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PlaneBuilder & Jets
Has anyone spent any time working on Planebuilder jets? I'm pretty familiar with how it works with propeller planes, but it's giving me some rather odd results when I try to work up a turbojet. I'm surprised, given that according to the designers notes, it's really more intended for jets than for props. Any pointers? Admittedly, I'm likely not going to need this as soon as I thought IF we decide to stick with the +0 years for jets, but still…..
You have to play around with the numbers a bit.
For the Jumo 004 you've got an sfc of 1.4/b/lbf-hr and a jet velocity of around 540m/s
Playing around with the numbers gives a bypass ratio of 1.45 and efficiency of 15%. Then use the diameter to scale the thrust up to 1980lb.
Planebuilder uses a fairly simplistic model.
The main problem I'm having is with altitude performance: I can get a sim of the Me-262 to have the right range, top speed, etc, but the blasted thing won't fly as high as the real thing.
I'd encountered a similar problem but hadn't gone for simming any historical jet aircraft. Ceiling is about 4000ft too low and speed seems overly optimistic in the transsonic region. The model for engine thrust against altitude isn't very accurate.
I started with a Gotha P.60, but since it never flew I couldn't calibrate it very well. There's plenty of good data on 262s, though, so I went there after I was seeing what I thought was odd in the P.60.
My 262 seems about right for speed, but the ceiling is about 6000 feet low.
I find the height issue affects just about every aircraft I've designed with planebuilder.
Maybe we should add a rough rule of thumb to add 4000ft to any planebuilder report?
In my experience, it over estimates ceilings for propeller aircraft. Most of the published data revolves around service ceilings rather than absolute. Service ceiling is usually defined as when rate of climb drops below a certain value, i've seen 100ft/m 500ft/m and 1000ft/m used.
What I've been using to calculate service ceiling was what the maximum altitude was to maintain cruise speed. I'll agree with RA, it seems to over-estimate ceilings for propeller aircraft, though that might be because of a difference in how we're using it to determine service ceilings.
Friday, April 19th 2024, 11:04am
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