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.

Ok, I am going to be the first to put my hand up and say I dont get this. I have only just started to study aircraft performance though.

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.

Just the opposite.
If CG moves aft (within allowed limits) there is less need for elevator 'input' (or force).

If it takes you "over" it doesn't mean you will fall out of sky or be unable to take off. Your performance will be gradually degraded the more you go "over" on the other side but you may still be able to fly just fine. Professional ferry pilots routinely get permission to take off with aircraft weights significantly over the MTOW. I am not advocating operating outside of published book values just explaining basic realities and physics of flying.

It was very clear in my training textbooks, I am surprised not in yours.

Ok, so clearly there is a gap here in my knowledge, and I guess I need to sort it out right about now!

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!

So far OK.

Yes, but precisely what KIND of input?
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.

Ok, so that makes sense now.

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.

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. I have not studied in 20 years, so not am I only learning about flying, I am also learning to study again!

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.

It is mentioned (indirectly) in places but you are right - it is not stressed. It is not stressed because it should be intuitive - if you could carry more by using the same amount of energy you would be violating basic laws of physics, last I checked all other forms of transportation require more energy to carry more cargo, it would be naive to think we could pull some sort of miracle in aviation. Wouldn't it be nice if 747 used less fuel than Cessna 172? :}

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.

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.

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.

Hah, your post made me go off and do a little more research in aircraft stability, which in turn led me to another pprune post that I learned 2 new things from! Maybe I should post that in the "can we learn anything new here" thread! See, us noobs take a lot from this place, even though we may not be able to contribute...

Anyway, I shall go away and study now, I have hijacked this thread enough. Thanks folks for the pointers.

That's very interesting. We have two 28's at our club, both essentially the same although one is heavier than the other due to a lot more avionic stuff being on board. I always assumed that the heavier one flew slower because it was heavier (bit of a no brainer!) but the CG on the heavier one is way forward of the lighter one. The lighter one is a good 15kts faster for the same power setting, I wonder if it's because of the reason you mention. I'm intrigued now and shall get fat blokes to sit in the back next time.

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.

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.

Perhaps but we were clearly talking about the vector sum of all these forces.

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. :eek:

MJ

Yes, it is an excellent source, I was once specially impressed with its explanation of lift using the theory of circulation.

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.

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.

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' :)

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.

Spoken like a true 21st century student.

Long time ago, people studied stuff because it interested them.


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

Not sure what you are suggesting there Lone Ranger, but I am not a 16 year old student just trying to pass an exam. I am doing this stuff as recreation well past the age of when I could make a career out of it. So if you are suggesting that I am not actually interested in what I am studying, I can confirm that you are wrong.

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.

I said "at cruise speed" for a reason. And I was thinking more C172 than B737.
"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 :)

No, it isn't flat in C172 or Piper Archer II (I fly both) and I immediately feel a lot more drag when hauling 3 extra people comparing to one.

Adding three extra people in a PA28 / C172 while keeping a "static CG" is beyond my skills.

I have flown many times with 3 passengers, plus camping equipment say in Archer II, impossible with 200 -lb people but quite doable with slender passengers (specially some ladies), both CG and total load was within limits, there are still slim people in this world, including the US. :}

The graph you showed isn't flat.
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

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.

And I would contend you engage in nitpicking and looking for something that is not part of 'typical' piloting experience - to fly at the same speed with heavier load does in fact requires more power.

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. :ugh:

I am looking at the graph for C172, it isn't flat, it in fact behaves quite linearly with angle of attack. :ugh:

I meant "up to - your maximum . . . "

olesak

Show us then . . . .

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.:rolleyes:

Porterhouse
Exactly.