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Elevator trim ineffective at high/low speeds,

Printed From: Just Flight Forum
Category: Just Flight Products
Forum Name: English Electric Canberra PR.9
Forum Description: In-development area
Printed Date: 12 Jul 2020 at 3:51am

Topic: Elevator trim ineffective at high/low speeds,
Posted By: Macoy_AbZ
Subject: Elevator trim ineffective at high/low speeds,
Date Posted: 13 Apr 2014 at 9:45pm

I'm currently trying the demo version of the Canberra PR.9 from Just Flight and, although I liked it a lot, I noticed that the elevator trim is almost ineffective at high/low speeds settings.

Example: At low power, flaps and gear up, 70% tanks, the lowest speed I could trim (full nose up trim) was 175knots. And at full power, full nose down trim, I was unable to keep the plane leveled using only the trim (in fact, the plane did a loop when I released the stick).

It's strange to control the pitch without the elevator trim so, I would like to know if this occurs on the Full Version I'm planning to buy.

Thanks a lot.

Posted By: snave
Date Posted: 13 Apr 2014 at 11:17pm
Please do read the manual: This is deliberate as it is a feature of the real aircraft..!

You can't complain about realism! Big smile

Posted By: Macoy_AbZ
Date Posted: 14 Apr 2014 at 12:21am
Thanks Snave! I'll read the manual.

It's a difficult plane to flight. Actually I think it's the most difficult aircraft I've flown in FSX.

I'm impressed with the Flight Model. This aircraft does have some characteristics I've never seem in FSX before.

Thanks a lot.

Posted By: snave
Date Posted: 14 Apr 2014 at 9:25am
Running out of elevator trim is the least of the issues, you haven't even started on the fuel system yet! (there is an easy way for that) - but once mastered it's one of the best `hand fly` jets I've ever encountered. 

Properly represents the transition period between the single-pilot manual operation of the 40's-50's to the increasing sophistication of flying `systems` from the 60's onward which required a crew of two or three to flick all the switches - tricky to represent on a desktop. 

It's a `marmite` aircraft  - you will love it or loathe it. Nothing in between! Best reason for a demo, ever. 

Posted By: Delta558
Date Posted: 14 Apr 2014 at 11:09am
The trim was probably one of the most difficult areas to work on in this flight model, as there are some interesting 'quirks' which you need to be aware of:

Firstly, to answer your comment on the high-speed trim, the PR9 runs out of forward trim at 450kts. As this is also the limiting airspeed for the aircraft, it shouldn't present too much of a problem. The trim can only be used to balance the elevator forces, it will not make the aircraft 'fly level'.

Low speed presents a more interesting range of problems. To start with, you trim neutral for take off and keep the stick fully forwards. This counteracts the tendency for the nose to rise as the aircraft accelerates. You may think 'trim forwards' but that will put the aircraft out of balance when airborne. The approach presents a slightly more complicated set of problems - as you have found, you'll be using quite a bit of nose-up trim as the speed decreases. When you lower the flaps, however, you will need to apply full nose-down trim and a bit of a push force on the stick. This is maintained until speed gets down to about 130 kts (dependant on weight) at which point you will need to start applying nose-up trim to allow yourself enough elevator authority for the flare. Power is maintained throughout the approach (approx 70%rpm). This is all well documented in the original PR9 pilots notes.

You need to keep on top (or even ahead) of the trim when flying this aircraft, as was the case with many aircraft from this period, but I think that when you are used to it it is fairly straightforward to fly.

Posted By: Paul Golding
Date Posted: 14 Apr 2014 at 12:29pm
Originally posted by snave snave wrote:

It's a `marmite` aircraft  - you will love it or loathe it. Nothing in between! Best reason for a demo, ever. 
Unlike 'marmite', the Canberra is very easy to love.

Posted By: Macoy_AbZ
Date Posted: 14 Apr 2014 at 2:20pm
Thanks a lot,

I'm loving the Just Flight Canberra, and I am probably going to buy it. But it has some characteristics that makes it hard to fly:

- Its ailerons. It's dangerous to use ailerons bellow 120knots. I spun 3 times in this situation. The only way to avoid it is to coordinate the turns with rudder. Any coordination error here and the aircraft drops a wing and stalls promptly. The adverse yaw is really strong, I've never saw this before in FSX.

- The approach. The entire landing process is hard. You have to keep the plane aligned. Drop the flaps, make trim correction, drop the gear, OMG, the plane is too slow and mushing, "CRASH". If you do something wrong here the only solution is to "slew" ('Y'). Even "going around" is hard here.

- The slow flight: The Canberra has a large wing, but it doesn't like to fly slow. Its ailerons become "rubbery" bellow 170knots.

It's a lovely work from Just Flight. But WOW, it is hard to fly.

Best Regards

Posted By: snave
Date Posted: 14 Apr 2014 at 6:18pm
It's not really, it's just that you are trying to apply other aircraft principles to a plane that doesn't respond well to them - It was a jet long before there were agreed standards for control response, it was designed by pencil and paper to fly at ultra-high altitudes (50,000ft plus) and it has a lot of quirks that, while they seem odd for this aircraft, actually match quite nicely to others of the day - Meteors, Vampires and the like.

Ailerons? Use them carefully and if required use the high altitude (lower gearing) at lower levels - as the manual states:

"The ailerons are light and effective, with good response throughout the speed range.
However, at speeds below about 200 knots it is important to use coordinating rudder and
to avoid large, coarse, aileron inputs because they generate significant yawing forces in
the direction of turn. In extreme cases, the rolling moment caused by these forces can
markedly reduce the corrective power of the ailerons, causing difficulty in preventing
bank angle increasing. It is particularly important to avoid coarse aileron usage in any
circumstances where large rudder deflections are already required to maintain control,
e.g. asymmetric flight.
At high Mach numbers and at high altitude, lateral oscillations are easily induced.
Selection of ‘low altitude’ aileron gearing at high altitude aggravates this tendency and,
for accurate flying, it is essential that the correct gearing is used. The use of ‘high altitude’
gearing at low altitudes is not detrimental to the handling characteristics apart from the
appreciably heavier aileron forces involved."

You don't fly this bird at 120Knots. 170-220 is the normal circuit speed, 150 the minimum safe single engine speed, so stay above 170 unless landing. Aileron problem solved!

The approach: Maintain high thrust level (70%) as the engine spool repsonse is slack at low rpm. You come in fast, stay fast, use the two-stage airbrakes and dump the flaps (which are nothing more than bloody big speed brakes really) to slow the old girl for landing. If you're mushing, you're way too slow.

So its actually reallt simple to fly - it flies nicely at or above 170 knots, so don't go any slower until the wheels are committed to the ground. Do that, and fly circuits at 220 or more and she's luvverly with visibility that modern passenger jets could only dream of!

Just a different mind set - or why it's a `Marmite` aircraft!

What you need to do is read the manual, then devote yourself to an evening of circuits and bumps (pattern work). Go faster than you would land a 747 at, and make sure the runway is long enough, that there isn't a significant crosswind component and you're ready to run out of trim at both ends of the speed spectrum and watch how she works her way under your skin...

Posted By: Delta558
Date Posted: 14 Apr 2014 at 7:35pm
Okay, I'll ask a question first - are you reducing the weight (fuel) and if so, are you following the manual to reduce it in the correct tanks? If not, there's your problem straight away - you are trying to fly and manoeuvre too slowly. As the Canberra loads, the AUW is a touch over 53,500lb; at that weight 120kts is only just above the threshold speed, so you do not want to be applying large amounts of aileron.
If you want to reduce the weight, firstly take centre and centre3 tanks down to 10% (which will bring you down to approx. 42,000lb). If you really want to throw it around at light weights, you can then drop the wing tanks down to 70% and get rid of the payloads (except for the crew, of course). At 40,000lb AUW, 120kts is your approach speed and 122kts is the speed at which, in a 60deg bank pulling 2.0g, the aircraft will stall.
Now to go to your specific comments -
The rudder, yes you will need to use it. The aircraft produces adverse yaw at all speeds, though it is apparently more noticeable at higher roll rates. At lower speeds it is described as 'a significant amount', expect it, deal with it and it's not a problem.
The approach - it looks like you're doing things in a bit of a rush, try the following:
Enter the circuit with airbrakes out, speed about 180kts and rpm mid-range. The speed should be decreasing very slowly. Drop the gear, then as speed gets down to 160kts increase the rpm slightly and drop the flaps. Trim fully forwards and increase rpm to about 70% (weight dependent). Flap extension takes about 15 seconds, so during that time you want to make sure that your speed has stabilised somewhere between 130 and 150kts. That's all on the downwind leg, flying in a straight line and gradually descending. All you need to do now is turn onto finals (one gentle turn from downwind) making minor adjustments to aileron and throttle. Don't try and keep it too tight, and fly the right speeds for the weight.
The slow flight - try it at a lighter weight. It bleeds speed quite rapidly, so at heavier weights you quickly find yourself mushing if not fully stalled. However, it will fly around quite happily at low speeds as long as you treat it for what it is - a sprightly and over-powered old aircraft that is in reality a development of a bomber, not a fighter. Roll rates are actually fairly constant up to about 350kts. If you take the earlier 60deg turn at 2.0g and add 10,000lbs to the AUW (that's a quarter of the weight from the original added on top) you're looking at a fair increase in the stall speed for what is not really that tight a turn.
I hope you buy it, but I also hope you give it the time it deserves to learn to fly it properly. That way, you'll get the most out of it and like I said before, once you get used to the quirks it is really simple to fly.
edit: sorry Snave, we appear to have crossed posts: I started typing before you posted, had my dinner and then finished off the post after!

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