Increase in Weight demands an increase in Power
Join Date: Sep 2011
Location: Mare Imbrium
Posts: 638
Likes: 0
Received 0 Likes
on
0 Posts
There is a bit of confusion possible (from reading some of the above posts). Remember there are three things at the back of the aeroplane:
- the tailplane (or horizontal stabiliser)
- the elevator - a movable control surface usually attached to the rear edge of the tailplane
- the trimmer - which can mechanically work in a numbr of ways, but effectively biases the elevator
All three of these can be at different angles to the airflow and exert different forces as a result.
- the tailplane (or horizontal stabiliser)
- the elevator - a movable control surface usually attached to the rear edge of the tailplane
- the trimmer - which can mechanically work in a numbr of ways, but effectively biases the elevator
All three of these can be at different angles to the airflow and exert different forces as a result.
Join Date: Dec 2013
Location: Paso Robles
Posts: 261
Likes: 0
Received 0 Likes
on
0 Posts
All three of these can be at different angles to the airflow and exert different forces as a result.
You might also want to check the weight and balace of your PA28s. Most need around 50-100lbs in the baggage area just to be within the foreward CofG limit with both front seats occupied. 
MJ
MJ
Join Date: Apr 2009
Location: Oakland, CA
Age: 72
Posts: 427
Likes: 0
Received 0 Likes
on
0 Posts
This is an excellent on-line free book if you want to dig a bit deeper into Stability, Balance, etc
Join Date: May 2013
Location: have I forgotten or am I lost?
Age: 71
Posts: 1,126
Likes: 0
Received 0 Likes
on
0 Posts
I seriously doubt that you would notice the difference in power required for cruise with different weights.
an aeroplane has two force couples at work.
thrust and drag
weight and lift
out on an arm at the back is the horizontal stabiliser.
lift can be taken as acting through the centre of pressure.
weight acts through the centre of gravity.
lift is pretty well locked in position by the geometry of the wing and moves forward and aft with angle of attack dependent on the characteristics of the aerofoil used.
weight will be affected by the position of the variable loads in the aeroplane.
it also can move during flight as fuel is burnt off.
the limits of where all this can sit and have the aircraft remain controllable were established during initial test flying of the aircraft, the results of which are simplified for the pilot in the weights and loads charts.
in flight if the two force couples balance out fully the horizontal stabiliser will sit out back needing to do nothing. all good and well but the trim will seem a little waffly.
if we move the cg slightly forward of the centre of pressure and use some downlift from the horizontal stabiliser we get a more stable trim.
there is more to it but you can figure it out from the texts.
an aeroplane has two force couples at work.
thrust and drag
weight and lift
out on an arm at the back is the horizontal stabiliser.
lift can be taken as acting through the centre of pressure.
weight acts through the centre of gravity.
lift is pretty well locked in position by the geometry of the wing and moves forward and aft with angle of attack dependent on the characteristics of the aerofoil used.
weight will be affected by the position of the variable loads in the aeroplane.
it also can move during flight as fuel is burnt off.
the limits of where all this can sit and have the aircraft remain controllable were established during initial test flying of the aircraft, the results of which are simplified for the pilot in the weights and loads charts.
in flight if the two force couples balance out fully the horizontal stabiliser will sit out back needing to do nothing. all good and well but the trim will seem a little waffly.
if we move the cg slightly forward of the centre of pressure and use some downlift from the horizontal stabiliser we get a more stable trim.
there is more to it but you can figure it out from the texts.
Join Date: Dec 2013
Location: Paso Robles
Posts: 261
Likes: 0
Received 0 Likes
on
0 Posts
I seriously doubt that you would notice the difference in power required for cruise with different weights.
What usually happens in my case I typically fly at the same power setting (close to max allowed per POH) and observe slower IAS with heavier loads at the same altitude.
Join Date: Jan 2008
Location: London UK
Posts: 517
Likes: 0
Received 0 Likes
on
0 Posts
For S&L cruise at a given airspeed, static CG, higher weight requires a higher lift coefficient, and so a higher angle of attack.
Power has to match drag at the new drag coefficient. So how much does the drag coefficient change by?
For a 'typical' aerofoil at cruise speed, not much.
for example:
File:CL, CD NACA632618.png - Wikipedia, the free encyclopedia
from Drag Polar - Wikipedia, the free encyclopedia
Of course, some aerofoils may not be 'typical'
Power has to match drag at the new drag coefficient. So how much does the drag coefficient change by?
For a 'typical' aerofoil at cruise speed, not much.
for example:
File:CL, CD NACA632618.png - Wikipedia, the free encyclopedia
from Drag Polar - Wikipedia, the free encyclopedia
Of course, some aerofoils may not be 'typical'
Join Date: Apr 2009
Location: Oakland, CA
Age: 72
Posts: 427
Likes: 0
Received 0 Likes
on
0 Posts
For a 'typical' aerofoil at cruise speed, not much
Last edited by olasek; 14th Feb 2014 at 18:17.
Join Date: Jan 2008
Location: Tr_no 688
Posts: 235
Likes: 0
Received 0 Likes
on
0 Posts
Thanks.
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.
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.
Long time ago, people studied stuff because it interested them.
I seriously doubt that you would notice the difference in power required for cruise with different weights.
Disagree, I have seen it many times.
What usually happens in my case I typically fly at the same power setting (close to max allowed per POH) and observe slower IAS with heavier loads at the same altitude.
What usually happens in my case I typically fly at the same power setting (close to max allowed per POH) and observe slower IAS with heavier loads at the same altitude.
I can notice it at the loading extremes in a microlight, but it is very much type dependant and I believe the former statement is true most of the time
Join Date: Nov 2013
Location: Oz
Posts: 242
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Lone_Ranger
Spoken like a true 21st century student.
Long time ago, people studied stuff because it interested them.
Long time ago, people studied stuff because it interested them.
I studied engineering at uni many years back as a mature age student, because I was interested. When I say I can learn this stuff in 3-4 weeks, it is because of my engineering background. And the BAK material is not that complicated, trust me on that.
Join Date: Jan 2008
Location: London UK
Posts: 517
Likes: 0
Received 0 Likes
on
0 Posts
What is "not much"?. For a typical aerofoil drag coefficient increases quite a bit with angle of attack (assuming constant speed) at typical angles of attack flown at cruise, even your own graph shows it. By the way - heavier load hits your pocket book, someone calculated that in typical airline flying a heavy male passenger (117 kg) costs airline about 65% more in fuel than an average 73 kg passenger - again cost attributed to extra drag.
"At cruise speed" 'typically' means a CL around 0.3 to 0.5.
The drag curve is 'typically' designed to be fairly flat for reasonable weights - like the graph I showed.
Not everything is 'typical', of course
Join Date: Dec 2013
Location: Paso Robles
Posts: 261
Likes: 0
Received 0 Likes
on
0 Posts
The drag curve is 'typically' designed to be fairly flat for reasonable weights - like the graph I showed.
Join Date: Dec 2013
Location: Paso Robles
Posts: 261
Likes: 0
Received 0 Likes
on
0 Posts
Adding three extra people in a PA28 / C172 while keeping a "static CG" is beyond my skills.
Join Date: Apr 2009
Location: Oakland, CA
Age: 72
Posts: 427
Likes: 0
Received 0 Likes
on
0 Posts
- like the graph I showed
You are making a mistake by looking at the portion of the graph where angle of attack is close to zero, this is NOT the place where typical flying happens (doesn't matter if it is Boeing or small Cessna), look at angle of attack around 5 deg, these are typical values and graph is no longer flat. Makes me think your are either non pilot or you missed some basic course of aeronautical knowledge. Also there are other better graphs which show this part of the curve in greater detail, you can actually find the exact graph for say Cessna 172.
http://www.aerospaceweb.org/question...rag-cessna.gif
Join Date: Jan 2008
Location: London UK
Posts: 517
Likes: 0
Received 0 Likes
on
0 Posts
Sorry, Porterhouse, my answer was a bit flippant.
The original post said that increased weight demands an increase in power, and I would contend that this is not strictly true, in the cruise. But a change in weight will very often move the CG position which most likely will change the power requirement. As many earlier posts have pointed out.
This isn't just academic: one important weight variation which typically does not change the CG is fuel.
The original post said that increased weight demands an increase in power, and I would contend that this is not strictly true, in the cruise. But a change in weight will very often move the CG position which most likely will change the power requirement. As many earlier posts have pointed out.
This isn't just academic: one important weight variation which typically does not change the CG is fuel.
Join Date: Dec 2013
Location: Paso Robles
Posts: 261
Likes: 0
Received 0 Likes
on
0 Posts
and I would contend that this is not strictly true, in the cruise
Join Date: Jan 2008
Location: London UK
Posts: 517
Likes: 0
Received 0 Likes
on
0 Posts
Olasek, the graph is fairly flat in the relevant region. Please re-read my post.
It is not completely flat, and there will be some extra drag, but nowhere near pro-rata with the extra lift required.
Also, this discussion is starting to get abusive, so I am out of here.
It is not completely flat, and there will be some extra drag, but nowhere near pro-rata with the extra lift required.
Also, this discussion is starting to get abusive, so I am out of here.
Thread Starter
Join Date: Jan 2012
Location: Not far from the edge of the Milky Way Galaxy in the Orion Arm.
Posts: 510
Likes: 0
Received 0 Likes
on
0 Posts
Doesn't adding to your maximum all up weight take you over the maximum all weight? More to worry about than power.
olesak
This is all on the first few pages of any aviation PPL textbook, really very elementary part of aeronautical knowledge.
Show us where it is write that an increase in weight demands an increase in power - just those words, all together.
Am not inventing the wheel. It is just not mentioned, at the beginning of a PPL/Basic training book. Which is the point of this thread.
Porterhouse
Disagree, I have seen it many times.
What usually happens in my case I typically fly at the same power setting (close to max allowed per POH) and observe slower IAS with heavier loads at the same altitude
What usually happens in my case I typically fly at the same power setting (close to max allowed per POH) and observe slower IAS with heavier loads at the same altitude