Thorp Air Command - T18.net

Supporting Owners, Builders and Pilots of the Thorp T-18 and its variants.
It is currently Thu Dec 14, 2017 8:19 am

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 49 posts ]  Go to page Previous  1, 2, 3, 4  Next
Author Message
flyingfool
PostPosted: Wed Oct 26, 2016 9:28 am 
Sr. Member
Sr. Member

Joined: Wed Aug 06, 2014 11:01 am
Posts: 183
Images: 7
My understanding is that fuel in the wings actually INCREASES the allowable G loading.

I'd like to understand how this is the case.


Top
 Profile Personal album  
 
Binder
PostPosted: Wed Oct 26, 2016 9:34 am 
Sr. Member
Sr. Member

Joined: Thu Sep 01, 2016 10:50 pm
Posts: 174
flyingfool wrote:
My understanding is that fuel in the wings actually INCREASES the allowable G loading.

I'd like to understand how this is the case.



Not to muck up this thread with speculation as I only have a minor in physics and nothing in aero engineering.

If the G load max is based on the loading of the wing spar to fuselage connection then putting weight inside the wings wouldn't effect this and actually remove the 29gal of fuel on the fuselage.

I think the post about gross weight being higher with fuel in the wings is actually meant to say that G loading can be higher. The weight is shifted to the wings thus not putting stress on the wing spar connection to the fuselage when pulling higher loads. The fuselage is basically dead weight off a moment arm from the lifting point (wings). The closer the weight to the area of lift the more force it can take before damage. I think this is based on the fact that weight isn't just added to the wings instead moved from fuselage to wings.


Top
 Profile  
 
fytrplt
PostPosted: Wed Oct 26, 2016 2:01 pm 
Sr. Member
Sr. Member
User avatar

Joined: Tue Oct 20, 2009 8:07 pm
Posts: 765
Images: 2
Location: Lakeland, Florida, USA
The wing spar doesn't need to carry the fuel load while in flight, however, taxiing is a different matter.

Fuel burn CG transfer in flight is not enough of a problem to reconfigure the plane. A couple of trim clicks about every fifteen minutes is plenty. Having a split fuel load like the RV's and the Cherokee with the attendant tank switching is a far more labor intensive cockpit mind load. My wing tanks feed into the main, so I don't have to worry about running a tank dry. All I have to monitor is whether the wing tanks are feeding and watching the fuel remaining in the main tank. I have an automatic shutoff for the wing transfer system, so I can't overfill the main.

The Cherokee fuel weight limit is probably more of a performance issue than a gross weight issue.

_________________
Bob Highley
N711SH
SN 835
KLAL


Top
 Profile Personal album  
 
James Grahn
PostPosted: Wed Oct 26, 2016 9:30 pm 
Sr. Member
Sr. Member

Joined: Sat Oct 24, 2009 7:16 pm
Posts: 920
Images: 0
Location: USA
In the 737, we have to burn center tank fuel before the wings. My understanding is that the spar load is increased with weight in the fuselage. However, any weight in your plane is part of gross weight. You and I do not know the exact structural limiting member at any given time. That's not to say you can't set your gross weight wherever you want. It's experimental after all. Bob is right in that JT did the math for Don Taylor. I have the original letter. He gave DT +3/-1 at 2200lbs. That was in a purpose built T18. Please respect the normal g load limits wrt gross weight.
Cubes


Top
 Profile Personal album  
 
Binder
PostPosted: Thu Oct 27, 2016 7:44 am 
Sr. Member
Sr. Member

Joined: Thu Sep 01, 2016 10:50 pm
Posts: 174
fytrplt wrote:
The wing spar doesn't need to carry the fuel load while in flight, however, taxiing is a different matter.

Fuel burn CG transfer in flight is not enough of a problem to reconfigure the plane. A couple of trim clicks about every fifteen minutes is plenty.


On my thorp even without baggage my cg starts to become rearward of CG range just under half tank. With any baggage it happens faster. So in my specific thorp with a 0290 the fuel burn significantly changes my CG into an unsafe range.


Top
 Profile  
 
flyingfool
PostPosted: Thu Oct 27, 2016 10:06 am 
Sr. Member
Sr. Member

Joined: Wed Aug 06, 2014 11:01 am
Posts: 183
Images: 7
^^^^

Something just doesn't seem "normal" or "right" with what you are explaining compared to the vast general experience of Thorps. Being outside of aft CG with half fuel remaining I do not believe is common at all.

I wonder if the math is wrong or maybe the datum used is not the normal place.

Something seems out of kilter.

Rear CG is nothing to fool around with as you are well aware and you are being careful which is smart. Somehow I think you'll get to the bottom of this. I suspect it is datum location and/or math.

Putting your plane on the scales and starting from scratch I think is the best solution. I would have an A&P, IA or some other experienced 3rd party look at the Thorp plans and do the CG calcs. My experience as an engineer is that I can work on a complicated math problem and know the result is suspicious, and being too close to the problem no matter how many times I go over and over the calcs, I cannot see the problem. While another co-worker engineer who is not familiar with the problem can find my error in a few minutes. A fresh set of eyes can do wonders! Sometimes we can get too close and we convince ourselves we did everything right. The can't see the forest for the trees scenario.


Top
 Profile Personal album  
 
Binder
PostPosted: Thu Oct 27, 2016 10:46 am 
Sr. Member
Sr. Member

Joined: Thu Sep 01, 2016 10:50 pm
Posts: 174
flyingfool wrote:
^^^^

Something just doesn't seem "normal" or "right" with what you are explaining compared to the vast general experience of Thorps. Being outside of aft CG with half fuel remaining I do not believe is common at all.

I wonder if the math is wrong or maybe the datum used is not the normal place.

Something seems out of kilter.

Rear CG is nothing to fool around with as you are well aware and you are being careful which is smart. Somehow I think you'll get to the bottom of this. I suspect it is datum location and/or math.

Putting your plane on the scales and starting from scratch I think is the best solution. I would have an A&P, IA or some other experienced 3rd party look at the Thorp plans and do the CG calcs. My experience as an engineer is that I can work on a complicated math problem and know the result is suspicious, and being too close to the problem no matter how many times I go over and over the calcs, I cannot see the problem. While another co-worker engineer who is not familiar with the problem can find my error in a few minutes. A fresh set of eyes can do wonders! Sometimes we can get too close and we convince ourselves we did everything right. The can't see the forest for the trees scenario.



I agree. I'm actually scheduled with the local EAA chapter to get it on the scales once I get comfortable flying it. Currently for what I am doing it's not of too much concern since I keep it topped off. I assumed it was most likely a math issue in the calcs since they look old as can be. It hasn't been checked since 1999.

I'm also wanting to move the battery from the baggage to the firewall to offset this as well. I figured I would weigh it and check the numbers then move the battery and re-check the numbers.


Top
 Profile  
 
Jeff J
PostPosted: Wed Jul 05, 2017 6:52 am 
Sr. Member
Sr. Member

Joined: Wed Aug 31, 2016 8:18 am
Posts: 276
Images: 0
Location: eastern OK
Jeff J wrote:
I like the battery position in the picture above but I have an oil filter occupying that space already. I found an extra ELT behind the seat so there is ~4 pounds that can disappear. Looking at the aircraft file, this bird appears to have always been heavier in back. The very first tail weight was 57 pounds. Current sheet shows 67 pounds. I will have to figure out the arm for the ELT and see what that does to the balance. It won't be a big change. I am thinking barely noticeable on paper.


FINALLY!!! I was able to weigh the aircraft yesterday. TW was 51.8 pounds. I know I haven't removed that much weight. The only thing I can think of to explain the discrepancy is the airplane was weighed then the weight was added to the engine without any kind of documentation. Total weight is 940.3. I haven't worked up a new sheet yet because I don't have the rest of the info needed but the hard part is over.

_________________
"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson


Top
 Profile Personal album  
 
Jeff J
PostPosted: Mon Jul 10, 2017 2:33 pm 
Sr. Member
Sr. Member

Joined: Wed Aug 31, 2016 8:18 am
Posts: 276
Images: 0
Location: eastern OK
I couldn't print the W&B sheet I had posted at the shop because they have an older version of Excel than what I run at home. I deleted the original post and am re-posting the sheet in the Excel 97-2003 format. As I stated in the original post, I left the numbers off of the drawing so people could write in their own. Please let me know if there are any errors.


Attachments:
T-18 Weight and Balance.xls [132 KiB]
Downloaded 42 times

_________________
"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson
Top
 Profile Personal album  
 
Jeff J
PostPosted: Mon Jul 10, 2017 5:20 pm 
Sr. Member
Sr. Member

Joined: Wed Aug 31, 2016 8:18 am
Posts: 276
Images: 0
Location: eastern OK
Well I already caught one error. The weight of 8 quarts of oil is not 4 pounds. In fact that probably counts as 2 errors since I was computing most rearward CG and had no intention of using 8 quarts in that slot. There should also be a note of the aircraft gross weight somewhere (there use to be a load chart that had it but I deleted the chart because it was for another aircraft).

_________________
"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson


Top
 Profile Personal album  
 
Fraser MacPhee
PostPosted: Wed Jul 12, 2017 3:55 pm 
Sr. Member
Sr. Member

Joined: Wed Oct 21, 2009 8:57 am
Posts: 629
Images: 19
Location: USA
I'm a bit short on time now - will look at your spreadsheet later, but you have only about 6 inches of CG range - You should have more range than that - 8 to 10 inches if I remember - Rich?.....John??......crickets.....

_________________
Fraser MacPhee
N926WM
Serial #279-1
Draper, UT


Top
 Profile Personal album  
 
Bill Williams
PostPosted: Wed Jul 12, 2017 4:17 pm 
Sr. Member
Sr. Member

Joined: Thu Oct 22, 2009 6:26 am
Posts: 737
Images: 0
Location: USA
Some where on this site is several W&B spread sheets


Top
 Profile Personal album  
 
James Grahn
PostPosted: Wed Jul 12, 2017 5:49 pm 
Sr. Member
Sr. Member

Joined: Sat Oct 24, 2009 7:16 pm
Posts: 920
Images: 0
Location: USA
The CG range is 15 to 31 inches, or 16 to 32 inches depending on what paperwork you believe.
Cubes


Top
 Profile Personal album  
 
bfinney
PostPosted: Wed Jul 12, 2017 8:51 pm 
Sr. Member
Sr. Member
User avatar

Joined: Thu Oct 22, 2009 2:24 pm
Posts: 394
Images: 6
Location: Auburn, WA USA
The data I have says 15% to 34% of wing chord (50") which equates to 7.5" to 17" from wing leading edge.

_________________
Bruce Finney
N18JF T-18C #262
Auburn, WA USA


Top
 Profile Personal album  
 
James Grahn
PostPosted: Wed Jul 12, 2017 11:50 pm 
Sr. Member
Sr. Member

Joined: Sat Oct 24, 2009 7:16 pm
Posts: 920
Images: 0
Location: USA
I should have said % not inches.
Cubes


Top
 Profile Personal album  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 49 posts ]  Go to page Previous  1, 2, 3, 4  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to: