Hello, question on WW2 plane
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Thread: Hello, question on WW2 plane

  1. #1
    Kurier auf Stube...pauke! NachtPiloten's Avatar
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    Question Hello, question on WW2 plane

    I have a Dornier 217 E2 that I am finishing up for CFS3. The plane's tail seems a bit heavy as it does not lift off the ground as I would expect (having never flown one or viewed a film I am guessing). So The E2 has a rear spoiler adding several feet to the aircraft and I know that may change the dynamics as compared to the normal length plane. That being said, what do I play with the have the tail raise a bit sooner?

    Also, the plane seems a bit too responsive for a bomber. Albeit it has a wingspan of 19 meters but it weights in at over 30000 loaded.

    Oh, BTW this is my first flight model in a while so be gentle ...:costumes:

    Thanks!!!!

  2. #2
    Could you take a screen shot of the AirWrench Balance tab and post it?

  3. #3
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    here it is

    I changed a few variables for the shape of the wings to better match the sweep and dihedral and the cog went crazy (was 24.2 now you see it is 12.4%). But the information is correct. I could move the model up a bit in max to make the cog more forward, but then the centerline would be more than 25% of the cord. This was an unusual plane with about a 5-6 foot tail section housing a spolier (dive break). The tail does come off at 140-150 kph which is a bit late but all in all it seems ok.

    I am having trouble understanding the critical altitudes for the engines too.

    1600 @ 1.42 ata @ SL
    1380 @ 1.3 ata @ 15100

    so what are the critical altitudes?

    The plane will only fly at 285 mph, according to documents should be around 315-320 at 17200.

    With the old worksheet you would adjust the SL speed to raise the high altitude speed, is that the same? Thanks!

    Oh, I have a wind tunnel graph of the airfoil NACA 2218, can't read it but would that be of use?

    Ted

  4. #4
    Hi Ted,

    Thanks for posting the screen shot. That doesn't look too bad. To get the CoG to 25% MAC, check the 'Direct Edit' button and zero the CoL.

    The dimension that's causing the late tail lift is the distance between the CoG and the main gear. It looks to me like it's over 1 meter from your screen shot. Go to the Contacts tab and set the Lon dimension for lines 1 and 2 to no more than 0.5 meters. That should bring the tail up a lot sooner.

    Hard to tell from your engine numbers exactly where the critical altitude is, but from your airspeed performance specs, as far as CFS3 is concerned, Critical Altitude on the Engine tab should be set to 17,200 ft.

    Then set Vmax at 17,200 ft on the Specs tab to 320 mph. My guess is that Vmax at sea-level would be around 280 mph.

    You can get an idea of where CLmax should be from the NACA graph, but those are typically infinite air foil charts that haven't gone through the corrections for the geometry of a finite wing.

  5. #5
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    Further questions

    Need some help on the v stab position and related measures. How is the position calculated?

    According to the wind tunnel charts I have cl max is 1.6 at 22 degrees AoA. I can't seem to edit this.

    Now that I have set wing sweep (main wing, horizontal, and vertical), dihedral the plane does not rotate until 125 mph. Prior to this she was rotating at around 110 which was where it should be. The Cl max is chart is now very different from what it was earlier with a max of 1.4 (did lower the test weight), where it was nearly 1.55. The wind tunnel tests show max cl at 22 degrees of AoA, not the 19 on Airwrench. And the clmax is slightly over 1.62 on the charts. Any help is good!

  6. #6
    What's the aircraft's clean stall speed at sea level for a given weight? (need the weight too )
    Ed Wilson
    Gauge Programmer
    Gauged Approach

  7. #7
    Quote Originally Posted by NachtPiloten View Post
    Need some help on the v stab position and related measures. How is the position calculated?
    The definition of 'H Stab Lon Position' is shown on the attached diagram. V stab position is measured the same way.

    According to the wind tunnel charts I have cl max is 1.6 at 22 degrees AoA. I can't seem to edit this.
    The only thing you need from the wind tunnel charts is the CL_max. I can explain how to get a critical angle of attack of 22 degrees, but I wouldn't recommend doing that for several reasons.

    Now that I have set wing sweep (main wing, horizontal, and vertical), dihedral the plane does not rotate until 125 mph. Prior to this she was rotating at around 110 which was where it should be. The Cl max is chart is now very different from what it was earlier with a max of 1.4 (did lower the test weight), where it was nearly 1.55. The wind tunnel tests show max cl at 22 degrees of AoA, not the 19 on Airwrench. And the clmax is slightly over 1.62 on the charts. Any help is good!
    This is difficult without seeing what values you're using in your flight model. Basically, 'Vsi: clean stall speed' and the 'Weight used to calculate estimated stall speeds' on the Specs tab are used to calculate CL_max. Set CL_max to 1.6 on the Air Foils tab calculator to calculate the stall speed, then enter the weight you used and the resulting stall speed on the Specs tab.

    Make sure the CoG is still at 25% MAC after your changes for wing sweep.

    Wing sweep will increase the critical AoA. Without sweep, you'd probably be getting around 15 degrees instead of 19. Higher critical angles of attack will increase the speed where the aircraft rotates.

    As far as rotation speeds are concerned, the area of the CL curve near zero AoA is more important than CL_max. Set a realistic cruising speed, then set Specified Pitch Attitude at Crusing Speed to zero on the Air Foils tab.

  8. #8
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    Thanks!!

    This will help!

  9. #9
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    Question Well

    Now. The tail lifts at 110 which reports say is true. The wheels come off at 120 with no bombs full fuel, which is about right. Now, how can the plane accelerate faster for take off. he plane barely reaches these speeds before there is no more runway. Forget it if you load it up with bombs. I have eaten so many tree branches I feel like well it not good. Everything else seems great.

  10. #10
    Quote Originally Posted by NachtPiloten View Post
    Now. The tail lifts at 110 which reports say is true. The wheels come off at 120 with no bombs full fuel, which is about right. Now, how can the plane accelerate faster for take off. he plane barely reaches these speeds before there is no more runway. Forget it if you load it up with bombs. I have eaten so many tree branches I feel like well it not good. Everything else seems great.
    This is an acceleration issue, which depends on the amount of low speed thrust available. The control factor AirWrench gives you to work with in this area is the climb rate - which also depends on the amount of low speed thrust available.

    What's the climb rate? I wouldn't be surprised if you can't find a value specified anywhere, and even if you do, it probably won't tell you the corresponding weight. In any case, trying increasing the climb rate by a few hundred feet per second on the Spec tab, then try your takeoff run again. It should be shorter. You will have to decide for yourself where the appropriate balance is between climb rate and takeoff run distance.

  11. #11
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    Ok

    Well, the climb rate for these planes is well .... they were as someone once said pigs! About 1000 ft/m on a good day. No weights but from what I have read 1/4 fuel, no weapons, no bombs, and one pilot!

    That is the trust scalar?

  12. #12
    Quote Originally Posted by NachtPiloten View Post
    That is the trust scalar?

    No, it's a lot more complicated than just a simple thrust scalar. The aircraft.cfg thrust scalar increases or decreases thrust at all speeds and altitudes. What's necessary here is to tailor the propeller tables so that the thrust produced is appropriate for low speeds (climb and takeoff) and high speeds (maximum speeds) at both low and high altitudes. There are literally thousands of calculations going on under the hood in AirWrench to generate the prop tables.

    Quote Originally Posted by NachtPiloten View Post
    Well, the climb rate for these planes is well .... they were as someone once said pigs! About 1000 ft/m on a good day. No weights but from what I have read 1/4 fuel, no weapons, no bombs, and one pilot!
    My reaction to that is - you're going to need a lot of runway, so if your model is taking a lot, it's probably more accurate than you realize. :d

  13. #13
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    Sparks ...

    You are probably correct. Trying to get 31000 lbs in the air on two engines with small wings. I have customized the take-off position on a couple of runways (to get the full length) never understood my MS gave up 3-400 meters of runway. This seems to help. I'll tinker with the climb rate and see.

  14. #14
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    Status ...

    Success ... I found some data on climb rate different from what I had, entered it and wow, I can take off using ALL of the 2000 meter runway with 3000 kg of bombs!!! Tail lifts at 110-115, wheels off at 125-130 (with a prayer) with flaps at the start position (27 degrees).

    I can tell you this I understand that being attacked by fighters must have made certain parts of your body pucker, but man speeding along at 125-130 mph and just missing those tree tops ...

    .... these guys were crazy!!! :isadizzy:

  15. #15
    NP:

    Speaking of flaps, you might consider the pitch moment of the flaps as a possible aid in lifting the tail if you feel this is still deficent.

    Cheers: T.

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