XP-39 II - The Groundhog Day Thread (1 Viewer)

[FLYBOYJ]

Delete No. 12. This chart is for the P-39Q, this item weighed 100lbs on most earlier models.
Move the IFF radio from the tail cone forward to just aft of No. 18. IFF radio not shown on this drawing, weighed approximately 120lbs, located about midway between No. 17 and the leading edge of the horizontal stabilizer. Or delete the IFF radio altogether and save another 120lbs, as the Russians did.

How far do you move the IFF? You need an exact distance in inches to know how its going to effect the c/g. It's not that simple, but please go on.

 

[fastmongrel]

Same for railroads. US went into WW II (2 years late) with 41,000 locomotives, 2 million freight cars and 230,000 miles of track.

An example of how things were similar but different. The UK at the same time had 29,000 locomotives, 0.75 million freight vehicles and 19,000 route miles 50,000 total miles of track. 1.2 billion passenger journeys and 92 million parcels. Clapham junction was (and still is I think) the busiest station in the world with a train movement every 5 seconds at peak times.

so much smaller but more intensively used. Mind you most British locomotives would have fitted into the tender of a UP behemoth with room to spare.

 

[FLYBOYJ]

My statements agree exactly. I stated that the nose armor was not needed on the P-39M and that the M was an early model with similar weight and weight distribution as earlier and later models. All the P-39 models were balanced whether they had the heavier 37mm cannon or the lighter by 140lbs 20mm cannon.

Then I said that Bell was able to balance the plane with varying weights for different components located fore and aft of the CG. When I said the plane I was referring to all the P-39 models that had different components. I can see how this could have been misinterpreted.

Let me be absolutely clear: P-39s (and P-400s) had different propellers, nose armor, nose cannons, radios (in different locations) etc that had different weights. Bell was able to balance all the different models with all these different components. The nose armor could have been deleted and the plane could have been balanced.

And the weight and balance charts that show this? As a teacher once told me, "show me the math!"

 

[DerAdlerIstGelandet]

How far do you move the IFF? You need an exact distance in inches to know how its going to effect the c/g. It's not that simple, but please go on.

I get the feeling that some people believe W&B and CG is a simple task, when there really is a science to it. You cannot simply take weight away from somewhere, and add some weight here. When I was on 2nd shift working as an A&P I had to work with QC quite a bit doing them after modifications had been performed. I always enjoyed it.

 

[Elmas]

The number of cars per 1000 people is not a false assumption, it is a fact. However it is just one fact out of many.
Omissis
Italy, France and Japan were even further behind.
Omissis

When at the end of May 1940 Italy was on the verge to declare war to G.B. and France, Italo Balbo was desperate, as in Italy public opinion clearly knew that U.S. would have not remained idle in supporting the British:
"The Duce has never seen the size of the phone book of New York!" he repetedly told his staff.
Probably the phone book of Rome in 1939 was some one hundred pages, that of N.Y. a couple of thousand if not more...

 

[Peter Gunn]

Gentlemen,

I believe you ALL are going about this issue of lightening the P-39 to get a better climb rate out of it the wrong way. I mean, I have no training in aerodynamics or engineering but ye gods and little fishes but you guys are dense. The answer is soooo simple:

I mean... DUH!

 

[FLYBOYJ]

I get the feeling that some people believe W&B and CG is a simple task, when there really is a science to it. You cannot simply take weight away from somewhere, and add some weight here. When I was on 2nd shift working as an A&P I had to work with QC quite a bit doing them after modifications had been performed. I always enjoyed it.

I pulled some other charts from what I can find on the internet. It seems that the P-39N used a %of MAC for it's CG envelope, the P-39Q uses inches aft of a datum line which I believe is at the nose of the aircraft. I see similarities between both models and can probably "interpulate" a C/G range. I have more info but let's see what our friend comes up with.

 

[wuzak]

I pulled some other charts from what I can find on the internet. It seems that the P-39N used a %of MAC for it's CG envelope, the P-39Q uses inches aft of a datum line which I believe is at the nose of the aircraft. I see similarities between both models and can probably "interpulate" a C/G range. I have more info but let's see what our friend comes up with.

I still want to know why the Russians didn't remove the nose armour after they removed the IFF.

And I still believe the IFF was kinda important for operators other than Russia!

 

[FLYBOYJ]

I still want to know why the Russians didn't remove the nose armour after they removed the IFF.

And I still believe the IFF was kinda important for operators other than Russia!

Hard to say but from I got from the P-39D W&B chart, the CG range was between 23 and 31 % of MAC. The Chart shows 3 loading configurations operating between 28.9 and 29.5 % of MAC which is not only a tight envelope but shows the aircraft tail heavy. My guess would be to make the aircraft more nose heavy.

 

[P-39 Expert]

Hard to say but from I got from the P-39D W&B chart, the CG range was between 23 and 31 % of MAC. The Chart shows 3 loading configurations operating between 28.9 and 29.5 % of MAC which is not only a tight envelope but shows the aircraft tail heavy. My guess would be to make the aircraft more nose heavy.

CG location is shown on the drawing you furnished in post #286.

 

[FLYBOYJ]

CG location is shown on the drawing you furnished in post #286.

Yes it is shown - So tell us, what's the value? You don't calculate W&B on a pictorial image with no value.

 

[P-39 Expert]

I still want to know why the Russians didn't remove the nose armour after they removed the IFF.

And I still believe the IFF was kinda important for operators other than Russia!

Yes the Russians removed the IFF radio that weighed about 120lbs from the tail section and didn't remove the 100lb nose armor. According to some folks on here the plane should have immediately fallen out of the sky.

Why would the IFF radio be important to the AAF and not the Russians? I don't know.

 

[pinsog]

An example of how things were similar but different. The UK at the same time had 29,000 locomotives, 0.75 million freight vehicles and 19,000 route miles 50,000 total miles of track. 1.2 billion passenger journeys and 92 million parcels. Clapham junction was (and still is I think) the busiest station in the world with a train movement every 5 seconds at peak times.

so much smaller but more intensively used. Mind you most British locomotives would have fitted into the tender of a UP behemoth with room to spare.

I read somewhere that German spies in the US reported back to Germany on the size of American locomotives like the Union Pacific Big Boy and the Germans simply thought they were lying.

 

[FLYBOYJ]

Yes the Russians removed the IFF radio that weighed about 120lbs from the tail section and didn't remove the 100lb nose armor. According to some folks on here the plane should have immediately fallen out of the sky.

It possibly can, but again the c/g calculation will show if this is possible.

 

[Ivan1GFP]

Yes it is shown - So tell us, what's the value? You don't calculate W&B on a pictorial image with no value.

Hello FLYBOYJ,

I did these calculations off the P-39Q chart a couple years ago when I was working on a design project for the P-39.
These numbers might be helpful in interpreting the values from the P-39Q chart:

The Leading Edge of MAC is at Fuselage Station 111.56 inch.
(LE Wing Root to LE MAC is 5.41 inch.)
MAC is 80.64 inch.
CoG at Basic Weight + 200 pound Pilot is at Station 135.88 inch.
That would make it 30.16% MAC.

I specified the pilot weight because it makes a significant difference and it ain't goin' nowhere without the pilot.

- Ivan.

 

[FLYBOYJ]

Hello FLYBOYJ,

I did these calculations off the P-39Q chart a couple years ago when I was working on a design project for the P-39.
These numbers might be helpful in interpreting the values from the P-39Q chart:

The Leading Edge of MAC is at Fuselage Station 111.56 inch.
(LE Wing Root to LE MAC is 5.41 inch.)
MAC is 80.64 inch.
CoG at Basic Weight + 200 pound Pilot is at Station 135.88 inch.
That would make it 30.16% MAC.

I specified the pilot weight because it makes a significant difference and it ain't goin' nowhere without the pilot.

- Ivan.

Excellent Ivan!!!

My calculations are about the same. I'm going to assume that the P-39Q carries the same CG range of 23.0 to 31.0% of MACas the P-39N. Assuming 23.0% of MAC = at station 111.56, 31.0% of MAC should be 140.98?

 

[FLYBOYJ]

Excellent Ivan!!!

My calculations are about the same. I'm going to assume that the P-39Q carries the same CG range of 23.0 to 31.0% of MACas the P-39N. Assuming 23.0% of MAC = at station 111.56, 31.0% of MAC should be 140.98?

P-39 Expert - If you understand any of this feel free to chime in.

 

[Ivan1GFP]

Excellent Ivan!!!

My calculations are about the same. I'm going to assume that the P-39Q carries the same CG range of 23.0 to 31.0% of MACas the P-39N. Assuming 23.0% of MAC = at station 111.56, 31.0% of MAC should be 140.98?

Hello FLYBOYJ,

Station 111.56 is Leading Edge of MAC, so it would be 0% MAC.
Here is how I arrived at %MAC:

Station in inches - Station of LE MAC / Length of MAC
so
140.98 - 111.56 = 29.42 inch
divided by 80.64 = 36.48% MAC

31% MAC would be 24.9984 inch so it would be
24.9984 + 111.56 = 136.5584 inch.

- Ivan.

 

[FLYBOYJ]

Hello FLYBOYJ,

Station 111.56 is Leading Edge of MAC, so it would be 0% MAC.
Here is how I arrived at %MAC:

Station in inches - Station of LE MAC / Length of MAC
so
140.98 - 111.56 = 29.42 inch
divided by 80.64 = 36.48% MAC

31% MAC would be 24.9984 inch so it would be
24.9984 + 111.56 = 136.5584 inch.

- Ivan.

Going back to check my math.

So the T/E of MAC would be Sta 192.2 (111.56+80.64)

So the most aft CG 31% of MAC is at 136.5584

so 23% of MAC would be 125.63~? (a rough calculation, I'm in a meeting)

 

[Ivan1GFP]

Going back to check my math.

So the T/E of MAC would be Sta 192.2 (111.56+80.64)

So the most aft CG 31% of MAC is at 136.5584

so 23% of MAC would be 125.63~? (a rough calculation, I'm in a meeting)

23% MAC would be 130.1072 inch
80.64 inch * 0.23 = 18.5472 inch from LE MAC
18.5472 + 111.56 LE MAC = 130.1072 inch

My Daughter would be screaming at me for significant figures right about now!

- Ivan.