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DHC-6 Twin Otter development

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Re: DHC-6 Twin Otter development

Postby dg-505 » Thu Mar 09, 2017 10:00 pm

Hi, thanks for the feedback.

The first three issues are known to me, can be done easily by changing the checklists a bit, i'll do that one day....

About the airspeed thing.
Most predestinated question: Did you forget to activate the pitot heat while the OAT fell below 0 deg C? :mrgreen:

The on-screen warning reads the indicated airspeed, i.e. the speed the ASI is showing. Imagine your copilot telling you that you are above Vne because the pitot tube got frozen, but in reality your true airspeed doesn't increase.


If you want to write an "official" review (e.g. for the website), would you mind to wait a few months with that? Because currently I'm in the process of giving the systems a major overhaul and I also have plans about some customization options regarding the interior. (See the wiki for a list of future plans) :wink:
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Re: DHC-6 Twin Otter development

Postby Thorsten » Fri Mar 10, 2017 6:41 am

Did you forget to activate the pitot heat while the OAT fell below 0 deg C?


Thinking the incident over last night, that was also the conclusion I arrived at. I honestly don't remember whether I did something to it on the ground as part of the checklist, but I probably did not.

The on-screen warning reads the indicated airspeed, i.e. the speed the ASI is showing


After doing some digging in the files, yes, I saw that as well.

We may have different philosophies here, but I guess that threw me off-track rather massively. My first idea was that maybe this is some kind of instrument malfunction, but since the limit check callout seemed to confirm what the instruments said, I decided to go by it.

I'm guessing in reality you'd have subtle sensory cues that an aircraft is getting overstressed - certainly in a glider you can tell from sounds, changed handling characteristics and g-forces that you're getting too fast, I'd assume there may be different vibrations in reality - same for stall warnings - and I've always pictured on-screen limit warnings as a stand-in for that as it's difficult to simulate.

Basically in the implementation, not only the instrument itself but also the on-screen callout gives you bad advice in the event of an instrument malfunction - so you need to be very sure you have diagnosed the problem correctly to not react in a dangerous way. That's perhaps not overly user-friendly - if the co-pilot doesn't give sound advice, I'd rather have him shut up rather than actively preventing me from flying safely.

That's also the way limit warnings work in the Shuttle - all warnings displayed on the avionics or the CWS matrix are based on simulated instrument readings, but all on-screen callouts are based on simulated truth (aka, there's no reason Mission Control or your Co-pilot should not be able to recognize an instrument malfunction...)

If you want to write an "official" review (e.g. for the website), would you mind to wait a few months with that?


I am fairly sure there'll always be a 'next project' - I don't think any of the planes which made it to the webpage were in any way 'finished' - it's mainly intended to get some attention and I guess users who like what they see will stay with the plane also for future versions.
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Re: DHC-6 Twin Otter development

Postby dg-505 » Fri Mar 10, 2017 1:12 pm

As far as I rembember, the checklists contains the pitot heat once in the "Engine start" and once in the "Cruise" checklist

Thorsten wrote in Fri Mar 10, 2017 6:41 am:your Co-pilot should not be able to recognize an instrument malfunction...

That's a good point, I will change that also in the future that the on-screen limit warnings refer to the "real" values.

Thorsten wrote in Fri Mar 10, 2017 6:41 am:I am fairly sure there'll always be a 'next project' - I don't think any of the planes which made it to the webpage were in any way 'finished' - it's mainly intended to get some attention and I guess users who like what they see will stay with the plane also for future versions.

Well, in that case..... go ahead! :wink:
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Re: DHC-6 Twin Otter development

Postby Thorsten » Fri Mar 10, 2017 3:50 pm

As far as I rembember, the checklists contains the pitot heat once in the "Engine start" and once in the "Cruise" checklist


After doing another test consciously looking at this - yes, it's in the checklist, and it seems to have been the cause of what I've been seeing before, making sure the pitot heater was definitely on at least caused no issue up to 12.000 ft.

So apart from the philosophical point whether the co-pilot gives always good advice or not, I think everything is fine.
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Re: DHC-6 Twin Otter development

Postby stuart » Tue Mar 14, 2017 10:08 pm

I hit exactly the same problem as Thorsten recently, and had an amusing couple of minutes in the cockpit struggling to understand how I could be climbing at 2500fpm and still over Vne!

I quite like that you don't get a "hint" from the co-pilot, that they are just as a fallible as you. Obviously the situation is different for the Shuttle with a Mission Control with additional data inputs, but in a small aircraft, the co-pilot will presumably be monitoring the same instrumentation as you and would warn you of overspeed based on the ASI reading.

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Re: DHC-6 Twin Otter development

Postby Thorsten » Wed Mar 15, 2017 6:12 am

I quite like that you don't get a "hint" from the co-pilot, that they are just as a fallible as you.


Perhaps it's in the wording.

I'd intuitively interpret Reduce airspeed to avoid overstressing the airframe! or Flaps deployed above maximum extension speed. as system messages, but Can you double-check your airspeed? I think we're going too fast. or Be careful with the flaps - we're too fast. as messages from a co-pilot.

Obviously the situation is different for the Shuttle with a Mission Control with additional data inputs


Actually I even have a distinction between messages from mission control (which you only get if you have a comlink established and are intentionally accuracy-blurred) and system messages (which are often limit callouts and based on simulated truth).
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Re: DHC-6 Twin Otter development

Postby Thorsten » Wed Mar 15, 2017 5:44 pm

Oh - here's another one: Just how do the skies work in YaSim? I got normal braking dynamics on a concrete runway, lots of skidding on ice, but the same amount of skidding on a dirt runway (basically impossible to slow down...) - and I did not expect that...
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Re: DHC-6 Twin Otter development

Postby Thorsten » Thu Mar 16, 2017 4:14 pm

... and yet another one (probably more of a question) - just how do you start engines with the version equipped with floats without rotating around wildly in the process? And how do you stop the plane in the water completely without shutting down engines?
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Re: DHC-6 Twin Otter development

Postby dg-505 » Thu Mar 16, 2017 5:15 pm

Thorsten wrote in Wed Mar 15, 2017 5:44 pm:how do the skies work in YaSim

I'm not really sure. If I understood it correctly Yasim uses some kind of material friction values which get defined by simgear to calculate ground friction of the gear (http://www.buckarooshangar.com/flightgear/yasimtut_landinggear.html). Variable via the Yasim FDM file are just two coefficients for static and dynamic friction. I down't know how Simgear computes the friction values for Asphalt, Snow, dirt, grass, etc. I believe it would take quite some time to tweak the values so that you get highly realistic behaviour on all surfaces.

Thorsten wrote in Thu Mar 16, 2017 4:14 pm:start engines with the version equipped with floats without rotating around wildly

1. Make sure that the propeller levers are at feather. That way the propellers generate the fewest thrust
2. Does it help if you have the gear extended while starting the engines? The extended gears generate drag hat should prevent you from turning around. But don't forget to retract the gear before takeoff or you'll never reach takeoff speed.

Thorsten wrote in Thu Mar 16, 2017 4:14 pm:And how do you stop the plane in the water completely without shutting down engines?

Did you try thrust reverse? (With power levers at idle, press the "DEL"-key to activate thrust reverse, then apply full throttle (watch the power levers move into the "reverse" section)


=============
Please let me know if that was helpful ;)
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Re: DHC-6 Twin Otter development

Postby Thorsten » Fri Mar 17, 2017 10:48 am

I down't know how Simgear computes the friction values for Asphalt, Snow, dirt, grass, etc.


I do - they're read in from materials.xml. I was more wondering how the skies are implemented to be different from wheels - in JSBSim for instance they could (mistakenly) not query the ground properties at all.

1. Make sure that the propeller levers are at feather. That way the propellers generate the fewest thrust
2. Does it help if you have the gear extended while starting the engines?


Not really... they tend to mitigate the problem, but you're still drifting madly. And the external power source submerged in water looks a bit silly.

It's usable in water, but has its quirks - some mode to tie the plane to a location would be good. Also, how are you supposed to turn in the water? Is there a rudder in the water provided? I ended up using differential throttle to just spin up one engine, but that gives a fairly large turn radius.

Also, there's something wrong with the implementation of rain splashes. Not only are there no splashes but just static drops (probably means the normal points the other way than you think) but also the direction the drops drift over the glass reverses for no good reason.
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Re: DHC-6 Twin Otter development

Postby dg-505 » Fri Mar 17, 2017 4:41 pm

Thorsten wrote in Fri Mar 17, 2017 10:48 am:how the skies are implemented to be different from wheels

If I read it correctly in dhc6.xml, resp. dhc6S.xml, they just have different static and dynamic friction coefficients, but unfortunately I don't know have enough experience with Yasim gears to provide a quick solution for this :cry:

Thorsten wrote in Fri Mar 17, 2017 10:48 am:the external power source submerged in water looks a bit silly

Yes, indeed :mrgreen: I'll correct that in the future

Thorsten wrote in Fri Mar 17, 2017 10:48 am:some mode to tie the plane to a location would be good

Dou you mean something like an anchor, or like a tie-down for a jetty/pier or quay?

Thorsten wrote in Fri Mar 17, 2017 10:48 am:Is there a rudder in the water provided?

Currently the floats don't have water rudders (and also I don't know how to implement them in Yasim). The best way (though a bit complicated) is indeed to use differential throttle: With one engine in forward mode and the other in reverse mode you can reduce the turn radius. But as said, it is quite inconvenient and requires some experience to handle it unproblematically.

Thorsten wrote in Fri Mar 17, 2017 10:48 am:Also, there's something wrong with the implementation of rain splashes.

Yeah, I know this issue, and it's really a bit annoying when flying in the rain. But until now I didn't understand the code well enough to tweak that...



And by the way, I really appreciate it that you give such extensive and detailed feedback!
A mile of road will take you a mile, but a mile of runway will take you everywhere.

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Re: DHC-6 Twin Otter development

Postby Thorsten » Sat Mar 18, 2017 8:10 am

Dou you mean something like an anchor, or like a tie-down for a jetty/pier or quay?


I'd assume they have an anchor or so, yes. But it's probably a YaSim limitation again - this'd all be cool to do in JSBSim where you can solve hydrodynamics directly, introduce external forces,...

But until now I didn't understand the code well enough to tweak that...


I guess I can have a look, shouldn't be too hard to fix (for some reason, the splash code seems a major stumbling point for many...) Mind if I push changes to that directly to FGAddon?

And by the way, I really appreciate it that you give such extensive and detailed feedback!


My pleasure.

Actually, I've discovered I'm spending way too much time in the ufo just looking at some rendering issue or shortly firing up the Shuttle for fixing something - so I'm trying to make myself fly a bit more. And I've always liked the Twin Otter, it agrees with my more adventurous style of flying, and it's a great plane for mountain flying. Now the graphics job on the cockpit is really outstanding, the level if systems simulations is very pleasant, and I really like goofing around with skies or floats.
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Re: DHC-6 Twin Otter development

Postby dg-505 » Sat Mar 18, 2017 8:49 am

Thorsten wrote in Sat Mar 18, 2017 8:10 am:Mind if I push changes to that directly to FGAddon?

Not at all. Please feel free to improve anything you want ;)

Thorsten wrote in Sat Mar 18, 2017 8:10 am: this'd all be cool to do in JSBSim

Hehe, for me it's already a hard fight just to get it flying in JSBSim. But yes, that's basically the plan.
Currently, I'm porting all the changes back to the Yasim version so that all updates work with both versions.
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Re: DHC-6 Twin Otter development

Postby scarymovie » Mon May 29, 2017 8:21 am

I noticed that when the DHC 6 takes off, it does not do it in a "natural" way (nose raises first followed by the body). Instead, it goes up vertically into the air during rotation. I am not sure if this is the behavior of a DHC 6.
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Re: DHC-6 Twin Otter development

Postby erik » Mon May 29, 2017 9:26 am

The DHC-6 is a VSTOL aircraft where the propellers generate extra lift. I wouldn't be surprised if this happens in reality too.

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