Twin Otter Flight Dynamics question

[darthbrooklyn 0]

When on final approach, i will be lined up almost perfectly with the runway at a +3 degree pitch, usually with a descent rate of about 500 to 700 ft/min... then when i cut the throttle as i cross the threshhold, the plane jumps from -500-700ft/min into a climb of about +500 ft/min which causes me to overshoot the runway... I know this is supposed to happen when adding flaps which increase the lift under the wing... but cutting the throttle???

Am I doing something wrong or is this a bug? Thanks

[Shaun Fletcher 570]

Hello darthbrooklyn,

Let me check that out.

[Finn 873]

Thats fixed in the upcomming 1.10 update

Finn "Wothan" Jacobsen

[Kofi 0]

In the interim,

Crank in a lot of nose-down trim at the same time as you throttle back and it's quite manageable...

[Finn 873]

If You are interested, the new flightmodel can be tested with the 1.02 unofficial update available in the FAQ/Updates section of the Aerosoft mainpage.

Only the -300 with old radios version have the new flightmodel though.

Finn "Wothan" Jacobsen

[WF442 1]

And it is a joy to fly the twotter with the new flight model.

[Finn 873]

Glad to hear.

I haven´t heard much from Mathjis the last few days, but I hope we will make progress this weekend.

If we are lucky, the 1.10 update might be ready next week, but we are still waiting on those .mdl files from Sibwings.

Finn "Wothan" Jacobsen

[Private-Cowboy 3]

Oh man, this next week thing is slowly becomming a running gag. :roll:

[Andy B. 40]

Oh man, this next week thing is slowly becomming a running gag. :roll:

Do you remember saying this:

Looking forward to the update. I doubt it will make friday though as was planned. But better make it right than quick. Well better do both but if only one, make it right.

lol :wink:

[Private-Cowboy 3]

I do and I still think that. But those constant delays are starting to get annoying anyway. And even more so since it does not look like much is done atm besides "waiting" for the models.

[pbearsailor 9]

I've got nothing to do with Aerosoft, but I tested the flight model changes and appreciate what they did. The fixes involve new models for all of the different Otters. I'd expect that's quite a bit of work. Absolutely cannot understand being "annoyed" at the time it's taking to make the fixes. :shock:

cheers,

steve

[Finn 873]

The problem is that the "center of lift" has to be moved in all models. To do this any point in the model, if You imagine that the Twotter is build as a grid, has to be moved accordingly. It is not a total rebuild of the models but close to.

One of the developers also went on holiday for a couple of days and just started to continue working on the models wednesday.

We are offcourse sorry for the delay, we really want to get it finished too. But since the 1.10 also will become the boxed version, we need to assure that we get it right.

Hope to could bring good news soon.

Finn

[Dougal 2]

I have to say, that even with the 1.02 applied, I still find the nose up & down issue on power application, way over the top. Surely the real bird can't be this hard to fly? So far, i've managed just 3 barely successfull landings at Aerosoft's Lukla. Probably out of about 15-20 approaches. Eeeek!

Do you guys have your FSX realism setting on 'hard'?

[pbearsailor 9]

I have to say, that even with the 1.02 applied, I still find the nose up & down issue on power application, way over the top. Surely the real bird can't be this hard to fly? So far, i've managed just 3 barely successfull landings at Aerosoft's Lukla. Probably out of about 15-20 approaches. Eeeek!

Do you guys have your FSX realism setting on 'hard'?

AFAIK, the flight model in the 1.02 is only upgraded in the 300 wheels with the old style radios. That model flies just right for me and the others do not as yet.

[Dougal 2]

AFAIK, the flight model in the 1.02 is only upgraded in the 300 wheels with the old style radios. That model flies just right for me and the others do not as yet.

So what were all the other files for in the folder?

[Finn 873]

The rest of the files are correcting other issues, like propeller wobble on some models, modern radios default to "indirect" mode etc.

Finn

[Dougal 2]

Thanks guys. Seems i have a problem with these updates..... After I apply them, the only 300 model remaining usable on my PC is the BA model. On the other two, the panel view dissapears, leaving just a closeup view of the rudder pedals, and what apears to be a brick wall!

Tried two reinstalls. Guess i need to wait for the update.

[Finn 873]

Hi Dougal

It is correct that the 1.02 update "breaks" the other two. That is due to the missing model files that are in the works now.

Thats also the reason we warned people that the 1.02 where meant for "tweakers".

Regards

Finn

[Dougal 2]

Ok, thanks for that Fin. I don't mind too much, now i've discovered the BA version fly's so well.

It does look a bit odd though....... BA flying in and out of Lukla

[Finn 873]

You can adjust the view from the "pedal view" with:

Sh+Enter

Sh+Backspace

Ctrl+Enter

Ctrl+Backspace

Sh+Ctrl+Enter

Sh+Ctrl+Backspace

But don´t worry the 1.10 update is close to be complete.

Finn

[Dougal 2]

Thank you very much. Much as I love this aircraft, i'm beginning to get frustrated by my own lack of turbo prop knowledge. Can anyone point to some good explanitory documentaion and/or tutorials for opperating such aircraft? I don't seem able to grasp the usage of CL & PL. I don't have the CH throttle quadrant, but i do have the yoke & pedals.

I've disabled the FSX default control, and use FSUIPC for calibration. I have PL 1&2 set to lever 1 on yoke, CL 1&2 to lever 2, and lever3 to mixture (both-on/off) Might i be better with indipendend PL control, and use 3rd lever for combined CL?

This AC is a beaut, and i'd love to fly it correctly.

Thanks

[Ken Stallings 1]

From everything I've learned, flying a PT-6 turbine equipped prop aircraft is very much like flying the piston engine powered prop aircraft that I have experience with.

The significant difference is the type of engine instruments used.

In piston engine aircraft you have manifold pressure and RPM gauges. Note: in aircraft with fixed pitch props you just have RPM gauges).

In turbine powered aircraft you have two gauges that relate to engine power -- torque and inlet turbine temperature (ITT). In the Twin Otter the torque gauge is at the top and the ITT gauge is near the bottom with the large red section.

The short answer is keep these two gauges out of the red zone at all time. Even on takeoff you should avoid overtorquing the engines. The torque gauges each have a slender red line on them. That is the limit of engine power you should command with the throttle levers.

The ITT gauges have a wider red arc area. Likewise you should avoid putting the engines into the red zone on these gauges.

The third primary gauge is the RPM gauge. This measures the rotational velocity of the engine crankshaft. However, it is directly tied to the rotational speed of the props. Not directly, because there are gears that separate the actual crankshaft from the prop shaft and in between a device called a governor that allows the pilot to manually set a given RPM, but that also makes fine automatic adjustments to the prop blade angles to maintain whatever RPM setting the pilot commanded.

For instance, in my fixed pitch prop equipped Cessna Skyhawk when I'm flying at cruise I set an RPM value. But if I'm flying along at 2400 RPM's in smooth air and hit a patch of turbulent air the RPM's surge up and down, sometimes wildly. I have to throttle down to keep from red lining the RPM values.

However, in a constant speed equipped aircraft like a Beech Baron or Bonanza, the governor stabilizes the RPM value at the 2400 RPM cruise setting. It does this by detecting potential changes in RPM and making immediate and automatic changes in the prop blade angles to stabilize the RPM settings. It is much more delightful flying this way.

You don't want to fly for long periods of time with the RPM setting at the red line value on the RPM gauge. However, for short periods of time like takeoff and landing you can. To takeoff, you set the prop condition levers full forward for takeoff.

Then, with the brakes firmly applied, you carefully and slowly move the power levers forward carefully noting the advance of torque and ITT. Once you reach the point just below red line for both torque and ITT you consider the takeoff power set and release brakes.

The plane surges down the runway and you monitor several items best as you can. First, you keep you view outside to ensure you are going down centerline and nothing gets in your way. You make occassional glances to your engine instruments (torque and ITT) to ensure you are not slipping into the red zone for either of them. If you are then you tweak the power setting less as required.

However, this doesn't normally happen if you set a judicious power setting prior to brake release.

Remember, the close to sea level and the colder the outside air is, then the more power the engine can produce. Therefore, taking off in winter from an airport in Florida you can set power well above redline if you cob the power levers full forward. This is called overtorquing the engines and this can cause turbine failure from overtorque or can cause engine damage from overtemperature due to exceeding ITT limits.

However, taking off from a mountain airport above 10,000 feet in summer, you might be able to set the power levers full forward without getting close to the torque or ITT limits. In fact, if you takeoff from Lukla Airport in the Twin Otter, this is exactly the case.

The other guage you glance at during takeoff roll is the airspeed gauge, since you want to rotate (pull the yoke to your stomach) when you reach Vr speed (rotate speed). You need to reference the manual to determine that speed, but it is normally between the thin red line and the thin blue line on the airspeed gauge in the Twin Otter (ask me another time what those are for).

Once you are airborne, and clear of any immediate obstacles ahead of you, pull back the prop condition lever slowly until you get below the red line on the RPM gauges. I use the 90% value during initial climb. As you pull this back, reference also the torque and ITT gauges to ensure as you retard the prop condition levers that it doesn't cause the torque or ITT values to go into the red. If that starts to happen then pull back the power lever as required.

BTW: This is where it's different for a piston engine. Because in a piston engine you advance full forward on the power lever as well has have prop lever full forward for maximum RPM values. So, in a piston aircraft you first retard the power lever (manifold pressure) to less than maximum value, normally around 25 inches of manifold pressure. But, again, consult the manual to determine what manifold pressure you need. Only after pulling the power back do you retard the prop condition lever to reduce RPM.

The manual will specify what combination of torque/ITT and RPM values work best for cruise. Some settings are for best performance over long time -- essentially the best power without damaging the engines. Other settings are for best economy (saving the most amount of fuel for a given distance traveled). And still other settings are for the best combination of power and economy.

Hope this helps.

Cheers,

Ken

[Dougal 2]

Many thanks for that Ken. Your time and effort is much appreciated. Should i be able to taxi the Twotter in ground idle, using only the prop levers? That's the understanding i have from the manual, but it doesn't work for me - the aircraft just sits there unless i apply power.

[fushi 0]

There is a valuable book:

Frank Delp; AIRCRAFT PROPELLERS AND CONTROLS, IAP Inc

Which is describing 'Hartzell Reversing Propeller System On The Pratt & Whitney PT6 Engine'.

Sincerely,

Kan-ichiro Fushihara

[Ken Stallings 1]

Many thanks for that Ken. Your time and effort is much appreciated. Should i be able to taxi the Twotter in ground idle, using only the prop levers? That's the understanding i have from the manual, but it doesn't work for me - the aircraft just sits there unless i apply power.

In turbine engines with a variable condition selector, yes, you can do that. But I believe the Twin Otter's fuel condition lever is either full on or full off.

What you are speaking of is called a dual stage turbine. I don't believe the PT6 engines on the Twin Otter feature that, but I could be wrong. Our resident Twin Otter pilot, pbearsailer may correct me on that.

On the C-130 we did have a dual stage turbine engine, we called it low speed and high speed. We normally taxied with one engine on high speed to provide generator power to the avionics while the other three were left in low speed.

Cheers,

Ken