Flight Characteristics of DHC6 Twin Otter

[Tom Goodrick]

This space is for discussion of how the flight characteristics of FS9 models of the DHC6 differ from the real one.

To begin with, we were discussing pitch response to flap extension.

[Tom Goodrick]

The following is a text by Lou Ross related to the DHC6 Twin Otter. I have copied it from the Blog section to this part of the Forum where discussion of this amount of detail is more appropriate.

Lou says:

"This is what I can give now. I'll do one or two quick flights today or tomorrow for more info. So, this is Part 1.
FS9; premieraircraft.com , Tamarik/Lab Air Wheeled, Windows XP Pro. I have two others from the Premier in livery of another virtual airline- no idea what the original model was, but these figures are essentialy the same.

1) Weight Errors. The enclosed description with the a/c lists the max weight as 12,000lbs. In FS, upper tool bar/airplane/fuel and load, the max weight is listed as 11,600 lbs. The real world max weight is 11,579 lbs.

2) Fuel Weight. The FS program gives a max fuel weight of 2,532 lbs of fuel. The real world a/c holds 2,475 lbs.

3) Fuel Distribution. FS list the fuel as a left and a right fuel tank (in the wings). The real a/c has a fore and aft tank in the bottom of the fuselage.

4) Basic Operating Weight. Is apparently too high. I distributed 14 pax, 2 pilots, added 1099.5 lbs fuel, and maybe 640 lbs baggage and was at max gross wgt- or a titch over. In the real a/c you can add 19 pax and 1000 lbs of fuel and baggage and be within the max gross weight.

5) Baggage. The default setting apparently puts 400lbs in the rear baggage compartment and 100 lbs in the nose. I'm sure the real baggage compartment holds more, maybe 600-800 lbs, but I don't remember, and I don't remember any baggage ever actually being loaded in the nose because there is virtually no room up there. The FS weight program shows the 2 compartments as the more rearward of parameters.

6) Fuel Shut-off Valves. On the Fire Panel, there are 2 fuel shutoff valves and 2 fire pull handles. Forget the fire pull handles as they are there for looks. The fuel shutoff valves are to shutoff fuel to the engine in case of engine fire (real a/c). In FS, these are set up to operate the fuel boost pumps, and they are automatically liked to the fuel lever on the overhead panel- why this fantasy was created is beyond me.

7) Fuel Levers. On the overhead panel, should operate as either ON or OFF. This is the simplest procedure I've come up with to at least get it as accurate as possible: Fuel lever OFF; note the fuel switches on the fire panel will go off. Very slowly move the fuel levers forward just until the fuel switches move to ON. Leave the levers in that position- as far to the rear as possible. The problem in FS is as you continue to move the levers forward, engine speed increases, which is not correct. Fuel levers do not control engine rpm in the Twin Otter.

8) Prop Torque. The relationship between Torque and prop rpm is totally incorrect. Take for example a torque setting from a low cruise to a high cruise setting. As you change the prop rpm, torque should change. In The FS program, torque remains fixed.

9) Prop Lever control is very poor.

10) The temps on the ITT gauge are way too high.

11) The beta system of prop control is totally absent. Probably an MSFS limitation.

12) Other minor errors. Common to all but a dozen or fewer a/c programs, not really important to many simmers. But here are a few: no DC Master Switch, no boosts pump switches, no bleed air switches, default MSFS radios.

13) Engine Sarting. Procedure is not correct, as is most other jet engines in FS. The correct procedure is to spool the engines, in this case as I remember, to 20%, then move the fuel lever to ON, the engine will ignite, and you must monitor ITT Temp to prevent a possible overtemp on start. MSFS starts the engines the same as piston engines, which would cause the engine to "melt off the wings" in the real world.

Flying Note: At this point I can only say that the flight dynamics is not at all good. Example, you should be able to set the flaps at 10 degrees, or 20 if you wish, props at 80%, add power and get airborne easily and climb. This program you can do it, maybe, but it is very difficult.

I'm not a programmer, but correcting the above problems would be a major advance in the operation of the Twin Otter. I suupose it would take some time to do so.

Will get to the flyig part As soon as I can.
lr."

[Tom Goodrick]

Okay. I will discuss each of these items mentioned above by Lou.

"1) Weight Errors. The enclosed description with the a/c lists the max weight as 12,000lbs. In FS, upper tool bar/airplane/fuel and load, the max weight is listed as 11,600 lbs. The real world max weight is 11,579 lbs."

Here I am confused. Are you sure these numbers apply to the DHC6? They sem more suited to the Beech 99. There may be different versions of the DHC6. I have two sources (FLYING and JANE'S) that list max takeoff weight as 12,500 lbs for the DHC6. The FLYING spec pertains to a 1985 model.

"2) Fuel Weight. The FS program gives a max fuel weight of 2,532 lbs of fuel. The real world a/c holds 2,475 lbs."

Here again we may have confusion between the DHC6 and the Beech 99. My FLYING spec has the fuel weight for the DHC6 at 3190 lbs and the fuel weight of the Beech 99 at 2466 lbs. In FS we must enter the fuel (in the FD files) in gallons and then the program converts it to lbs using a conversion factor that is also in the FD files so it can be changed. Jet fuel is notorius for varying in density depending on additives like anti-icing.

My version of the FS model has a conversion factor of 6.60156 (in the .air file). Many people use 6.7. My aircraft.cfg file shows 193 gal in each main tank and 45 gallons in the two aux tanks located in each wing. That totals to 3142.3 or 3189 using 6.7 lbs/gal. I think I will change the fuel weight factor to 6.7.


"3) Fuel Distribution. FS list the fuel as a left and a right fuel tank (in the wings). The real a/c has a fore and aft tank in the bottom of the fuselage."

Are you saying the aux tanks should be in the fuselage or are these the only tanks? I'd be very surprised if they are the only tanks, especially since the engines are mounted on the high wing. Fuel in belly tanks would depend on a strong pump. When carrying passengers, aren't the aux tanks seldom used?

"4) Basic Operating Weight. Is apparently too high. I distributed 14 pax, 2 pilots, added 1099.5 lbs fuel, and maybe 640 lbs baggage and was at max gross wgt- or a titch over. In the real a/c you can add 19 pax and 1000 lbs of fuel and baggage and be within the max gross weight."

It is nice to use BOW but we seldom find that in the commonly available specs. We see empty weight instead and add the crew weight as well as all payload. I have 7441 lbs empty weight and always include 200 lbs for each pilot. In my file I have only 8 rows for two pax each. I reduced nose baggage to 20 lbs and put 640 lbs in the back baggage so each of the 16 pax on a commuter flight can bring their full allowed baggage. I only flew on a DHC6 once (in Houston, 1979). But on some commuter flights some baggage had to be carried on a later flight. This works all right if you have an MTOW of 12500 lbs.

In FS we have to use the published specs rather than a particular value for a certain aircraft.

"5) Baggage. The default setting apparently puts 400lbs in the rear baggage compartment and 100 lbs in the nose. I'm sure the real baggage compartment holds more, maybe 600-800 lbs, but I don't remember, and I don't remember any baggage ever actually being loaded in the nose because there is virtually no room up there. The FS weight program shows the 2 compartments as the more rearward of parameters."

I don't think you are reading this right. Just because baage areas are the last listed does not mean they are both positioned aft. Their position is determined in the aircraft.cfg file where X, Y and Z positions are given for all stations carrying loads (including people).

"6) Fuel Shut-off Valves. On the Fire Panel, there are 2 fuel shutoff valves and 2 fire pull handles. Forget the fire pull handles as they are there for looks. The fuel shutoff valves are to shutoff fuel to the engine in case of engine fire (real a/c). In FS, these are set up to operate the fuel boost pumps, and they are automatically liked to the fuel lever on the overhead panel- why this fantasy was created is beyond me."

In FS various panel designers do these details in various ways. I don't have any fuel shutoff valves. But when I tap the certain pair of keys, the fuel shuts off.

"7) Fuel Levers. On the overhead panel, should operate as either ON or OFF. This is the simplest procedure I've come up with to at least get it as accurate as possible: Fuel lever OFF; note the fuel switches on the fire panel will go off. Very slowly move the fuel levers forward just until the fuel switches move to ON. Leave the levers in that position- as far to the rear as possible. The problem in FS is as you continue to move the levers forward, engine speed increases, which is not correct. Fuel levers do not control engine rpm in the Twin Otter."

I finally understood this question as I was flying the DHC6 this evening. What you are calling "fuel levers" Beech calls "condition levers." I use the "standard FS turboprop" engine control gauge borrowed from the default King Air 350. It sounds like you have the same one. The three levers are actually labeled "Power, Prop and Condition levers" (shown in vertically spaced letters to the left of each lever. I understand from reading about turboprops that even the same engine can be given different controls in different applications. The effect of these controls depends on whether or not the engine is "flat rated." The lable "Fuel Cutoff" applies not to the whole lever but to the bottom position of the lever which trips a cutoff switch. When I do a start using Cntrl-E, the condition levers go to Low Idle. I leave them there for the duration. Some people recommend theses levers be set in the High Idle position on final. But the standard takeoff position is Low Idle. To shut down I use Ctrl-F7 which I defined to cut off the fuel on jets. I see now that you can cutoff the fuel on the turboprops with this set of controls using the same controls as used on mixtute in piston engines. (That is just for convemience of the programmers.)

Power output is proportional to torque multiplied by prop RPM. If prop RPM is held constant, then only the torque will vary when you change power. But you can also change power if the torque remains fixed when the prop RPM is reduced. Thus power will change on my gauge if only the prop level is used to reduce RPM. It is common practice on Beech turboprops to take off at the max RPM which may be 1700 as it is on my DHC6 (is 1700 correct?) and then to reduce that to 1500 for cruise to reduce the cabin sound level. If the torque is left high, this reduces the power about 12%.

I understand we should not be seeing N1 in a turboprop. N2 is always proportional to RPM. In a turboprop we should see Ng. I don't know what the difference is between N1 and Ng. But N1 is a parameter we can read from the sim.

"8) Prop Torque. The relationship between Torque and prop rpm is totally incorrect. Take for example a torque setting from a low cruise to a high cruise setting. As you change the prop rpm, torque should change. In The FS program, torque remains fixed."

This is the engine torque appliked to thne prop as discused above. But doen't this change depending on the type of engine control system used? For example I am suspect for an engine with a flat power rating this is true. But for a general turboprop engine this may not be true. On some new aircraft like the Meridian, the prop RPM is fixed at 2000 RPM. Throttle controls the torque only.

I made a gauge that shows turboprop power in % of max power. It does this by reading the % torque and the % turbine speed and multiplying the two. I use this in setting the power for cruise, climb, descent, etc because we don't have any operating manuals showing all the power-setting curves.

"9) Prop Lever control is very poor."

I suppose you are referring to the oscillation in the prop RPM when you try to change it from one setting to another. I am troubled by this too. It is caused by an inappropriate setting of the prop inertia. How do we know when we have the right sensitivity setting? Should it quickly settle on the new value with no oscillation at all?

I was noticing a lot of oscillation when I tried changing cruise RPM from 1700 to 1500. I changed the prop moment of inertia in the aircraft.cfg file from 120 to 12 and the problem went away. It is now very easy to set an exact RPM. If that's how it should be, I'll gladly leave it that way.

"10) The temps on the ITT gauge are way too high."

This can be controlled easily within the .air file. But how sensitive should this be? I know that 800 degrees is a normal operating limit (though it varies with some versions of these engines). The default setting will give you 860 or so if you use full throttle on takeoff. Don't you normally have to monitor this and adjust it on takeoff to keep the temps below 800? I just changed the scale factor from 1 to 0.8 and now I see less than 800 with full throttle through climb. Am I cheating to make it easier?

We also have indications from another pilot that the variation of ITT with altitude is not right. FS shows it always decreasing with altitude while this other pilot says it often increases and that the increase determines what power settings you should use in climb. If so, we don't see that and I can't figure how to make it happen.

"11) The beta system of prop control is totally absent. Probably an MSFS limitation."

This is not true but it depends on what controls you use. I use the keys F1-F4 to control the throttle rather than a lever on the yoke panel or joystick or by mousing the picture of a lever on the screen. As I begin a flare, I have the throttle almost at zero and then I tap F1 as the wheels touch to make sure the throttle is at zero. Then I can get beta pitch by holding F2. I see a digital indication on my panel of the throttle position so I can adjust the amount of beta pitch I have. This can be done on turboprops while flying. There are a few challenging places in the Alps where this comes in handy.

"12) Other minor errors. Common to all but a dozen or fewer a/c programs, not really important to many simmers. But here are a few: no DC Master Switch, no boosts pump switches, no bleed air switches, default MSFS radios."

I always have a DC master on my panels, others don't. Few of us make our own gauges. (A switch is a "gauge" and must be programmed in C). We canabalize other people's panels to grab switches and special gauges. But there are not many turboprop panels to choose from.

"13) Engine Sarting. Procedure is not correct, as is most other jet engines in FS. The correct procedure is to spool the engines, in this case as I remember, to 20%, then move the fuel lever to ON, the engine will ignite, and you must monitor ITT Temp to prevent a possible overtemp on start. MSFS starts the engines the same as piston engines, which would cause the engine to "melt off the wings" in the real world."

I use Ctrl-E to start the jet and turboprop engines. This equivalent to hitting the auto start button on most jets. When I do that, I get exactly the proper engine start procedure you describe.

"Flying Note: At this point I can only say that the flight dynamics is not at all good. Example, you should be able to set the flaps at 10 degrees, or 20 if you wish, props at 80%, add power and get airborne easily and climb. This program you can do it, maybe, but it is very difficult."

With some adjustment this is possible but a little back pressure on the stick is often needed "to unstick the nose wheel."

"I'm not a programmer, but correcting the above problems would be a major advance in the operation of the Twin Otter. I suupose it would take some time to do so.

Will get to the flyig part As soon as I can.
lr. "

You don't have to do any programming to make these changes. You simply edit text documents that tell you in plain English what the name of each parameter. You edit in a change to the number and save the file. Then the aircraft flies with that change. We'll give you a few pointers to get you going. See the next thread I add below.

Tom Goodrick

[Tom Goodrick]

Here are some notes about editing the "FD files." There are two of them that control the flight performance of an aircraft model. One is the ".air file" or "airfile." (Its actual name is name.air where "name stands for the aircraft model dame.) The other is the aircraft.cfg file. (That is its actual name.) When you edit these files, you look up parameter names, see the existing value, and edit that value as if you were editing a mistake in a letter your are writing. It is not really programming. For example to change the jet fuel conversion factor from 6.6 to 6.7, you find "fuel conversion factor = 6.60215 in the airfile and change it to "= 6.7" and save the .air file. To set a new max gross weight you look up "max gross weight = 12500" in the aircraft.cfg file and change it to "= 11576" or whatever.

The .air file in general has some parameters that are pretty deep in aerodynamic theory. But many are not. For example it is the only place you can adjust the drag of the landing gear. Just look up that parameter and either increase or decrease the parameter and then save and check the effect.

The aircraft.cfg file was intended as a "user adjustable" file. It has pretty clear parameter names. You do need to be a little careful but, if you jot down the original value, you can always go back and reset it.

The .air file must be edited using a speacial program written for that purpose. The one I use is "AAM" or Aircraft Airfile Manager. You can find it on many FS sites. It is free. In the .air file, most parameters are given with just a single value. But some values are set by a chart. Changing these is possible but care must be taken. An example of this is the lift coefficient chart showing variation with angle of attack. This is where you would set the stall speed and the cruise pitch angle in a very indirect way. Details are described in many other articles on this Forum.

The aircraft.cfg file is just a text file. You can use NotePad to edit it. You can tell your Windows Explorer to use NotePad to open it when you double click on it in the Windows Explorer list. (The same applies to the panel.cfg file which you may also want to edit to install a new gauge on the panel or delete and old one.)

The aircraft.cfg is always the final authority in any case where the same parameter is includie in both files. In many cases a parameter is given by an exact numerical value in the .air file but can be modified in a relative sense in the aircraft.cfg file. Forexample, aircraft profile drag is given as a coefficient in the .air file and then can be modified with a "scalar" or simple factor in the aircraft.cfg file.

It is best to be methodical about editing FD files. Keep a notebook handy and jot down original values. Try several values and see which works best. Often, the "right" value is unknown.

[louross]

Okay:
Point-
1) Is correct as I stated.
2) I am probably wrong here, the 2,475 if the correct fuel weight for the B99.
3) Is correct as I stated. Boost pumps are used.
4) The a/c I have from Premeir has something like 24-27 load staions (rows).
The a/c carries 19 pax. You can use EW rather than BOW, but the final result is the same. Yep, MSFS doesn't know BOW. 170 is standard pax and pilot weight. Add xxx for pilot baggage. On occasion we had to send baggage on a later flight- bad wx requiring an alternate.
5) Could be. I didn't check for any CG movement when I put weight in the second to last load station. Maybe that station is for pax, and the first one is for nose baggage. But, I think the program shows the first statin as pilot at 170 lbs. Not 100% -will have to check.

7) Oh, you need to upgrade. The power control quadrant is the B99. Completely different from the DHC6.
B99 is - The "fuel condition lever" has OFF, Low Idle, High Idle. In real world use the fuel condition lever is always in low idle. On occasions high idle is used only for stating in certain conditions. This concerns the PT6A engine. Forget this FS nonsense that high idle is necessary for taxying, takeoff, descents or whatever other fantasies. Again, this is for PT6A engines- only.
8) Is correct as I stated. Again, we are discussing the PT6A engine on a DHC6, with a bit of discussion about the B99. This info applies to no other engine or a/c. I know nothig about the Saab, Dash 8, Garrett, etc.
In the real a/c, power for each realm of flight is set primarily by torque. Temp can be a limiting factor, usually in the summer, N1 is seldom a limiting factor. The first company I was with (DHC6) did not use a calculator for torque computings. They ran it to the max. The other company (B99) did use a torque computer. Actually it was a chart, not a computer. As stated, (DHC6) we used 80% prop rpm for takeoff, cruise, and approach. Upon touchdown, the PNF moved the prop levers forward. If an engine failed, PF feathered the dead prop, then moved the other prop forward as necessary, depending on the flight realm. However, the procedures were different when I went to the other company flying 99, props were full forward on T.O., reduced for climb, and reduced at cruise (PNF). PNF moved them forward on touchdown. The 80% DHC may have been company procedure, but it was very adequate; the B99 procedure was necessary.
9) You are basically correct. Prop rpm sould be smooth, definite, immediate. Essentially exact. King Air procedures could be different from the B99.
10) Whatever the gauge shows for the limit, is the limit. The problem occurs at high field temps in the summer, in which case your limiting factor in setting takeoff power is the temp- you will not reach the max torque setting. However, the temps were never in the yellows during cruise.
11) Whatever you say. What real world a/c have you flown with beta? My statement is correct as written. You can move the power levers to idle when landing. If you hold F2 the props go into reverse. I'm not talking about reversing. I'm talking about beta control.
13) I don't use CNTL + E, so if it works, great.
lr.

[louross]

Just a quick point, after thinking about it, I think you have a misunderstanding about beta. Beta has nothing to do with idle and reverse thrust. And apparently it is unique to turboprops, and probably unique to the PT6A because it is a free turbine, but again, I'm not 100% on that beause I only know the ops of that engine.
Beta is a situation where prop pitch is controled by the power levers- not the prop levers. The props are full forward and while taxying the position of the power levers sets the prop rpm. This is not modeled in MSFS.
lr.

[Tom Goodrick]

Okay. I had thought beta was the same as reverse thrust. That is what I had read in magazines.

Regarding the fuel tanks, I understand you are saying that the only tanks are in the fuselage. Are they on left and right sides feeding each engine? How does fuel use affect the CG?

[louross]

The fuel tanks are fore and aft, not right and left. I don't remember any specific issues concerning CG with them. As I remember, we had no problems with CG in the DH6. There was an issue with the 99 in that if you were light the CG was too far forward and you had to ask the few pax to move to the rear. If you were at gross weight, you had a rear CG problem, if I remember correctly, so baggage was loaded in the nose. Actually I left the company with the Otters in order to go with the company flying the 99's. Seems to me fuel loads affected the 99's CG, but not the Otter's. The 99 to me was more of an airplane, and had its peculiarities. The Otter was pretty straight forward, very true, and got to be rather boring. I always thought of it as a heavy 182.
Apparently the magazines you had read were wrong. I see a lot of mistaken ideas out of the various flight sim forums.
lr.

[Tom Goodrick]

Let's go back to the subject of "the beta system." My concept is based on the fact that, in the aeronautical engineering business, we call the prop pitch anngle "beta" and use the greek letter to show it. Your concept is not completely correct. In the case of taxiing it may be appropriate. A casual read of my sources indicates Beta is reverse thrust. But a more careful read (which I had not done before) indicates there may be a distinction. When the levers in a real aircraft are lifted over the gate, the first position brings BETA and then further movement gives reverse thrust. This tells me that BETA is a slight negative thrust condition and that additional pitch change brings stronger negative or reverse thrust. The fact that BETA comes after the gate shows it is not something one uses casually in flight.


You wrote:
"11) The beta system of prop control is totally absent. Probably an MSFS limitation."

I wrote:
"This is not true but it depends on what controls you use. I use the keys F1-F4 to control the throttle rather than a lever on the yoke panel or joystick or by mousing the picture of a lever on the screen. As I begin a flare, I have the throttle almost at zero and then I tap F1 as the wheels touch to make sure the throttle is at zero. Then I can get beta pitch by holding F2. I see a digital indication on my panel of the throttle position so I can adjust the amount of beta pitch I have. This can be done on turboprops while flying. There are a few challenging places in the Alps where this comes in handy."

You responded:
"11) Whatever you say. What real world a/c have you flown with beta? My statement is correct as written. You can move the power levers to idle when landing. If you hold F2 the props go into reverse. I'm not talking about reversing. I'm talking about beta control."

You added:
"Just a quick point, after thinking about it, I think you have a misunderstanding about beta. Beta has nothing to do with idle and reverse thrust. And apparently it is unique to turboprops, and probably unique to the PT6A because it is a free turbine, but again, I'm not 100% on that beause I only know the ops of that engine.
Beta is a situation where prop pitch is controled by the power levers- not the prop levers. The props are full forward and while taxying the position of the power levers sets the prop rpm. This is not modeled in MSFS."



Edward H Phillips is a respected aviation journalist and an experieced pilot who has written many pilot reports for AVIATION WEEK AND SPACE TECHNOLOGY. In an article on the Beech King Air C90B in September of 1992 he wrote the following:

"The main gear touched down smoothly, followed by the nose gear. Once again, the C90B had proven itself a docile machine. I lifted the power levers up and moved them back into the beta range for reversing and applied the brakes to slow the aircraft before exiting the runway."

The text book VAN SICKLE'S MODERN AIRMANSHIP has long been a respected source of infomation on aeronautics and aviation. I use the 5th edition edited by John F Welch, printed in 1987. In the section on flying higher-performance aircraft and "Turboprop Controls", this text says:

"The power levers, when lifted over a 'gate' or stop, provide 'BETA' control and propeller reversing."

And, referring to the Beech B200 King Air:

"Landing techniques are normal. After the aircraft slows and the nose gear is allowed to make runway contact, the power levers may be lifted over their stop gates and retarded into the BETA range for rapid deceleration. For maximum stopping effect, the power levers are moved farther aft through BETA into the reverse position, and aggressive braking action is applied. If he uses reverse thrust, the pilot should bring the levers out of reverse at 40 knots, to minimize likelihood of foriegn object damage (FOD)." {Italics are in the text.}

It would be great if someone explained the term BETA a little better. I have seen it referred to simply as negative pitch.

You added: "Apparently the magazines you had read were wrong. I see a lot of mistaken ideas out of the various flight sim forums."

This is a little strong. We try.

[Tom Goodrick]

"BETA" is the greek letter aeronautical engineers commonly assign to the prop pitch angle (when not using it for aircraft sideslip angle). Here are data I found when playing with the power adjustment on DHC6 while parked at idle. I have added my TPower Panel as a pop-up gauge showing several power parameters.

TAPS_____BETA____________THRUST
+2 (F3)__15.0 degrees____348 lbs
0 (F1) ___1.0_____________-9.5
-1 (F2)___0.8_____________-15.4
-2 _______0.7_____________-21.5
-3 _______0.5_____________-27.5
-4 _______0.3_____________-33.8
-5 _______0.2_____________-39.6
-6 _______0.0_____________-45.8
-7 ______-0.1____________-150.2
-8 ______-0.3____________-159.0

"TAPS" are the number of taps on the indicated key (F1, F2, F3) needed to change beta from the value after F1 is tapped setting the power lever at the "off" position. The gauge reads these values from the sim and displays them. I use this gauge for testing engines. (Anyone can download it from my web site.)

[louross]

I'm not going to argue the point. Whatever you want to think is okay.
lr.