heavy, requiring tail weight - a practice I just can't live with. I have no problem squeezing the tank in to shorten the nose, but that magic "minimum length" is something I have to figure out first.

All ideas are welcome.

check out this article:

http://www.clstunt.com/noselong.htm

Ron,
I've read that article, and generally agree with Len's statements...
The only thing that "bothers" me is the change in the prop's giroscopic effect depending on the nose length during a sharp turn (corner). As in here: http://www.clstunt.com/noselong.htm#prop

LAST EDITED ON Aug-03-04 AT 08:47 PM (CDT)
 
The moment arm is reduced with a shorter nose, Right? This should be a good thing, Yes?

Also, see the article on the "Bar Bell" effect. I think there was a thread recently that talked about heavy engine vs. light engines and the product of inertia. my quick search didn't turn it up...

Mike, the nose length does not cahnge gyro stabilizing effect. The longer nose moves CG front wnat makes another, but still stabilizing effect. So therefore that relation bigger prop->shorter nose.

>check out this article:
>
>http://www.clstunt.com/noselong.htm

This said what I couldn't.

Heh heh heh..

Glancing over the topics in the Forum, "scratch:nose" stood out..

I have noticed that most scratch built airplanes are nose heavy. I think they are designed to be nose heavy on purpose because its much easier to correct the balance by adding tail weight. I have learned to build my fuselage tail section about 2" longer than called for on the plans. I build and finish the entire airplane and leave off the stabilizer, fin, and elevator. I build the stabilizer, fin and elevator as an assembly then set the assembly on the tail end of the fuselage. I slide the assembly back and forth until I find the place where the assembly balances CG in the correct place. If the fuselage is 1/2" too long I cut off about 3/8" and check balance again. I may have to check balance and cut off the fuselag 2 or 3 times to get it right. Once I find the correct fuselage length and balance I glue the stab, fin, elev. assembly on.

I recently have learned to do the same thing when glueing the fuselage to the wing. I balance the wing on center with a predetermined amount of weight attached to the inboard wing tip. I slide the fuselage toward the outboard wing tip until the weight of the fuselage balances the wing. This usually puts the fuselage off center about 3/8", 1/2", 5/8" it depends on the size of the airplane and the weight of the fuselage. I remove the wing tip weight on the inboard wing and NO wing tip weight is needed on the outboard wing. The outboard wing will be heavy equal to the amount of weight that was attached to the inboard wing to make it balance.

No weights are needed on the finished airplane, tail or wing tip so the finished airplane will be lighter weight.

Gary,
thanks for sharing your methods of balancing; while those may be OK for trainer-level aircraft, I'd hesitate to do that on competitive equipment. Changing tail moments and inboard/outboard wing panel spans will result in predictably different flying characteristics, which may cancel out the advantages of saved balance weights.
What I was trying to determine (by the popular vote method) is whether CG balancing using the nose length as a variable will leave the flying qualities unchanged.
The topic must be either too boring or the answer too obvious to everyone else, as it's not not getting nearly as broad of a response as the ones on Ebay prices or controversial judging practices.

>What I was trying to determine (by the popular vote method)
>is whether CG balancing using the nose length as a variable
>will leave the flying qualities unchanged.

Yes, pretty much. Read the "Imitation" article.

BTW- making the nose longer or shorter *does not* increase or decrease the gyroscopic effect. Torque is torque.

I don't think that's what Leonard article says, either. I think the premise is that you can mitigate the negative effects of the larger prop on turn performance by running the CG back. That's hasn't been my experience at all, but that's a matter of opinion.

>I have noticed that most scratch built airplanes are nose
>heavy. I think they are designed to be nose heavy on
>purpose because its much easier to correct the balance by
>adding tail weight. I have learned to build my fuselage
>tail section about 2" longer than called for on the plans.
>I build and finish the entire airplane and leave off the
>stabilizer, fin, and elevator. I build the stabilizer, fin
>and elevator as an assembly then set the assembly on the
>tail end of the fuselage. I slide the assembly back and
>forth until I find the place where the assembly balances CG
>in the correct place. If the fuselage is 1/2" too long I
>cut off about 3/8" and check balance again. I may have to
>check balance and cut off the fuselag 2 or 3 times to get it
>right. Once I find the correct fuselage length and balance
>I glue the stab, fin, elev. assembly on.

If someone's "scratch built" plane is nose heavy it was simply a mis calculation in design. Most of the top flyers are flying planes that could be called "scratch bilt". But when one refines a design you can design it such that it will come out nearly "on the money". The only exception may be if you are using an engine package that is considerably heavier than what was used on the original design.

Your idea of changing the tail moment in the way you describe would be very bad if this were to be a serious stunter. Changing tail moments does affect the way the airplane will fly. And since the finish adds considerable weight behind the CG in comparison to that in front, I don't know how you can assess this accurately at this point anyway. On the first Still Stuka I bulit I was tempted to add lead to the tail before covering it as I felt it was going to come out nose heavy. After covering and finishing it, however, it came out just fine.

>I recently have learned to do the same thing when glueing
>the fuselage to the wing. I balance the wing on center with
>a predetermined amount of weight attached to the inboard
>wing tip. I slide the fuselage toward the outboard wing tip
>until the weight of the fuselage balances the wing. This
>usually puts the fuselage off center about 3/8", 1/2", 5/8"
>it depends on the size of the airplane and the weight of the
>fuselage. I remove the wing tip weight on the inboard wing
>and NO wing tip weight is needed on the outboard wing. The
>outboard wing will be heavy equal to the amount of weight
>that was attached to the inboard wing to make it balance.
>
>No weights are needed on the finished airplane, tail or wing
>tip so the finished airplane will be lighter weight.
>
>Gary Weaver [email protected]

Here I really have problems if you are building any type of serious stunt plane. Tip weight is needed even with assymetry. Now, if you are working with a profile and mounting the engine and tank outboard, then this becomes your tip weight. I have had profiles that did not need any additional tip weight. (You will note I said "additional"). I also had a couple where I built heavy tip weight boxes, and the box, itself, became the weight. But to balance the plane by moving the fuselage in and out?? I don't think so.

>The only exception may
>be if you are using an engine package that is considerably
>heavier than what was used on the original design.

Most of the plans by MAN, Hobby Helper, Air Trails, others show a Fox 35 engine which is very light weight and has no muffle. Plans never specify plastic of wood prop, wood props are typically half the weight of plastic. I have been using OS engines with mufflers, and McCoy 35 which are all heaver than the Fox 35 engine. You are right the engine weight is probably the problem.

>When reading people's opinions on the subject, I've come
>across several different statements, such as "nose moment
>should only be long enough to balance the tail weight" and
>"nose should only be long enough to squeeze the tank in". I
>have, however, also heard several people refer to some magic
>"minimum length", below which the flying characteristics of
>the airplane will change.
>Most of my planes, if built per plans, come out way, way
>nose heavy, requiring tail weight - a practice I just can't
>live with. I have no problem squeezing the tank in to
>shorten the nose, but that magic "minimum length" is
>something I have to figure out first.
>
>All ideas are welcome.

Since I wrote the one article that people have alluded to, I didn't answer until now. But since you want more answers, I will say it again. there is no magic "minimum length" for the nose moment. Tail moment, along with areas, etc., does make a difference, and, when comparing airplanes, you will find a range in which all successful designs will fall. Going too long or too short on the tail moment will add certain undesireables to the plane that will make it less suitable for stunt. But the nose is there for balance. A longer nose with a lighter engine and a shorter nose with a heavier engine, even though each may balance the same, will fly different, but that is still the answer you are seeking. Don't go to a shorter nose moment and purposely add nose weight (a bad idea) or a longer moment and add tail weight ( even worse) because of some preconceived "magic numbers". The "magic numbers" are there for balance. They are "magic" because they work in keeping one from adding extra dead weight for balance. But you are not going to make or break the plane by lengthening or shortening the nose for some supposed "magic" reasons.

If your planes are coming out way nose heavy, it is probably because you are using a heavier engine package. This is especially true of older designs where small, light weight engines without mufflers were originally used. Some of today's power plants are a bit heavier, and, when combined with a muffler, might offer enough incentive to shorten the nose somewhat if you are able. (Paul Walker's P-51 does not have a scale-like nose length, you will notice.) Just be aware that Old Time rules do not allow the shortening of the nose and for classic it may result in some loss of fidelity points.

To take your thought back a few more years Leonard, remember that many OTS designs were created when heavy, low powered sparkers or glows were used and you needed a much larger engine to pull the plane. Noses were short then. A modern engine in one of those sometimes still requires nose weight.
I think the longer nose came about when the lightweight Fox .35 (and other engines of that period) came into use. Perhaps some of the longer nose came from it looking cool and different for the day. Then it became the norm.
This is just speculation on my part from things I've seen and read. I didn't start flying until the mid 50's when the longer noses were already "in".
Following your thoughts, I guess that's why it may be better in many cases to try for a "period" engine for a design.

I do not know if there is some minimal nose length, but certainly too short nose, or better too small side area front of CG can lead to yaw unstability. Just rudder or side area aft of GC helps to point nose where it belongs, but too much can lead to overstearing and together with natural frequency of lines (if they come close together) can lead to oscillation. I know one model which needed to enlarge the front area by small vertical fins over and under the nose. That solved all problems with yaw stability much better than rabe rudder. (again, on that ONE model)

Mike,

I think the long and short of it is that airplanes with different length noses have different handling characteristics. Airplanes with short noses seem to start and stop turning very quickly and easily. I rather like the characteristic and build many short nose airplanes but am also attracted to the occasional long nose Mustang. The longer nose airplanes can be trimmed to fly as well but they "feel" different in corners. Either do fine. I don't think you can build a nose too short as long as you can balance it.