Increase in Weight demands an increase in Power

[Natstrackalpha]

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.

[dirkdj]

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.

[India Four Two]

I prefer to think of it as:

More weight = more lift.
More lift = more drag.
More drag = more thrust.

[fujii]

"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.

[Andy_P]

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.

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 (http://avstop.com/ac/flighttrainghandbook/effectofloaddistribution.html) but I need to go digest it a little longer.

[porterhouse]

moved CoG aft and therfore you need increased elevator input to keep the aircraft level. Just the opposite.
If CG moves aft (within allowed limits) there is less need for elevator 'input' (or force).

[olasek]

Doesn't adding to your maximum all up weight take you over the maximum all weight? 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.

Although this seems obvious it is rarely stressed in most training documents.
It was very clear in my training textbooks, I am surprised not in yours.

[Andy_P]

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

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!

[olasek]

I though rearward CoG means plane pitches upSo far OK.

requiring elevator input to pitch it down. 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.

[Andy_P]

Yes, center of gravity is always forward of the center of lift in conventional (non-canard) airplane.

Ok, so that makes sense now.

[olasek]

This is all on the first few pages of any aviation PPL textbook, really very elementary part of aeronautical knowledge.

[India Four Two]

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.

[Andy_P]

This is all on the first few pages of any aviation PPL textbook, really very elementary part of aeronautical knowledge.

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!

[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.

[porterhouse]

Although this seems obvious it is rarely stressed in most training documents. 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? :}

[Andy_P]

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.

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.

[Tarq57]

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. ...
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.

[Andy_P]

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.

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.

[thing]

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.

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.

[dirkdj]

See How It Flies (http://www.av8n.com/how/)

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.

[Heston]

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.

[porterhouse]

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.

[Mach Jump]

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

[olasek]

This is an excellent on-line free book if you want to dig a bit deeper into Stability, Balance, etc
Yes, it is an excellent source, I was once specially impressed with its explanation of lift using the theory of circulation.

[dubbleyew eight]

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.

[porterhouse]

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.

[24Carrot]

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 (http://en.wikipedia.org/wiki/File:CL,_CD_NACA632618.png)
from Drag Polar - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Drag_Polar)

Of course, some aerofoils may not be 'typical' :)

[olasek]

For a 'typical' aerofoil at cruise speed, not muchWhat 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.

[Lone_Ranger]

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.

Spoken like a true 21st century student.

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.


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

[Andy_P]

Spoken like a true 21st century student.

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

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.

[24Carrot]

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.

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 :)

[porterhouse]

The drag curve is 'typically' designed to be fairly flat for reasonable weights - like the graph I showed.
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.

[24Carrot]

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

[porterhouse]

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. :}

[olasek]

- like the graph I showed
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/aerodynamics/drag/drag-cessna.gif

[24Carrot]

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.

[porterhouse]

and I would contend that this is not strictly true, in the cruiseAnd 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.

[24Carrot]

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:

[olasek]

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

[Natstrackalpha]

Doesn't adding to your maximum all up weight take you over the maximum all weight? More to worry about than power.

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

olesak

This is all on the first few pages of any aviation PPL textbook, really very elementary part of aeronautical knowledge.

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
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

Exactly.

[9 lives]

The concept that carrying more weight will require more power being applied may be an over simplification. Yes, adding the proportionate increase in power will keep everything else more or less in line, but it's not the only way.

For a certified aircraft, the handling and performance information you are provided, will be applicable at the maximum permitted weight, unless it says otherwise. Of course, in most cases, the performance charts allow the pilot to select information with weight as a factor. But, if in doubt, keep it within the weight and C of G limits, and you'll be fine.

The maximum certified weight will be based on one or both of structural capacity or performance. An efficiently designed aircraft will limit at both very close to each other. Or else, you have more of one of those than you really need.

For aircraft where performance was in excess, sometimes a small structural improvement will allow a higher gross weight - a C 182 can have a higher gross weight when an STC'd wingstrut change is installed.

For aircraft with excess structure, like the venerable C 150, if you increase performance (with.... yes, more power) a heavier gross weight is possible - hence the C 152 with 70 pound heavier gross weight, because it's about 10 HP more powerful.

But otherwise, all other things being equal, if you want to carry more weight in a power plane, some more power will be your best solution - more fuel burn...

[ChickenHouse]

I am a tid surprised. Even aircraft have to obey the basic principles of physics, or?

We all learned during our very first lessons on the physics of flying about the different forces enabling flight. One of the most prominent force pairs is a recall from school physics, called "Weight&Lift" and governed by gravitational forces down. Some may remember that an almost ancient guy with a second good application for an apple, Newton, came up with the grandiose idea, that Force equals mass times acceleration (the best idea for an apple use came from a snake, just to be complete and indeed ignoring Steve Jobs).

A Plane needs to overcome gravity to fly, or (hard to imagine different)?, where the acceleration is equal to earth gravitational acceleration. Even our little planes do not get their force from The Force, but simple from the power drawn from the engine. So, to keep a plane up in the sky when it gathered weight, it has to use more force = power, or am I oversimplifying? (Of course I am talking status within flying envelope)

[mm_flynn]

This seems to be surprisingly complex !

As an example (using the Cessna Graph and some guessed weights)

IF 2000 lbs and 100 knots is 'typical', this will probably have an angle of attack of 5 and CD of 0.05

IF one adds 400 pounds (which is a lot in to an aircraft previously 2000 all up), you will need to increase the angle of attack to 6 at a constant airspeed) and increase the Cd to 0.06 (20%) which will result (if we keep the power constant) in a speed reduction of 6% (cube root of 1.2). It will be slightly less because we always cruise above our minimum drag speed, so the Cd will be somewhat reduced due to our slower speed reducing parasitic drag more than the increase in induced drag from the higher angle of attack needed at slower speed (vs the above calculated effect of the higher AoA needed for increased weight).

HOWEVER, In many planes the increase in weight will also result in the CG moving aft, which as previously discussed reduces the drag due to the reduced downward force on the tail. So for many aircraft, the difference between light+forward CG and heavy+aft CG could be negligible.

[Natstrackalpha]

Exactly, (again) Chickenhouse has got it as have some other learned folk here - - as chickenhouse and porterhouse have correctly declared without the academia thereof

Load your 172 or PA28 or 152, A320, A330, B747, DC10 chocka block with weight - up to the max perameters etc - or not, just for this academic experiment and try to fly as you would if you were with full tanks and no payload and no other baggage, with same power setting

~and

You would notice a decrease in airspeed in the heavier aircraft. Assuming then that you could actually fly the heavier aircraft at that power setting (and speed) - assuming you could control the aircraft and that it did not feel too sloshy you would be grappling for speed, never mind the handling.

Now, if you apply full power AND hope for the best, in this experiment, for we are experimenting here guys (and fair maidens:) then, you might, might get an increase in airspeed and with a lower angle of attack, (providing you are not too heavy, baby) and hopefully the handling characteristics would be perhaps tighter affording greater flying control of the aircraft. [if not then land the bloody thing and consider you got away with it]

In short -

For a substantial increase in weight - you need an increase in power.

The manuals of all of the above aircraft give you the graphs and the tables so you can all go and work it out for yourself before you fly - which is what you do.

But, as we all train in an aeroplane with no payload apart from the instructor or flight crew but no pax or payload - and we already know what is written in the performance tables - and even though we know that weight is opposite to lift and the less weight you have the more lift you get - in short.

No one stands there in front of you all in class, and announces that for an (a substantial) increase in weight you WILL need an (a substantial) increase in power.

Nor is it writ, in them there books like you say on the first pages either. Obvious - simple and true.

[9 lives]

No one stands there in front of you all in class, and announces that for an (a substantial) increase in weight you WILL need an (a substantial) increase in power.

... Because that is an incomplete thought/statement.

I will not need an increase on power, if I will accept a decrease in performance.

I have flown many aircraft at weights greater that the certified gross weight, as much as 130%, without the benefit of more power. I just expected less performance, (and I limited G) in accordance with the overweight authority for the flight.

So, no, I will not stand in front of class and assert that more weight requires more power - it just requires a "trade back" of something.

[Natstrackalpha]

Yes, you are right. An increase in weight requires an increase in power or you will lose something like, controllability, performance, predicted attitude, stall perf, therefore turning perf - and somewhere in all of this is dangerousness - just flying along having traded speed, performance, stall speed, to see what happens - else why fly in that config?

I thought we flew to a performance figure/parameter rather than "see watcha got"

That being said - if you don`t get what you want . . . .with say . . .full power plus the attitude you want then it is time reconsider your options. Power + Attitude = Performance.

I remember one sad time at a touring air show departure event thang - this single engined aircraft took of from the UK and climbed out turned left and promptly went side ways into the ground from about 200 feet - heavy and full of fuel.

Step Turn - I don`t know why (because I am not very intelligent) anyone would want to fly with a trade off rather than go for a perf. Once we went heavy and there was no trade off or power increase (can`t have been that heavy then . . maybe not) once we went heavy and there was a big trade off. Why trundle along like that - I landed back on - some mother else took it. Flying slow and slushy (not mushy) on purpose to demonstrate is one thing, flying slow and slushy because that is all you`ve got is not a place to be. I am not talking about commercial operations stuffed full of mail - even so I have been there too and whilst my nose was comfortably pressed onto the windscreen (joking . . ) there was no deficit in performance, admittedly it was a twin turbo-prop - I am talking about flight training SEP here.

[9 lives]

and somewhere in all of this is dangerousness - just flying along having traded speed, performance, stall speed, to see what happens - else why fly in that config?

I don`t know why ... anyone would want to fly with a trade off rather than go for a perf.

Operationally, a pilot might choose heavy weight over performance, if that heavy weight were to be ferry flight which could not be accomplished at all within the regular weight. Otherwise, the modification of a landplane to be a skiplane, floatplane or amphibian will increase weight, and decrease performance, but is an accepted compromise, based upon increased utility. A C 172 landplane owner, who is entirely content to carry two people and bags might choose a 182 amphibian to maintain that capability, as the 172 amphibian might not have full fuel capability with that load. But neither Cessna is achieving the performance that it would have as a wheelplane.

to see what happens

Test pilots aside, no pilot is flying within the limitations of a certified aircraft to "see what happens". The data is there, and design compliance has been demonstrated for the proposed weight, with stated power.

[Natstrackalpha]

@ Dubbleyew 8

Yer not lost - I stuffed a large tool kit in the back (which put my back out for a year in physio) took 3 pax did the working out in the charts - WAT and all - and got up there. In the 172 I used 2350 RPM and whereas I would often get 102 knots or more indicated - on this day I barely got 82 knots flat out - waiting for the airspeed to increase and it did not. So, I landed back on - sod that. (the highest power setting I have used in my life to maintain the approach profile - interesting)

Yes, although clean I was stuck with that airspeed.

========================================================



@ Step Turn Yes. Step Turn I see you point and this is precisely my point when you mention design compliance. Well, Design Compliance is not the issue - because it does not state the airspeed you will achieve if heavy.

They DO say, on the charts that at a certain altitutde and temp and a certain power setting you will achieve an average of . . . Xknots . . . (an often lower figure with significant altitude increase and also a recommended Full Throttle setting0.

My point was to demonstrate that there is not one maxim of more weight equals more power. There is an examiner around who is not light in weight at all. In the 150 and with top fuel we are on the edge and, depending on the day (WAT) we might not even load max fuel.

My point was demonstrated in the above example of the 172.

However, as clearly seen - even using full power did nought to . . improve the situtation.

Had I have used normal cruise power - we would not be having this conversation.

[9 lives]

Nats, I'm not sure of the point you're tring to make. The aircraft either will be within weight limits, or it was overloaded. If within weight limits, it has met the requirements for climb performance, in standard atmoshpere. Yes, you might eventualy find a WAT value, at which a sacrifice must be made. This is why people purchse more powerful aircraft, or turbo equipped aircraft - but that would be comparing apples to oranges here.

Fly the plane within limits, it will give you "book" performance. That's what it was certified to do....