North American B-25C Mitchell Mk.II

[Ivan]

A couple guys around here already know that I have been working on a B-25C Mitchell for quite a while.
The visual model has pretty much been done for a couple years.
The hangup has been the flight model for which I simply cannot get some effects to work.

For each of us, there is a set of characteristics that define an aircraft for us. Those characteristics may be visual, armament, performance, markings, animation, etc. A Corsair without the wing bend just isn't a Corsair. A Spitfire without elliptical wings isn't a Spitfire. A P-51 Mustang can't be a slow plane.

For the B-25 Mitchell, one visual characteristic that defines the aircraft for me is the anhedral of wing outboard of the engines. This appears to me to be a rather peculiar arrangement. In reading up on this, I found an explanation: The original B-25 prototype had dihedral on the outter panels. This was changed to anhedral so that directional adjustments on the bomb run could be made with just rudder trim and the aircraft would not roll.

I wanted to reproduce this effect but could not get the aeroplane to NOT roll with rudder use. A couple days ago, I finally decided to isolate each relevant AIR file parameter and experiment with it independently and not down the effect. I finally got a reasonaby behaving flight model IN THIS regard.

My original B-25C had a steerable nose wheel. I found out that the B-25 actually had a non-steerable castoring nose wheel. Steering was done by differential braking and probably also by propeller effect on the twin rudders.
The problem was that as soon as I made the nose wheel non-steerable, I was not able to control the aircraft on the ground.
Differential braking does work but it is not easy on this plane. I was able to tune the effects on single engine tail draggers, but those same variables had no observable effect on the Mitchell.

Finally, I decided to isolate each change from a steerable Macchi C.202 to a non-steerable B-25C. The change was fairly easy to spot though I have no solution: As soon as the second engine was added, the propeller effect on the rudder disappeared. To restore proper ground handling, this aeroplane will get back its steerable nose wheel. If anyone has a workable alternative, please let me know.

The last problem which hopefully will be simple is tuning the propeller to match the engine output. Hopefully this will be easy.

- Ivan.

 

[smilo]

this is great news!
and she's a real beauty.
you know how much i love the twins.
i will try not to get too excited, though.
i've been waiting for this sweetheart
for a very long time.

now, if we can only keep Dave
from drooling all over the place.

 

[No Dice]

Would an SOH staff member, please bring a mop and bucket to the B25C forums, we have a drooler.

Dave

 

[womble55]

In my various books, the important one, I can't find, they state that the outer wings were level whereas the inner wings kept the original dihedral. Only the first ten, I believe had the full span dihedral.

 

[Ivan]

Last night I spent a fair amount of time testing out a new propeller (off the stock Hurricane Mk.I) and found that althought the sea level speed was increased slightly (about 5 mph), the speed at height was decreased. After a few hours of trying to make things work, I came to the conclusion that the original propeller was probably better all around.

FWIW, my Horsepower and manifold pressure readings were the same as my last full test back in 2008, but the level speeds were down about 3-5 mph at all heights. Perhaps this is because the testing protocol is a bit different. I don't remember how long I was waiting for speeds to stabilise back then, but now, I only wait about 20 seconds, so if I write slow, the aeroplane may occasionally gain another mph by the time I get back to set up for the next altitude.

Hello Womble55,
The attached image is lifted from the Pilot Training Manual for the B-25. I know it LOOKS like anhedral and that is how they describe it. You kind of caught me here. I admit that the wing angles were pretty much set by eyeball rather than dimensional drawings. I wanted to capture "the look" here.

- Ivan.