How bad is a little be of tailheaviness?

astroboy · May 21, 2003, 02:35 AM

I finally finished the Beech 18 I've been working on since last summer, only to find that I made a serious error in placing the main pack. With it pushed all the way forward, and with the receiver pack and ESC shifted from their original locations as far forward as they will go, the model still balances about 1/4" aft of where the plan says it should. The structure that holds the front end of the main pack is inaccessible. Being a twin, the nose is also not that accessible if I had to add weight up front, which I would be extremely reluctant to do in any case. The wingspan is 49.5", the length 35."

I guess my question is how likely is it that that degree of tailheaviness will be a problem? I don't want to trash 10 months of work on the first flight, but I really don't want to tear the model apart to get to the battery holder, either. Help, Ollie, help!

Thanks,
Jeff

Ollie · May 21, 2003, 05:04 AM

Please try to get over the idea that there is a correct CG that depends only on the plane's design. Every plane has a range of acceptable CG locations. The width of that range may be on the order of 5 or 10 percent of the average wing chord. Where the CG should be located in that range depends on the purpose of the model, the flying style of the pilot and the pilot's ability. Usually the CG shown on the plans is a safe place to start flight testing to tune the CG to the model's purpose and pilot as well as to compensate for any minor variations in the building of the model.

The proceedure is to start testing with a safe, forward CG and a means of adding and removing small increments of nose weight between test flights. The following assumes a constant throttle setting. (Thrust offset adjustments should be made to correct any undesirable response to throttle changes) As nose weight is added, the plane becomes more pitch stable, the pitch and yaw response is less and the decalage or up-elevator trim needs to be increaseed a bit. As nose weight is removed, control response increases, and decalage or up-elevator trim needs to be reduced a little. When the CG is at the aft limit of the acceptable CG range, the plane will go where it is pointed after the controls are neutralized, without any tendency to return to smooth, level flight. When the CG is near the forward limit of its acceptable range the control response will be sluggish and the plane will correct strongly in pitch and yaw when the controls are neutralized. A trainer model will have a CG which tends to be biased to the forward end of the acceptable range. An aerobatic model will need the CG to be at or, very near the aft end of the range. Pilots with skill, experience, good eyesight and quick reflexes tend to prefer CG's more aft but still in the acceptable range.

MikeSt · May 21, 2003, 12:35 PM

Quote:

Originally posted by Ollie

When the CG is at the aft limit of the acceptable CG range, the plane will go where it is pointed after the controls are neutralized, without any tendency to return to smooth, level flight.

That is "neutrally stable" right ? After a gust the plane would then adopt the new attitude .. correct ?

What about if c.g. is past the aft limit?

If I understand correctly, you are then in an *unstable* configuration. The plane would adopt a new attiude, but then keep on increasing the tendency to climb / dive .. is that right?

Mike

PeterH · May 21, 2003, 11:37 PM

I think you've built a model with little tolerance for mistakes. Heavily tapered wing, heavily loaded airframe. Being a twin, the nosecone is available for ballast-hauling. I strongly encourage it. If this is Tom Hunt's Beech built with his vacuumformed parts, then the nosecone is accessible by either removing it , ballasting, and reinstalling, or drilling a hole in the bottom of the nosecone and adding lead birdshot and epoxy, covering the hole with a circle of covering material.

A noseheavy airplane doesn't fly well, a tailheavy airplane doesn't fly for long.

astroboy · May 22, 2003, 02:35 AM

Thanks, Ollie. The critical bit of information in your post that spurred me to action was the "5 or 10 percent of the average wing chord." Assuming the lower value, even if the CG location on the plan was near the front end of that range, then my model was at best near or at the back end. I studied the model a little more closely, and decided that perhaps I could do something to move the pack farther forward--which I'm in the process of doing now--without tearing the fuse apart.

Peter, you're right. It is Tom Hunt's plan, but even though it's going to be significantly lighter than his target weights(I'll probably post pictures of it this weekend in Scale Modeling), I'm still loathe to add any dead weight to it. I'll adjust the CG, if necessary, by moving the pack forward or aft.

Jeff

Terry S · May 22, 2003, 07:15 AM

If you start flying with ballast in the nose and remove a bit each flight, you may find you dont need to move the battery !

Terry
UK

astroboy · May 22, 2003, 04:36 PM

Terry:

I considered that, but even if it turned out that I could remove all the nose weight, those critical first flights would be made with extra weight in the model.

Jeff

Robbie d · May 23, 2003, 09:11 AM

is that the big radial engined twin????

astroboy · May 28, 2003, 04:50 PM

Robbie:

It does have radials, but I'm not quite sure what you mean by big. The full-scale has a wingspan of about 47.5'; mine looks like this:

http://www.rcgroups.com/forums/showt...hreadid=120370

Jeff

shaneyee · May 29, 2003, 03:03 AM

Could you reconfigure your battery pack? I moved the CG on a wing I had by folding the aft half of the battery pack over the forward half. Of course, I had to resolder a few joints to do it.

Shane

astroboy · May 29, 2003, 05:07 PM

Shane:

I don't think I'll have to, now. I managed to get the pack moved forward far enough that the model balances at the spot indicated on the plan. I won't tell just how the pack is currently secured, though, as it would best described as "wacky." Hopefully the new ESC will come by Saturday(see the above-mentioned thread for the reason why I need a new one), so I can do some taxiing tests before taking it out to the field when I'm on vacation week after next.

Jeff

May 21, 2003, 02:35 AM	#1
astroboy Superhero, Third Class Join Date: Nov 2000 Location: Carson City, NV Posts: 689	How bad is a little bit of tailheaviness? I finally finished the Beech 18 I've been working on since last summer, only to find that I made a serious error in placing the main pack. With it pushed all the way forward, and with the receiver pack and ESC shifted from their original locations as far forward as they will go, the model still balances about 1/4" aft of where the plan says it should. The structure that holds the front end of the main pack is inaccessible. Being a twin, the nose is also not that accessible if I had to add weight up front, which I would be extremely reluctant to do in any case. The wingspan is 49.5", the length 35." I guess my question is how likely is it that that degree of tailheaviness will be a problem? I don't want to trash 10 months of work on the first flight, but I really don't want to tear the model apart to get to the battery holder, either. Help, Ollie, help! Thanks, Jeff Last edited by astroboy; May 21, 2003 at 01:59 PM.

May 21, 2003, 05:04 AM	#2
Ollie Registered User Join Date: Apr 2002 Location: Punta Gorda, FL Posts: 4,949	Please try to get over the idea that there is a correct CG that depends only on the plane's design. Every plane has a range of acceptable CG locations. The width of that range may be on the order of 5 or 10 percent of the average wing chord. Where the CG should be located in that range depends on the purpose of the model, the flying style of the pilot and the pilot's ability. Usually the CG shown on the plans is a safe place to start flight testing to tune the CG to the model's purpose and pilot as well as to compensate for any minor variations in the building of the model. The proceedure is to start testing with a safe, forward CG and a means of adding and removing small increments of nose weight between test flights. The following assumes a constant throttle setting. (Thrust offset adjustments should be made to correct any undesirable response to throttle changes) As nose weight is added, the plane becomes more pitch stable, the pitch and yaw response is less and the decalage or up-elevator trim needs to be increaseed a bit. As nose weight is removed, control response increases, and decalage or up-elevator trim needs to be reduced a little. When the CG is at the aft limit of the acceptable CG range, the plane will go where it is pointed after the controls are neutralized, without any tendency to return to smooth, level flight. When the CG is near the forward limit of its acceptable range the control response will be sluggish and the plane will correct strongly in pitch and yaw when the controls are neutralized. A trainer model will have a CG which tends to be biased to the forward end of the acceptable range. An aerobatic model will need the CG to be at or, very near the aft end of the range. Pilots with skill, experience, good eyesight and quick reflexes tend to prefer CG's more aft but still in the acceptable range. Last edited by Ollie; May 21, 2003 at 05:07 AM.

May 21, 2003, 11:37 PM	#4
PeterH Registered User Join Date: Jan 2001 Location: Granby, CT, USA Posts: 559	TAILHEAVY BEECH 18 I think you've built a model with little tolerance for mistakes. Heavily tapered wing, heavily loaded airframe. Being a twin, the nosecone is available for ballast-hauling. I strongly encourage it. If this is Tom Hunt's Beech built with his vacuumformed parts, then the nosecone is accessible by either removing it , ballasting, and reinstalling, or drilling a hole in the bottom of the nosecone and adding lead birdshot and epoxy, covering the hole with a circle of covering material. A noseheavy airplane doesn't fly well, a tailheavy airplane doesn't fly for long.

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May 22, 2003, 02:35 AM	#5
astroboy Superhero, Third Class Join Date: Nov 2000 Location: Carson City, NV Posts: 689	Thanks, Ollie. The critical bit of information in your post that spurred me to action was the "5 or 10 percent of the average wing chord." Assuming the lower value, even if the CG location on the plan was near the front end of that range, then my model was at best near or at the back end. I studied the model a little more closely, and decided that perhaps I could do something to move the pack farther forward--which I'm in the process of doing now--without tearing the fuse apart. Peter, you're right. It is Tom Hunt's plan, but even though it's going to be significantly lighter than his target weights(I'll probably post pictures of it this weekend in Scale Modeling), I'm still loathe to add any dead weight to it. I'll adjust the CG, if necessary, by moving the pack forward or aft. Jeff

May 22, 2003, 07:15 AM	#6
Terry S Registered User Join Date: May 2003 Location: Bridgwater, UK Posts: 568	If you start flying with ballast in the nose and remove a bit each flight, you may find you dont need to move the battery ! Terry UK

May 22, 2003, 04:36 PM	#7
astroboy Superhero, Third Class Join Date: Nov 2000 Location: Carson City, NV Posts: 689	Terry: I considered that, but even if it turned out that I could remove all the nose weight, those critical first flights would be made with extra weight in the model. Jeff

May 23, 2003, 09:11 AM	#8
Robbie d Crazy Cessna Owner Join Date: May 2001 Location: Adelaide, South Australia Posts: 1,350	is that the big radial engined twin????

May 28, 2003, 04:50 PM	#9
astroboy Superhero, Third Class Join Date: Nov 2000 Location: Carson City, NV Posts: 689	Robbie: It does have radials, but I'm not quite sure what you mean by big. The full-scale has a wingspan of about 47.5'; mine looks like this: http://www.rcgroups.com/forums/showt...hreadid=120370 Jeff

May 29, 2003, 03:03 AM	#10
shaneyee Registered User Join Date: Aug 2002 Location: Singapore Posts: 747	Could you reconfigure your battery pack? I moved the CG on a wing I had by folding the aft half of the battery pack over the forward half. Of course, I had to resolder a few joints to do it. Shane

May 29, 2003, 05:07 PM	#11
astroboy Superhero, Third Class Join Date: Nov 2000 Location: Carson City, NV Posts: 689	Shane: I don't think I'll have to, now. I managed to get the pack moved forward far enough that the model balances at the spot indicated on the plan. I won't tell just how the pack is currently secured, though, as it would best described as "wacky." Hopefully the new ESC will come by Saturday(see the above-mentioned thread for the reason why I need a new one), so I can do some taxiing tests before taking it out to the field when I'm on vacation week after next. Jeff