posted 01-10- 08:56 AM               

Could somebody tell me if I am interpreting this right. As far as I can tell, a constant speed prop will be simulated. The blades will change pitch until the pitch of the 75% radius location reaches PropMinPitch or PropMaxPitch.

The reason that I ask is that, following this thread: http://www.fightersquadron.com/ubb/Forum1/HTML/003602.html
I thought I would try to take a look at the Mossie's prop to see if I could learn anything.

Since I don't know how Open Plane handles the PropMinPitch and PropMaxPitch I approximated it by scaling between 0 and (Max Velocity)/(Max RPM). (I put in 400 MPH for Max Velocity and used the Engine MaxHP value to get an reasonable first guess).

I shoved in the values into Excel with some algebra and plotted the AoA VS velocity at different RPMs for each element. This seemed to work!

Anyways, from what I can tell, the inner portion of the Mossies prop is not stalling but is actually at NEGATIVE AoA for full throttle RPM and velocities less than about 350 MPH. (ie the prop is giving reverse thrust for most of the Mossies flight speed).

If the pitch of the first station is increased to about 70 degrees you can get positive thrust starting at 200 MPH and the negative thrust below that speed will be reduced, increasing the low end acceleration. Alternatively, the chord can be thinned out (but that still leaves 17% of the prop working backwards)

The problem with doing this is that Tailslide's calibration of the Mossie's Max Speed and Climb Rate will be thrown off.

Also, my calculations are in no small way dependent on what the PropMaxPitch and PropMinPitch are doing.

On the bright side, checking out the Mossie's prop has helped me understand why real props are usually cylindrical near the hub (Before I thought it was just structural)

-Bishop

posted 01-10- 09:28 AM               

quote:

---

Could somebody tell me if I am interpreting this right. As far as I can tell, a constant speed prop will be simulated. The blades will change pitch until the pitch of the 75% radius location reaches PropMinPitch or PropMaxPitch.

---

As far as I know, that is correct. You can implement a fixed pitch prop by setting the min and max to the same value- that uis what we do for the WWI aircraft. But there are problems with the engine quiting in certain situations, mainly at idle...

posted 01-10- 02:35 PM               

That might explain things a bit.

I was responsible for the 5.1 and 5.2 FM on the mossie and it was those damn props and negative posts that made me drop FM work.

I actually measured the chord at regular intervals on a real mossie prop and plugged the values into the sm. I don't know if they have the same values now but with my R/L measurements and a bit of experimenting with the tips the mossie was really slow to accelerate but climbed too fast.. I tried hard to kill the climb rate while increasing acceleration but never succeeded, maybe you have the answer bishop, I wonder if you could publish the results of your spreadsheet in a web page and then we could check with MH exactly how PropMinPitch etc figure in the equation.

I always felt the props where the single biggest grey area of FM design in OP. We can get data on areas, chords, airfoils etc but I've never found decent info on props.

posted 01-10- 02:51 PM               

Some of the WWI planes would climb to fast relative to their top speeds. I could never lower the climb rate enough without lowering to top speed too much. I used all TailSlide's chart stuff - but I could never get the performance specs I needed. For WWI, the pitch is constant- but in my mind the other data (the twist and airFoil data) was the key... but where to start?

posted 01-11- 01:32 AM               

The tip sections are important for acceleration and rate of climb.

Outer mid sections transition into cruise speeds.

Inner sections for higher speed flight.

Propellor information on the 'net is real tough to come by, that's for sure!

posted 01-12- 07:22 AM               

"Alternatively, the chord can be thinned out (but that still leaves 17% of the prop working backwards)"

Forget I said that. Thinning out the cord would require the next chord value to be increased to keep the area the same, which would require the next one to be reduced, which would require....

Also the 17% actually refers to 17% of the length of the blade... turns out it's only 8% of the blade area.

Elric: I was wondering where somebody could get prop data. (I'm picturing you sneaking a really big ruler into a museum ..."what's under your coat sir?" )
The values I put in the spreadsheet were from the MkXVIII on your site. I'll mail you the spreadsheet so you can check it out.... the calculations are very simplified but it still gives an idea of how changing the pitch values affects the angle of attack.

Sv: Tailslide's chart stuff??? May I ask what this is and where one might get a hold of such a magical source of information???

Pang: Whatever you know, it's more than me. Thanks a heap for the link. I was able to get ahold of the paper used to make those prop calculations. I'm going to go over it this weekend and hopefully learn a little more...

posted 01-12- 09:04 PM               

A plane without a supercharger should only have something like 1/2 the power at 15,000 as at sea level. A plane with a supercharger would have 5% more power than at sea level.

If you search back I posted a document that gives you step by step instructions to get your weight distribution, total weight, handling, sustained climb, speed at altitude calibrated for your plane. It's called the FM FAQ should be easy to find.

If SV has done some testing with prop twist I can add that too but I haven't run into a situation where I needed to alter the default values.

TS

posted 01-12- 09:19 PM               

TS

posted 01-13- 03:54 AM               

I'll search for that document.

It's *because* you have to tweak the props to change the engine performance that I want to look into it a little more. Right now I don't understand it very well but I want to learn.

*If* my calculations are right, then increasing the chord of the innermost section could actually decrease low speed performance becuase the airfoil is at negative angle of attack (ie 200 mph, at max engine RPM gives an angle of attack for the innermost section of about -18 degrees... the angle of attack doesn't reach -5 degrees until the RPM drops to about 60% of the max.) Wouldn't increasing the pitch at the hub have the same effect as increasing the chord but would work for a larger range of engine RPM?

posted 01-13- 05:12 AM               

When I posted this topic I was hoping to find some luckless volunteer who would try this out for me

Is this the FAQ you were referring to: http://www.fightersquadron.com/ubb/Forum7/HTML/000810.html ?

I remember several months ago I was trying to download an FM document you had written from the OPProjects website but the link was dead... is this what I was trying to get?

posted 01-13- 10:47 PM               

Yes, this is the document that was on the OPP site.

posted 01-13- 11:34 PM               

The engine stuff needs allot of work, I wonder why it was implemented as a second class object? With all the great physics modeling, why is altitude not taken into account? Why is the engine sound tied to the throttle instead of the RPM?

These must not be the hardest updates to make, right?