Increase in Weight demands an increase in Power
To maintain cruise speed an increase in all up weight demands an increase in power.
So, if you cruise along at 2350 rpm at your usual indicated airspeed for the cruise then add fat and heavy friends plus several pounds of baggage to your max all up weight then your 2350 which you may normally cruise around at will be insufficient to give you the same attitude or anywhere near the same airspeed. Although this seems obvious it is rarely stressed in most training documents. An increase in weight demands an increase in power. |
Higher weight means more angle of attack means more drag means more power required. What is so difficult about it?
Moving weight aft (while remaining in CG limits) will also help since the tail will have less downforce which means less weight to be carried by the wing. Speaking of conventional tail here, not canards. |
I prefer to think of it as:
More weight = more lift. More lift = more drag. More drag = more thrust. |
"add fat and heavy friends plus several pounds of baggage to your max all up weight"
Doesn't adding to your maximum all up weight take you over the maximum all weight? More to worry about than power. |
Originally Posted by dirkdj
Moving weight aft (while remaining in CG limits) will also help since the tail will have less downforce which means less weight to be carried by the wing. Speaking of conventional tail here, not canards.
I thought moving weight aft, moved CoG aft and therfore you need increased elevator input to keep the aircraft level. Just found this: Effect of Load Distribution but I need to go digest it a little longer. |
moved CoG aft and therfore you need increased elevator input to keep the aircraft level. If CG moves aft (within allowed limits) there is less need for elevator 'input' (or force). |
Doesn't adding to your maximum all up weight take you over the maximum all weight? Although this seems obvious it is rarely stressed in most training documents. |
Originally Posted by porterhouse
Just the opposite.
If CG moves aft (within allowed limits) there is less need for elevator 'input' (or force). This is how my brain is seeing it. I though rearward CoG means plane pitches up, requiring elevator input to pitch it down. What am I missing? Is the CoG always forward of the CoP when in the envelope? Here I was thinking all confident, hoping to be able to do my BAK exam in 3-4 weeks time! |
I though rearward CoG means plane pitches up requiring elevator input to pitch it down. If the plane pitches up it means its nose goes up but its tail goes down. If the tail goes down what do we do to make the tail go back up? We lower the downward force on the elevator - which means less 'input. The force on the elevator is always down. Yes, center of gravity is always forward of the center of lift in conventional (non-canard) airplane. |
Originally Posted by olasek
Yes, center of gravity is always forward of the center of lift in conventional (non-canard) airplane.
|
This is all on the first few pages of any aviation PPL textbook, really very elementary part of aeronautical knowledge.
|
Andy,
Competition glider pilots will often use ballast to put the CG at the aft limit in order to reduce trim drag and thereby improve performance. |
This is all on the first few pages of any aviation PPL textbook, really very elementary part of aeronautical knowledge. |
OK, good luck with your study.
Your previous post confused me when you said your were 'all confident' hence my thinking you already spent a great deal of time studying the material. |
Although this seems obvious it is rarely stressed in most training documents. |
Originally Posted by olasek
OK, good luck with your study.
Your previous post confused me when you said your were 'all confident' hence my thinking you already spent a great deal of time studying the material. I am confident I can do it in 3-4 weeks!! Don't worry, still think I can. I am going to do a practice BAK test tomorrow, that should isolate those area where I need to put in more effort. I did read my text book cover to cover a couple of months back so have the basics covered, but I have only really started to knuckle down into the study this week. |
Originally Posted by Andy_P
(Post 8314063)
Yup, I am still in the elementary stage, hence the reason I put my hand up and asked the question. FWIW, I just looked up my text book and can confirm that is says most conventional aircraft have the CoG foward of the CoP. ...
Pitch excursion (due turbulence, say) down, speed increases. Downward "lift" on tailplane/elevator increases. Mainplane lift increases. Plane pitches up. Some types, like the PA28, are very stable longitudinally. You can see it easily by moving the tailplane through its full range on a preflight. So in stable trimmed flight there's quite a lot of downward force produced by the tail. Friend and I flew a 181, 2 up, on a quick 30 mile trip. When I climbed into the back seat, and he re-trimmed to compensate, it was good for another 10kts IAS. |
And the primary reason for that is longitudinal stability. Pitch excursion (due turbulence, say) down, speed increases. Downward "lift" on tailplane/elevator increases. Mainplane lift increases. Plane pitches up. Anyway, I shall go away and study now, I have hijacked this thread enough. Thanks folks for the pointers. |
Friend and I flew a 181, 2 up, on a quick 30 mile trip. When I climbed into the back seat, and he re-trimmed to compensate, it was good for another 10kts IAS. |
See How It Flies
This is an excellent on-line free book if you want to dig a bit deeper into Stability, Balance, etc. Will give you a much better understanding without excessive math. |
All times are GMT. The time now is 18:54. |
Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.