Peter

But what causes you to need to trim? When you are trimmed straight and level at say a lowish power setting and you advance the power levers fully forward what happens that causes the aircraft to need trimming to maintain the altitude? What is the initial thing that has to happen? Before as many claim you pitch for speed? (which is not the true picture!)
We are trying to dissect something into black or white chunks rather than the whole hence why I repeat both are wrong there can technically only be one right?
Trimming while vitally important is a bit of a Red Herring in this discussion some aircraft dont even have a trim wheel.

Pace

A final thought. If a pilot understands the lift equation, he will have a far better idea as to how an aircraft works.

The aircraft then climbs because the engine power is being translated into extra potential energy. The engine power has to go somewhere, and it cannot go into an increased speed because the aircraft prevents that (ignoring cases where pitch attitude is very power sensitive because the thrust is excessively out of line with the hull).

Peter

Not trying to be funny with you ;) but what makes it want to climb? It is important.

Pace

Pace; Power tries to accelerate the aircraft, the aircraft is trimmed for a speed therefore if the speed tends to increase, the aircraft will climb to maintain trimmed speed.

If you displace a correctly trimmed convential aircraft from its speed then release the controls it will return to its trimmed speed after a few phugoid excursions without any control inputs. Try it sometime in the cruise.

Just physics :)

Exactly.

M

As you add power the aircraft will accelerate (speed) as it accelerates the increase in airflow will cause lift as simple as that. The primary action of adding power to a trimmed aircraft in level flight has to be an acceleration and hence increased airflow over the wing.... as simple as that.


What the heck has that got to do with it try doing that and at the same time shove in a fistful of throttle and see what happens?



Pace

Lots of aircraft that doesn't work on :) a change in power in an aircraft that has a moving stabiliser that sits inside the slipstream as a trim method will not hold the same speed. A large change in power will produce a large change in attitude that will almost certainly correspond to a large change in speed.

The pitch change is caused by a change in the lift moment created by the difference in pressure across the tail and the wings right? So if the trim method automatically compensates for power changes in the way that it does in the tomahawk, warrior, C152/C172 etc and TB series then that principle works (a relatively small trim tab that isn't that greatly affected by more airflow relative to the size of the tail, with the exception of the tommie which has an elevator that lives outside of the slip stream so only Airspeed initiates a change in trim). But in the case of the cub for instance it's the AoA of the entire stabiliser that changes so a change in power alters the lift balance between the wings and the tail in a larger way, it requires a different attitude towards trimming but it does still come down to the same basic principle that if you desire a certain RoD and Airspeed then you must set the power and attitude to achieve that and trim to maintain it without having to push and pull your way down :).

Probably not a great explanation but I think that's what Pace is getting at.

If it's not clear what I mean is this,

In the cub, if I trim for 65kts with zero power and then apply full power, I will not retain 65kts, the nose will pitch up way too much and the aircraft will slow down quite rapidly and no amount of waiting will bring it back to 65kts, it will wallow around the stall at 47-48kts I must push quite hard to keep the attitude to maintain 65kts and will consequently climb, I then trim to hold the attitude.

Trimming for an airspeed as such works for a boat load of aircraft though :)

There's no one method that works for everything but it all starts at the same place.

Power + Attitude = Performance, Select Hold Trim.

Pace: One last post from me despite your petulant response........

On all aircraft the Lift Vector is displaced from the Weight Vector.
The Thrust Vector is usually displaced from the Drag Vector.

The act of trimming resolves the moments between the four forces to achieve a stable situation. Change any one parameter and the aircraft will need to be retrimmed if it is to continue to fly at the same speed as before. Do you still not see why the application of power will tend to make a conventional aircraft climb (stick-free)??

Not sure I'm with you there. The act of trimming the elevator only resovles the difference between turning moments created by the lift vectors of the stabiliser/stabilator and the wing. Basically you're changing the effective angle of attack on the tail to maintain a given attitude. A change in power will only necessitate a change in trim for a given airspeed if the tail plane generates more or less lift, which would require a change in relative airflow - the primary change of this comes from either the slipstream or from a change in trim :).
So if the tailplane lives outside of the slipstream the aircraft ought to settle at the same airspeed as it was trimmed for at any power setting. The tomahawk does this beautifully (as does the T tail arrow). The Jetstream does this really nicely as well :) although I haven't really had the opportunity to investigate all it's characteristics.

The application of power will ultimately lead to a pitch change but not necessarily a speed change or a change in RoC/RoD it depends very much on the design and position of the tailplane and the trim system.

this is of course my opinion :) and not necessarily correct.

And of course where the propulsion unit sits. In my plane if i add power i get a strong pitch up moment and speed will decay. If i reduce power there will be a strong pitch down moment and speed will increase. Simply because the thrust line is way below the CG. So any power change will lead to a trim change for the same speed.

Meikleour

Totally but that is the point I am making it is increasing speed or airflow from an increase in power which achieves that! The trimming for speed is secondary.

My apologies for being petulant "bad hair day :E

Pace

Silvaire

I totally agree with your comments but the OP started the thread taking about not agreeing with pitch for speed. My point is that he is partially right as both concepts pitch for speed or power for speed in their own right are wrong.
If you look at my previous posts I talked about Gliders in still air relying totally on pitch for speed and light powered aircraft more towards pitch for speed. aircraft like the lightning are at the other end of the scale power for speed being able to vertically go up to 40K on thrust alone.
With especially students the emphasis has to be pitch for speed for just the reasons you point out.
Having said that why does the Citation I fly use pitch for descent on the glideslope autopilot engaged and thrust to control speed? Obviously that works too.
It is really energy management two sources of energy from the airframe in pitch and from the engine. Both are best suited in different situations and relying on skills for one alone is asking for trouble.
Somehow I dont think I am getting my point over very well!! :ugh:
It is the juggling of both sources as needed which is the correct way.

Pace

Silvaire

I totally agree with your comments but the OP started the thread taking about not agreeing with pitch for speed. My point is that he is partially right as both concepts pitch for speed or power for speed in their own right are wrong.
If you look at my previous posts I talked about Gliders in still air relying totally on pitch for speed and light powered aircraft more towards pitch for speed.
With especially students the emphasis has to be that way for just the reasons you point out.
Having said that why does the Citation I fly use pitch for descent on the glideslope autopilot engaged and thrust to control speed? Obviously that works too.
It is really energy management two sources of energy from the airframe in pitch and from the engine.
It is the juggling of both sources as needed which is the correct way.

The old joke of the student sitting on the end of the runway on his first solo pumping the elevator up and down. His instructor approached to ask what he was doing? "I am pitching for speed but not going anywhere ;)

Pace

That's a very different situation because the aircraft is artifically constrained into a lateral+vertical flight path.

When you are flying a coupled ILS, the autopilot pitch and roll servos are driving the ailerons and the elevator to maintain the localiser+glideslope. The pitch trim servo is concurrently trimming out the elevator trim tab so as to minimise the pitch servo torque. And the yaw damper is also doing its own thing with the rudder...

The autopilot doesn't care what speed you are doing. It will drive the control surfaces purely to maintain the LOC+GS. You don't have an autothrottle...

This brings many gotchas e.g. it is easy to end up on the back of the curve and then you may not have enough power to recover while maintaining the glideslope. Turkish 737 in Amsterdam... Thompson did it with a 757 at Bournemouth but they got away with it.

You get funny things happening when on autopilot e.g. you are flying along and suddenly your airspeed has shot up. But your altitude is constant (obviously). This is because you are in an updraught, so the AP pitches down to maintain altitude, so the airspeed goes up as a consequence. Or the reverse, in a downdraught.

But I am sure you know all this. The autopilot scenario is a total redherring.

In a 152, which the OP seems to be flying, there's nothing wrong with "point and power". All you have to do is aim at the runway, and the ASI tells you what to do with the power.

On the other hand, on a nice calm day, it's quite satisfying to be able to set one power setting, or no power at all, and make it all the way down only using flaps and attitude to control the speed. As long as you meet Vref, it's all good.

Peter

Its funny how I am totally agreeing! but not if you get my Gist! Yes I do know that ;) But dont agree the autopilot is a red herring.

It shows that in descent pitch for descent rate power for speed works I would even go as far as stating that in some aircraft pitch for climb! power for speed works too!

In another situation such as a high AOA you may need not only pitch but power too to save the day! What is your stall recovery? pitch alone?

Ie it just confirms that my argument that neither method pitch for speed or power for speed is right and I hold that you are asking for trouble following one method.

It has to be pitch for energy power for energy and both are so linked that you cannot divorce them. It should be a skillful blend of the two!!!

Pace

I think the OP would be best advised to listen to his instructor! Sorry EGKB but you are demonstrating all the signs of " I know best", you will get away with it 999/1000, but that arrogance will see you in a smoking hole one day I'm afraid....you are 17 years old with very,very low hours, listen to the qualified instructor who is teaching you how not to kill yourself or your unfortunate passengers..I am 31, & have held my PPL for 7 years, my instructor was 23, so we are not all old miserable gits!


However, if you carry on as you are...
EGKB, Coming to you via an AAIB report within 5 years, I would put money on it :(

Silvaire

Do they actually allow modern students to do potentially dangerous things like slipping nowadays? Thought the training was all incipient this and that and just aircraft drivers at the end not pilots?
Its ok just stirring :E Again! A joke ;)

Pace

I got taught slipping during my PPL and find it both great fun and useful. I can't imagine being able to do consistently good pfls without it and like Silvaire, I like being able to stay high until final approach.

That said, a few people have seemed surprised when I used the technique...

To me, the fact that a normal plane climbs, at a relatively constant speed, when power is increased, is simply the consequence of the pitch stability mechanism which every certified plane has.

The elevator has what is called decalage which AIUI means that the turning moment due to the change of elevator AoA about the pitch axis has to exceed the turning moment due to the change of wing AoA about the pitch axis. If you didn't have this pitch stability (which is in effect a negative feedback control system where airspeed is the setpoint) then the plane would be virtually uncontrollable manually.

I think that the cases where you are flying at say 65kt, trimmed, at low power, and increasing power causes the plane to nearly stall, are because one is flying on the back of the curve at 65kt (on that particular type). Otherwise, can a plane with such a large moment (due to power changes) about its pitch axis get certified?

Peter

I think we get lost in the detail and the science rather than the basics,
We have two energy sources available to us potential energy we have created by dragging the airframe into the sky and the energy from our engine.
The glider having no power plant as such has to be towed ,carapolted or whatever into the sky to get the potential energy to be able to control airspeed by pitch on the way down.
Let me use the Citation as an example as using power for speed and pitch for speed and them both being right in a climb where we are not using the potential energy from the airframe.
Low level at Max N1 I may climb at 2500 fpm. I may elect to only want to climb at 1000 feet per min. Climbing at 1000 fpm I may decide that I don't want to go too fast so I reduce thrust and now am using power to control speed ,
High mid 30 FLs I am at max Ni coaxing a climb and only have pitch to control airspeed,
Eventually at max N1, climb rate is decreasing speed is decreasing AOA is increasing drag is increasing and I have nothing left to play with,
I have all the potential energy available but as I am still trying to coax a climb I am stuffed on going higher.
So as I say pitch for speed or power for speed are your tools which are for your use, both are important and both create the complete picture of controlling the aircraft
Coming Down from FL 350 I know have tons of potential energy available and tons of thrust available. I juggle the two energy sources maybe closing off engine power and just using the potential energy in the airframe to stop breaking VNE .Hence why Hence why saying I belong to the pitch for speed or power for speed brigade is such a nonsense ! Sorry for grammer written on I phone .

Pace

Good points Pace and Peter, though perhaps on the edge of being relevent to the original post.

We have a very simple, basic question, posed by a new pilot of yet to be proven receptive attitude.... who has been absent from the discussion lately (since I posed a challenge to his (her?) self described knowledge......). EGKB?.....

Good discussion but miles away from the point. I see the speed with pitch in the PPL as an exersize which needs to be mastered early in the course and repeated in other exersizes until it is second nature. He may use the combined method eventually.

It's like the flight at minimum speed exersize. Flying around with a high power setting on the wrong side of the power curve. It is only a demonstration. Not an everyday technique.

D.O.

Pilot DAR?

Absent from the discussion? You mean eating, sleeping for 8 hours, and working....

I can't sit at my comp 24/7.

And people STILL don't listen. I am using the technique the instructor mentioned. I think that's the 4th or 5th time I've mentioned that!

Dont Overfill

I do take your point and have stated the same in all my posts.
The glider only has pitch for speed on still days.
Attach a tiny motor to said glider and you still have pitch for speed with some power for speed but the dominant principal has to be pitch for speed.
Our original poster who is BACK :D needs to keep away from a high AOA and high drag situation thinking he can control his speed with power because he probably cannot.
He nevertheless has to be aware of both power sources and their relevance as he may at some time require not just pitch for speed but power too.

Pace

Well I have taken away a few things from this dicussion and no, I'm not a troll. That term is thrown around too loosely!!!

The auto throttle pulled back the power because of a radalt failure telling it the aircraft had landed (or something like that) while the autopilot was maintaining the glideslope.

Peter

To me that is a lesson in never trusting any Gizmo autopilot or otherwise or being lulled into a false sense of security as these things have a habit of going wrong when you least expect.
Hence why piloting skills are so important even in our day of technology ;)
Flying older Citations I tend to get more system and autopilot failures than I want :{ all good for the soul ;)

Pace

I've had more autopilot failures than I can recall, but I still think that during the 99.9% of the time that the system works, you are a safer pilot using it and using the now copious spare brain power to monitor everything, than to hand fly and perhaps make a major cockup in another department.

I can hand fly in IMC fairly well, and have hand flown the TB20 most of the way down to Santorini (LGSR) and back, as well as parts of some N European airways flights, but I make simple mistakes during hand flown IR training (instructor or examiner in the RHS watching and issuing instructions) which I have never made when flying on my own, using automation at my discretion. In fact I have never made a major error or had any significant self inflicted "event" flying on my own, IFR, in CAS, and that's a good enough record for me...

The bit about letting the autopilot stall the aircraft when going down the ILS is just stupid. One can dress it up (the way that 1,000,000 posts elsewhere have tried to dress up the AF447 muppet-pilots) but the fact remains that if the pilot is not watching the speedo then all bets are off, and this is true on AP or off AP. Airspeed awareness is the absolutely #1 thing in flying. Well, alongside obstacle clearance ;) I suppose.

Peter in that situation, the aircraft flirts with the stall but the stall in an L18C cub is barely a stall, it's more of a light buffet with a bit of nose down which unstalls the aircraft very quickly - it's not a CofA aircraft any more, but it was and there are ones just the same that are so I see your point. 60-65kts in the bottom of the curve as opposed to the speed unstable section but it is an extremely draggy machine.

Nonetheless a change in power doesn't automatically cause the aircraft to pitch for the speed that it has been trimmed to, equally if I'm flying a little faster and trimmed for say 75kts (top cruise speed being about 85) and pull the power completely it accelerates fairly quickly and I think it's not wise to assume that you will automatically pitch for an airspeed if you're in trim and change power but it is a technique that does work very nicely for a hell of a lot of aircraft and is unquestionably the basis for understanding the couple between, pitch, power, trim and airspeed.

It might be an interesting Q whether trim=speed (regardless of power) only when on the right side of the curve?

I think that is self evidently true, because if you let the speed decay too far then you are looking at the scenario where a given power setting is capable of holding either of the two speeds and, if you were careless enough to let the speed decay to the lower one, that's where you will end up.

Peter

You don't have to justify yourself ! This is only a forum discussion and I know you are an excellent committed and very detailed pilot!
You also have a wealth of knowledge which exceeds my own so please don't take any of this discussion the wrong way.
Sometimes I will post to encourage discussion just my style so no offence ;)

Pace

Nope, does't work that way at a higher speed with cruise power set if closing the throttle the aircraft pitches down and accelerates beyond that speed.

I'll do a few tests at altitude next time I get the chance to see if it settles down slowly to the trimmed airspeed.

I wonder whether he has done the slow flight exercise yet, where you discover that to fly as slowly as possible you need full power?

Yes, but you still have to vary pitch to maintain just above stall speed or 5 knots above or however they teach you...

Fair enough, I have been similarly absent (including crossing the Atlantic).

Execellent.

Are you keeping your mind opened to the wisdom of others? I sense that some [very experienced] participants here think otherwise. It's up to you, they don't have to fly with you! Yours to win or loose - they don't really care that much!

And, what would you like to tell me about the surprize I had during an early STOL takeoff in the aircraft for which I offered the control force charts? If you were to fly that plane, what would you be expecting you'd experience? Might you run out of something?

It definitely depends on the aircraft type. For example IME the Cherokee 140 will maintain a trim speed while using throttle to adjust descent rate much better than will a Warrior.

Pilot DAR?

Family visit? Or you were flying across?

I still think you guys are a bit too "black and white" if you will. If don't use the technique you'll crash and die, and aaib will have reports on you and well laugh... lmao