Throttle for height - elevator for speed

[IanR]

I hope this is the right place for this question from a beginner.

 

I've noticed that some modellers make adjustments to their planes to prevent zooming upwards when applying full throttle. They appear to be trying to achieve a constant atitude irrespective of throttle setting. I believe they usually make these changes to the engine's/motor's thrust angle - perhaps they also alter the main wing's angle of incidence, but I can't remember.

 

What puzzles me is this - have I got this wrong and is there something else going on and, if I'm right, what happens to "throttle for height - elevator for speed?"

 

In my very, very limited experience I have applied differant throttle settings for differant heights. What are these other lads doing?

 

Confused

Ian

 

[Ben B]

In reality you use both throttle and elevator to control both height and speed.

 

You want throttle to increase height by increased lift from the wing, you don't really want the throttle to change the wings angle of attack (that's the job of the elevator). Otherwise if you kept the plane full throttle the plane would draw a parabolic curve until it stalled (or went vertical!).

[Terry Walters]

Hi Ian,

When on the approach for landing you should use elevator for speed and throttle for height. Just like pilots of full size a/c!

 

Terry

[Frank Skilbeck]

Posted by Terry Walters on 29/01/2012 17:47:31:

Hi Ian,

When on the approach for landing you should use elevator for speed and throttle for height. Just like pilots of full size a/c!

 

Terry

Except on gliders

[Biggles' Elder Brother - Moderator]

When of course its the otherway round

 

BEB

[IanR]

I seeeeeee - no, I don't, not really.

 

Ben, you've lost me, mate.

 

Terry, so everyone keeps telling me.

 

But I'm still puzzled - if I adust my model to ensure nil increase in height at full throttle, then how do I increase height, via the throttle, when coming in to land.

 

Or could it be that I'm not trying to increase height when coming in to land but only to maintain height, to avoid undershooting the runway? Could that be it? I can see how that would work.

 

Still confused

Ian

[Frank Skilbeck]

Ian

 

You aren't confused, at least not yet, the reason the mantra throttle for height and elevator for speed is taught, is that as a plane slows down, to maintain lift the wings angle of attack has to be increased and this will in turn increase drag and cause it to slow down more, meaning you have to further increase the angle of attack causing it to slow down even more unless you increase the throttle.

 

For a constant angle of attack the lift increases with speed.

 

What the modellers are trying to do by adjusting the engine thrust line (or wing incidence) it counter the turning moment between the engine thrust line and the main center of drag (i.e the wings), so on a high wing aircraft the wings are holding the plane back and the engine pulling it forward, because there is a vertical separation the tendency is the the aircraft to rotate nose up. By adding some down thrust this creates force pulling the nose down, which counteracts the forces trying to rotate the nose up. If you have the engine on a pylon above the wing then you usually add some upthrust to the engine to stop it from nosing down when you add power.

 

It's also possible to use the elevator to compensate for this, and on a couple of electric gliders I have mixed some down elevator in with throttle to minimise the nose up effect on increasing power (until the modeller has had a chance to change the motor thrust line)

[Biggles' Elder Brother - Moderator]

OK, "being serious" head on.....

 

Its all about trimming. There are personal preferences at work here - so no hard and fast numbers. For me I tend to want my model to fly straight and level at about 2/3 throttle if I can. You can usually assume on any model you'll be flying at this stage in your flying career that the down thrust and wing incidence are correct.

 

So fly your model - how does it go at 2/3 throttle or there abouts? A little nose down maybe? OK, put in a couple of clicks of up elevator. Still nose down? Open the throttle a click or two. Level now? If so are you happy with the where the throttle is to give you level flight? If not - ie if you think its too open, then add a little positive wing incidence or reduce the downthrust a mite - and try the level flight test again.

 

The objective is to end up with the model flying straight and level on 2/3 throttle. Once you have this then (leaving the elevator alone for the moment) any throtlle setting higher than 2/3 will result in the model climbing. Any throttle setting below 2/3 will result in the model decending.

 

So, you want to land? OK put your throttle back to say 1/3, the model will start to decend. BUT,...it will also start to accelerate as it loses height. Now you have a problem, if you use the elevator to level out now - which you are going have to do in order to not "land" nose down - then the excess speed will cause too much lift - and you'll zoom. How do you deal with this?

 

Well the answer is as you reduce the throttle feed in some up elevator - the up-elevator increases the drag which acts a "brake" to stop the model accelerating too much. Very important point here - don't use the elevator to "hold on to the height" - let the model decend, that's what you want to do. Just use enough elevator to stop it gaining too much speed.

 

The "trick" now is to balance these two effects. Throttle open a tad - you'll decend more slowly - and hence extend your approach glide. Throttle down a tad and she'll decend more quickly. Ease in a little up-elevator and the model will slow down, relax out the up-elevator and the model will speed up.

 

It takes a lot of practice! But eventually you will be able to balance these two effects so that you can put your model down on the strip exactly where you wnat to!

 

What Ben says is spot on - rememeber these:

 

Yes, the throttle speeds you up - but the main effect of that is to generate more lift so you climb. In that climb you shed the gained speed - therefore think of the throttle as the way to climb and decend.

 

The evelator controls the pitch - as the wing is fixed to the fuselage it therefore in effect controls the angle of attack. A higher angle of does produce more lift - but it also produces a lot more drag! So the main effect of up elevator is to slow the plane down,

 

If you are flying straight and level and simply pull up-elevator - without changing throttle - yes you will climb - a little. Then you'll slow down and start flying straight and level again but at a higher angle of attack and slower. The only way to get a real, lasting, increase in height at the same speed is with more throttle while you climb and then throttle back to "cruise" when you are at the new height you want!

 

BEB

[Ian Easton]

Interesting stuff and I've been wondering about it lately. My son and I have been practising taking flying shots of our models and I'v e noticed in the photos of slow fly-bys that my models had a bit of up elevator (and yes they were trimmed level at about 2/3 throttle). I was wondering if perhaps my models had a CG issue even though they appeared to fly well, but the above explanation about elevator position makes sense to me now. - Thanks guys!

[John Muir]

BEB and Frank are quite right, Ian, but I understand what you're saying.

 

 

Some models have downthrust designed in not to remove unwanted pitching with sudden throttle openings as Frank described (and as it should be), but to stop the plane climbing when the throttle is opened. Excessive downthrust 'pulls' the nose down when the throttle is opened, reducing the wing angle of attack and resulting in acceleration but no climb. It's a form of automatic trimming if you like. I think this is an old fashioned way of doing things, maybe even a hangover from free flight days, and maybe isn't done so much these days. I never understood it myself as it seemed to me that an aerobatic model would spend a lot of time upside down when the downthrust would be working against you. On trainers and the like we should be trimming as BEB describes, but I think there's still some extra downthrust on some models to reduce the climb rate when the throttle is opened.

 

With a plane set up like this, you are quite right, more throttle does not mean more height, the plane's altitude does not change until you give it a nudge with the elevator. Which, as Ben B pointed out, is what we tend to do in reality anyway.

 

John.

Edited By John Muir on 29/01/2012 20:01:05

[Doug Ireland]

Throttle for rate of descent, Elevator for Airspeed! Other way round and you've messed up your approach. Gear up, flaps up and go round again!

[IanR]

I seeeee - yes, I do, really.

 

Its all about personal preference but throttle height/elevator speed holds whichever way round you prefer - as long as incidence angles aren't ridiculously out of wack (as I discovered this pm whilst messing about with Phoenix)

 

John, "...the plane's altitude does not change until you give it a nudge with the elevator..." clinched it for me.

 

Uncle BEB, (you don't mind if I call you Uncle do you: I've never had one and trolling through the forums looking at your posts - almost invariably replies to queries - makes me think of you as a kindly uncle, which is daft really as I'm probably old enough to be your uncle. Still...) I read elswhere your answer to a beginners query re effective landing procedure and you said that he should do it this easy way, for now, and you'd outline the proper, more difficult, way later. Well after the brilliant explanation above, I'm thinking that it must be the "proper" way. I'll be setting my Foamie Wot up per you 2/3 throttle procedure.

 

Thanks everyone - very much.

Ian

 

("Its been a good day," he thinks, as he climbs the wooden hill to bed)

[Steve W-O]

As has been said, the only time the saying is 100% correct is on approach, the rest of the time it is a balance.

 

If you were at 30000' over London and wanted to get to New York quicker, and used the elevator to increase speed, you'd end up in the drink before New York.

 

If you were about to touch down, but were short, and you used the elevator to gain height, you would stall, (assuming you were at the correct speed to start with). One landing on a 320 recently, we landed on full throttle, as we were short and the pilot had to make the runway. It was an interesting landing, what I would call a Flight sim landing.

 

I think the "golden rule" came about to stop pupils stalling on approach.

 

On a model, I think a way of practising BEBs balancing act is to practice descents in a slightly nose up attitude, where the throttle will not be at zero

[Steve Hargreaves - Moderator]

The saying is probably more applicable to full size aircraft too....our models have so much power in comparison that often we can make them climb vertically.....

 

Try that in a Cessna 152....

[Tony Smith 7]

Posted by Steve W-O on 30/01/2012 04:56:34:

....

If you were at 30000' over London and wanted to get to New York quicker, and used the elevator to increase speed, you'd end up in the drink before New York.

...

Unless you increased power to maintain altitude, which comes back to the same thing. I suppose what you'd really do is to increase power first, then retrim to maintain level.

[Biggles' Elder Brother - Moderator]

Posted by IanR on 29/01/2012 21:50:28:

 

Uncle BEB, (you don't mind if I call you Uncle do you:

 

Ian you can call me anything you like as long as you don't call me early!

 

Posted by IanR on 29/01/2012 21:50:28:

I read elswhere your answer to a beginners query re effective landing procedure and you said that he should do it this easy way, for now, and you'd outline the proper, more difficult, way later. Well after the brilliant explanation above, I'm thinking that it must be the "proper" way.

 

Correct

 

Posted by Steve W-O on 30/01/2012 04:56:34:

On a model, I think a way of practising BEBs balancing act is to practice descents in a slightly nose up attitude, where the throttle will not be at zero

I agree. On the simulator this is easier if you pick a heavier model and set the wind in the sim so that you can land directly into a say 3-5mph breeze.

BEB

 

[Vic8]

Posted by IanR on 29/01/2012 17:24:07:

I hope this is the right place for this question from a beginner.

I've noticed that some modellers make adjustments to their planes to prevent zooming upwards when applying full throttle. They appear to be trying to achieve a constant atitude irrespective of throttle setting. I believe they usually make these changes to the engine's/motor's thrust angle - perhaps they also alter the main wing's angle of incidence, but I can't remember.

What puzzles me is this - have I got this wrong and is there something else going on and, if I'm right, what happens to "throttle for height - elevator for speed?"

In my very, very limited experience I have applied differant throttle settings for differant heights. What are these other lads doing?

Confused

Ian

Glad you posted this topic, I am finding the thread very helpful.

Equally confused.

[Martin Harris - Moderator]

Can I give you a very simple analogy:

Imagine you're on a push bike.

Steep hill (elevator), goes fast down it - not much pedalling (throttle) if any required to maintain speed.

Steep hill (elevator), goes slower and then stops going up it unless you pedal (throttle) like crazy to maintain speed.

In flying, you're trying to acheive a balance between AoA and power to either climb, fly level or descend at (usually) a selected airspeed. The main control which influences the rate of climb or descent is the throttle, the main control over airspeed is the Angle of Attack which is controlled by the elevator. On a typical light aircraft, trainer, scale model etc. opening the throttle will result in a climb with little (if any) change in airspeed due to factors designed in such as thrust lines, incidences etc. Closing it a bit will do the opposite.

[fly boy3]

Hi all, throttle and elevator control come into their own during landings. Many many years ago I was told a good approach will be followed by a good landing, Reduce throttle be begin decent, right height, right direction (into wind) right angle of decent, will acheive a good landing most times, If you are short, blip throttle to move forward, if too fast, go round again,Use elevator to flare out. I spent many flying sessions only practicing landing approaches using throttle and elevator. Pays off in the end. Cheers

Edited By fly boy3 on 03/04/2012 11:02:39

[Biggles' Elder Brother - Moderator]

While we are "in the mood for analogies" here's another one. Remember this simple rule

"The higher you are the more energy you have".

Don't worry about why, just accept that it is true.

Now, energy can't be "detroyed", it just changes form that all. So to climb higher, in a plane or on Martin's bike, you must put energy into the system. In the case of Martin's bike we put the extra energy in by pedalling, for a plane we open the throttle and burn more fuel.

In exactly the same way, if we come down, then the system has to give us that energy back. It usually does so by us gaining speed as we descend.

Can you see now why simply pulling back on the stick - ie inputing up elevator - can't deliver a true sustained climb? Because it puts no new energy into the system. Its like trying to bicycle uphill without pedalling. It won't work.

Similarly can see now why a plane's natural tendancy is to speed up when loosing height? The energy we no longer need (because we are not so high) has to go somewhere. So its first choice is to go into speed. But that's ratther inconvienent for us. We don't want the model to speed up. So we need to find another outlet for the energy. The alternative outlet we find is drag. By putting in up-elevator we increase the drag and so the model now has to work harder against the air - and this provides a handy way of dumping the energy without speeding up.

So now the elevator controls the speed. Let the nose come down, and we descend with acceleration, speeding up. Hold the nose up a bit, we descend with lots of added drag to act as a brake and possibly even slow down. Ultimately, if the elevator can't make enough drag to control the speed we fit the aircraft with flaps to help it make even more drag. And if that isn't enough we apply crow brakes and spoilers and all sorts of drag devices. But that's another story!

BEB

[Former Member]

[This posting has been removed]

[Martin Harris - Moderator]

That last part rings some very distant bells where I remember being told that airliners and military jets "behave differently" on the approach in this respect.

I don't think I could ever quite grasp why this should be but I'm wondering if it is a function of the relatively small corrections required due to the higher approach speeds and consequently smaller proportional corrections required for atmospheric deviations i.e. although there is no aerodynamic difference, with the greater inertia and slower power change responses it is easier to correct height by pitch and accept the slight speed change which can be corrected over an averaged period when making small corrections to a glide path?

I suppose that there could also be differences in rigging incidences between this type of aircraft and typical models, light aircraft etc. which would influence this behaviour?

Edited By Martin Harris on 04/04/2012 13:15:54

[Former Member]

[This posting has been removed]