Pitch up when IAS engaged for descent

[highlander_821 8]

Hi all.  New owner of the Twin Otter.  Very nice plane, having fun learning it. 

The only difficult issue I am having is that I get the pitch up when I try to engage IAS mode for descent. I also get it if I dis-engage ALT Mode.   It does settle back down after a few seconds and works normally, but by then the speed has bled off around 15 knots or so.  So if I engage IAS mode at around 160 kias, and immediately reduce power,  I am down to around 140 kias when it finally decides to start descending.

I have installed the Hotfix 1-4 which claims to fix the pitch-up problem, but I'm still having the issue.   I have watched a couple of Froogle's tutorial flights on Youtube, and he is able to press IAS when in ALT Hold mode, and the plane does not pitch up and it starts to descend as soon as he reduces power.   What could be the difference between the plane he is flying and the one I am flying, lol?

 

Thanks!

 

 

[expire 7]

Hi Highlander

Excuse my laziness in asking you this and not researching myself - but when did you buy and download your Twin Otter?  If it was recently would Hotfix 1-4 not have been incorporated?

 

I ask because I am having trouble with 'nose up' and cannot get a speed higher than about 110 kts. (In fact my question to the forum immediately precedes yours).  I wonder if I need to look at Hotfix 1-4?

 

(I blew the engines up yesterday in my quest for speed).

 

All the best

 

Mike

 

[MarkHurst 173]

20 hours ago, highlander_821 said:

The only difficult issue I am having is that I get the pitch up when I try to engage IAS mode for descent. I also get it if I dis-engage ALT Mode.   It does settle back down after a few seconds and works normally, but by then the speed has bled off around 15 knots or so.  So if I engage IAS mode at around 160 kias, and immediately reduce power,  I am down to around 140 kias when it finally decides to start descending.

 

 

Can you give an exact sequence of events that causes the faulty behaviour, preferably from before you engage the autopilot to the point hwere you get the problem. Then we can try to repeat it or perhaps spot some error of procedure. I wonder, for example, if you are tweaking the elevator trim or the yoke while the AP is engaged? You shouldn't be doing that. I'm not sure how IAS hold is used in real life except for one procedure describe elsewhere in this forum, which is to select it on climb-out before engaging the AP. For a descent I would guess you should be on ALT hold, reduce the power and when the speed falls to what you want, switch to IAS hold. Then reducing the power should begin a descent. From memory I have found that the IAS hold doesn't want to hold anything higher than about 120kt, or maybe I'm doing something wrong...

[highlander_821 8]

On 11/14/2016 at 0:06 PM, MarkHurst said:

 

 

Can you give an exact sequence of events that causes the faulty behaviour, preferably from before you engage the autopilot to the point hwere you get the problem. Then we can try to repeat it or perhaps spot some error of procedure. I wonder, for example, if you are tweaking the elevator trim or the yoke while the AP is engaged? You shouldn't be doing that. I'm not sure how IAS hold is used in real life except for one procedure describe elsewhere in this forum, which is to select it on climb-out before engaging the AP. For a descent I would guess you should be on ALT hold, reduce the power and when the speed falls to what you want, switch to IAS hold. Then reducing the power should begin a descent. From memory I have found that the IAS hold doesn't want to hold anything higher than about 120kt, or maybe I'm doing something wrong...

I take off, get trimmed properly, retract flaps, re-trim if necessary, set climb power, then engage AP with heading mode selected.   Then when I reach around 95-100 kias, I engage IAS mode and set the alt alert button.   The plane levels off at my selected altitude, I set cruise power, cruise to the point where ATC wants me to descend.  I set the alt alerter to the alt I want to descend to, then hit IAS.   The plane pitches up a few degrees, I reduce power, it eventually starts to descend, but not until it has bled off around 15-20 knots of speed, due to the unwanted pitch up.   At no time do I manipulate the yoke, trim controls, or the pitch knob on the yoke.  

On 11/14/2016 at 5:36 AM, expire said:

Hi Highlander

Excuse my laziness in asking you this and not researching myself - but when did you buy and download your Twin Otter?  If it was recently would Hotfix 1-4 not have been incorporated?

 

I ask because I am having trouble with 'nose up' and cannot get a speed higher than about 110 kts. (In fact my question to the forum immediately precedes yours).  I wonder if I need to look at Hotfix 1-4?

 

(I blew the engines up yesterday in my quest for speed).

 

All the best

 

Mike

 

I just bought it when I heard about the sale a week or two ago.

 

[MarkHurst 173]

19 hours ago, highlander_821 said:

I set the alt alerter to the alt I want to descend to, then hit IAS.   The plane pitches up a few degrees, I reduce power, it eventually starts to descend, but not until it has bled off around 15-20 knots of speed, due to the unwanted pitch up.

 

I see the same behaviour, but only when the aircraft is flying faster than about 122kias. If I trim for slower than that and do the same thing, the IAS hold works like you are expecting. I have concluded (as I said earlier) that there is an upper speed limit for the IAS hold. There also seems to be a lower limit of about 88kias. I can't say if this is realistic, but it is plausible that the limits might be connected to the amount of elevator trim available.

[highlander_821 8]

Thanks Mark.  I'll play with that this weekend if I get the chance.   I'll also review froogle's Youtube video and see if he is engaging IAS for descent at or below 122 kias or so.  In his videos, there is no pitch up when he engages IAS for descent.

[highlander_821 8]

For what it is worth, I get a similar reaction if I try to descend using the pitch wheel on the yoke.   ALT hold cancels as soon as I rotate the pitch wheel to "down", but the aircraft pitches up and bleeds off my speed before settling down and pitching down as commanded.

[highlander_821 8]

And just in case anyone from Aerosoft is reviewing this thread, I am running the Twin Otter in FSX-SE under Windows 7 64.    Not sure what other information would be relevant in this case.

 

[highlander_821 8]

On 11/16/2016 at 1:25 PM, MarkHurst said:

 

I see the same behaviour, but only when the aircraft is flying faster than about 122kias. If I trim for slower than that and do the same thing, the IAS hold works like you are expecting. I have concluded (as I said earlier) that there is an upper speed limit for the IAS hold. There also seems to be a lower limit of about 88kias. I can't say if this is realistic, but it is plausible that the limits might be connected to the amount of elevator trim available.

I've done a few more flights.  I don't really see what you are seeing in terms of IAS always settling on 122 kias or thereabouts.   For me, it just seems to subtract around 18-22 knots from whatever speed I am going when I engage.  Enage at 160kias, it eventually descends and holds speed at ~140 kias.  But always with the unwanted pitch up, which is what causes the speed to bleed off. 

Kind of a shame, really.   I am pretty much ready to love this plane in every other way, it is a real beauty and sounds and otherwise performs top-shelf.  But this is a real buzzkill.   There is no way that the actual aircraft does that.  Passengers would scream their heads off!

 

I would like to see if Aerosoft has anything to say or any suggestions for a workaround or something.

[MarkHurst 173]

3 hours ago, highlander_821 said:

I don't really see what you are seeing in terms of IAS always settling on 122 kias or thereabouts

 

Oh well, maybe I have it wrong. I'll try again a bit more rigorously when I next have the FSX PC up.

[expire 7]

Sorry to intrude again Highlander, but I am not sure that everything is as it should be with SE.  As I mentioned above, I am having a pitch up and speed  problem caused by the propeller pitch malfunctioning (as far as I can gather).

 

Are you using FSUIPC by any chance?

 

If so, the next time you get the problem try opening FSUIPC and pressing 'OK' on the Propeller Pitch calibration page.  Things return to 'normal' for me when I do this.  (I used to go through through the while business of recalibrating the pitch but I have found that this is not necessary).

 

Mike

 

 

[highlander_821 8]

13 hours ago, expire said:

Sorry to intrude again Highlander, but I am not sure that everything is as it should be with SE.  As I mentioned above, I am having a pitch up and speed  problem caused by the propeller pitch malfunctioning (as far as I can gather).

 

Are you using FSUIPC by any chance?

 

If so, the next time you get the problem try opening FSUIPC and pressing 'OK' on the Propeller Pitch calibration page.  Things return to 'normal' for me when I do this.  (I used to go through through the while business of recalibrating the pitch but I have found that this is not necessary).

 

Mike-

 

--

You are not intruding.  I welcome your input.   I had a look at the relevant FSUIPC section, and didn't really understand what I was looking at.   My other two turboprops that have prop levers seem to work fine with the Z axis on my CH yoke, so I don't think I have an issue in that department anyway.

I did try something different today that seemed to have good results.  I fueled her up at about 50%, using equal quantities in both AFT and FWD tanks.   When I went to descend, I was going about 150 kias, and engaged IAS after cutting power just a little.   No sudden pitch up, and the plane immediately responded to a further reduction of power and descended and held about 145 kias.   So I'm going to play with fuel loads for a few flights and see what happens.  

[highlander_821 8]

So I can confirm that the fuel load is a workaround for this pitch up issue that I am having.   If I load both the fore and aft tanks with 750 gallons each, and leave the others empty, I get correct behavior with the IAS mode for descent.