Propeller questions

[PJ_Godzilla]

I wonder what its quarter chord sweep is compared to the leading edge sweep.  Does anyone have a scale drawing of the wing that measurements could be made off of?

I found this ( http://www.albentley-drawings.com/me262.htm ) and also fired off a request to the SturmVogel people...

[drgondog]

The reference sweep angle quoted is usually the sweep angle for the 1/4 chord line..

[Stoney]

The reference sweep angle quoted is usually the sweep angle for the 1/4 chord line..

Yeah, but all the "soft" information sights like Wiki are showing the leading edge sweep angle, which is only good for analyzing the mach cone sweep.  I haven't imported those .jpg that PJ linked into my CAD program, but it looks like the 1/4 chord is less than the leading edge.  

[EDIT] From my CAD program, it looks like the 1/4 chord sweep is 16 degrees...

[Stoney]

So.. do any AH props occasionally go supersonic?

Well, its possible.  There is two components to the "helical" prop tip speed.  One is the rotational velocity alone, which is a function of rpm and diameter.  To add the forward speed of the aircraft, we have to use a formula:

Vtip = Sqrt(V^2 + Vtip(static)^2)

Where V = forward velocity of the aircraft and Vtip(static) = the rotational velocity of the tip.

Vtip(static) = pi*rps*diameter

rps = revolutions per second

*velocities for V need to be expressed in feet per second

So, if we know the diameter and RPM of a prop, and the forward velocity of the aircraft, we can compute the helical tip speed to see if it approaches or exceeds Mach 1.  Remember though, that the speed of sound changes with temperature and altitude.  Luckily, in AH, we use standard temperatures and standard lapse rates, so mach is easily predicted using standard dynamic pressure tables.

For the P-47M at 2700 engine RPM (equal to 1350 propeller RPM) and 300mph at sea level using the above equations, we find (if I have my prop reduction gear numbers correct):

Vtip(static) = 3.14*22.5*12 ~ 848 fps
V = 440 fps

Then

sqrt(440^2 + 848^2) = 955 fps.  Mach 1 at sea level/standard conditions = 1116 fps.  Vtip = .86 Mach

Now if that same plane comes out of a dive and is carrying 500 mph then:

V = 734 fps

and

sqrt(734^2 + 848^2) = 1121.  1121/1116 ~ Vtip of 1.005 Mach

[PJ_Godzilla]

 Remember though, that the speed of sound changes with temperature and altitude.  

root-gamma-are-tee, if I recall correctly...

Still, per usual, you've calc'ed with aplomb.

Not to highjack but you might be the guy to ask (actually, probably a COADER for this one)... and it came up in FSO last week so I said i'd check. I kind of assume the AH flight models use lumped masses for several of the main lumps of each aircraft. One of those is, I assume also, the fuel tanks. Does the AH model update, in time steps, the mass in the fuel tanks? I'm assuming it must certainly calc the overall mass but also the location associated with each discrete lump (for the inertial props).

[Stoney]

root-gamma-are-tee, if I recall correctly...

Still, per usual, you've calc'ed with aplomb.

Not to highjack but you might be the guy to ask (actually, probably a COADER for this one)... and it came up in FSO last week so I said i'd check. I kind of assume the AH flight models use lumped masses for several of the main lumps of each aircraft. One of those is, I assume also, the fuel tanks. Does the AH model update, in time steps, the mass in the fuel tanks? I'm assuming it must certainly calc the overall mass but also the location associated with each discrete lump (for the inertial props).

I have no idea...  With some luck, this discussion will be compelling enough for HiTech to make an appearance, and he can educate us.

[Stoney]

Actually, I made a mistake above, as I used 12 feet for the diameter of the P-47M prop.  The correct diameter is 13 feet.

So:

Vtip(static) ~ 918 fps versus 848 as I used originally.  At 500 mph and sea level Vtip would then = 1175 fps or Mach 1.05

[PJ_Godzilla]

Actually, I made a mistake above, as I used 12 feet for the diameter of the P-47M prop.  The correct diameter is 13 feet.

So:

Vtip(static) ~ 918 fps versus 848 as I used originally.  At 500 mph and sea level Vtip would then = 1175 fps or Mach 1.05

I jinxed you. But, of course, I wasn't checking the physicals of the ac anyway.

[moot]

Somehow didn't expect the math to be so straight forward..  Thanks Stoney.

With some luck, this discussion will be compelling enough for HiTech to make an appearance, and he can educate us.

And maybe add supersonic prop sounds to the game

[Stoney]

Somehow didn't expect the math to be so straight forward..  Thanks Stoney.And maybe add supersonic prop sounds to the game

Well, determining the speed is pretty straightforward.  What would be very difficult is to model the effects.  Prop efficiency is such a ethereal thing sometimes I'm not even sure what kind of performance penalty would be incurred, or if it would even be worth the trouble to determine/model.  The other thing is that we have rock-solid RPM in-game, with no potential for overspeed.  I don't know if a P-47 could maintain 2700 RPM in a 500 mph dive in real life.

[moot]

But the props' mach sound would only depend on its speed, right? 

[hitech]

root-gamma-are-tee, if I recall correctly...

Still, per usual, you've calc'ed with aplomb.

Not to highjack but you might be the guy to ask (actually, probably a COADER for this one)... and it came up in FSO last week so I said i'd check. I kind of assume the AH flight models use lumped masses for several of the main lumps of each aircraft. One of those is, I assume also, the fuel tanks. Does the AH model update, in time steps, the mass in the fuel tanks? I'm assuming it must certainly calc the overall mass but also the location associated with each discrete lump (for the inertial props).

Yes for consumables, I.E. weapons and fuel.
No for parts of the airplane. I.E. loose a wing tip neither the mass nor the CG change.

HiTEch

[colmbo]

  The other thing is that we have rock-solid RPM in-game, with no potential for overspeed.  I don't know if a P-47 could maintain 2700 RPM in a 500 mph dive in real life.

I'm thinking the potential for overspeed does exist.  I've noticed in a high-speed power off descent in the P47 (either D40 or M, can't recall which it was) the RPM was up around 2000 or so even though I had rolled the RPM back to minimum, once I had slowed the RPM dropped down to around 1000 so it appears that airflow is "pushing" the prop above where I had it set.

I would think you'd have to get well over 500IAS to overspeed the prop.  The minimum governing RPM on the Ham Standard is down at 1200 so the prop is able to put quite a bit of pitch in to slow it....no practical experience diving a Jug at 500IAS...just guessing.  In real life I've never had a prop gain RPM.  When hauling jumpers we used a low RPM setting with high airspeed (well, high for a Cessna -- 170 +/-) and didn't notice any RPM change.

[FLS]

Yes for consumables, I.E. weapons and fuel.
No for parts of the airplane. I.E. loose a wing tip neither the mass nor the CG change.

HiTEch

That's interesting. What changes then just the lift and drag?

[PJ_Godzilla]

That's interesting. What changes then just the lift and drag?

Not just that. While it's true that weight would decrease such that lift to counter gravity would decrease (as would any induced drag), the other significant impact of changing masses is the inertial properties. Recall those as the rotational analog of mass - Tau = I*Omega just like F=M*A. These are simplified reps of vector equations, of course. In any case, Inertia is a function of mass and it's distribution. Hence, all other things equal, you're likely to get more pitch authority if you minimize your pitch axis inertia. Anyway, you can look at all the equations of motion here ( I pull a more or less random source, there are many good texts on this):  http://books.google.com/books?id=AUWCXVepMKEC&pg=PA107&lpg=PA107&dq=aircraft+equations+of+motion+linearized&source=bl&ots=sN1j0dlEoH&sig=ajyS9c3YsxbBZUu905sSTYXpJ1Y&hl=en&ei=SwiNTaiHCejp0gH2xJ2-Cw&sa=X&oi=book_result&ct=result&resnum=6&sqi=2&ved=0CDUQ6AEwBQ#v=onepage&q=aircraft%20equations%20of%20motion%20linearized&f=false

and you can see the inertia terms by their I notation. They go through a nice linearization in this text I searched up at random and arrive, further down, at some linearized forms (see 4.38). It's easier to get an intuitive feel for the impact of inertia terms from those.