Building a MIG-21PF - using a Zvezda or RV as a base? (Renamed)

[Kari Lumppio]

Hello!

I wrote "Undeneath one can find a fuselage similiar to the ealier F types". This meant and means the spine tank is add-on to fuselage (shape and geometry) similar - but not the same - as MiG-21F. Please do not misunderstand me. Yes, there is equipment bay behind the cockpit and forward of the fin, too. And the spine tank is between them.

 

Finnish MiG-21 BIS had several pushrods with part number beginning 66 (or even E-66? It was 29 years ago). I think E-66 is the oldest version of the MiG-21? I would not be surprised if some mid-fuselage frames remained the same, too. Perhaps relocated, likely not.

 

What I would like to know (and was asking) is if their intakes were optimized for different angle of attack. Nose tilt 3 degrees down for BIS. For me there is something different in the MF and BIS intakes and I would like to put finger on it.

 

Cheers,

Kari

[KRK4m]

On 2/21/2018 at 10:23 PM, Kari Lumppio said:

What I would like to know (and was asking) is if their intakes were optimized for different angle of attack. Nose tilt 3 degrees down for BIS. For me there is something different in the MF and BIS intakes and I would like to put finger on it.

 

Cheers,

Kari

Using an hour of the weekend time (the Museum is open 9-17 in winter and on all the other weekdays these are my office hours 20km afar too) I have measured the front fuselages of 21PF, PFM, M, MF and Bis. And the facts are as follows:

1. The movable green nose cone housing radar is identical for all five versions 
2. There are two variants of steel intake lip there: PF, PFM, M and MF have 89cm diameter, while Bis has 92cm. Moreover in all variants prior to Bis the lip is 10cm wide and its coning angle follows the whole nose shape, while at Bis there is a visible kink between the 6cm long lip and the fuselage front part. As in Bis the lip front edge is moved 40mm rearwards compared to the M/MF it is easily understood that its diameter is 30mm greater. The lip itself is riveted to the fuselage with 2 rows of 112 rivets each in PF/PFM/M/MF and 2x120 in Bis.  
3. At the sideview all dimensions are the same: 94cm between rear edge of lip and front edge of aux intake door, 202cm between rear edge of aux intake door and the end of fuselage first panel, 160cm length for cockpit upper side panel, 57cm length of debris deflector plate divided 23+34 for 1st and 2nd fuselage panel respectively
4. However there is a notable difference on the fuselage underside: all five a/c measure 88cm between rear end of front u/c bay and datum line of fuselage 1st side panel and all feature the same 124cm long undercarriage doors, but the distance between the rear edge of steel intake lip and front u/c bay forward edge is 99cm for BOTH Bis and M/MF and 101cm for the PF/PFM. Thus it looks that if really the nose part axis of 21Bis is tilted 3 degrees down compared to the 21F, the modification has been made twice - some 2 degrees at the "big mouth" 21PF/FL/PFM introduction and then 1.2 degrees (2cm lower lip edge backwards move at some 90cm intake diameter equals 1.2 degrees) at the R/S/M introduction (the assymetric overnose probe can be the external feature there). But there is NO difference in attack angle between the M/MF and 21Bis. 

There is a visible difference in nose shape between the M/MF and Bis, but it applies only to some 50-60cm long area between the intake lip and the front u/c bay. Bis looks less conical (more bulbous there). But one end of this area (i.e. the fuselage at station described as the forward edge of cockpit canopy) feature the same diameter for both variants, while the forward edge of intake lip is just 3% larger (0.016" in 1/72) which is barely visible. 

To be sure I should also measure the top fuselage distance between intake lip and front of the cockpit canopy. But heavy snowfall today and 7 degrees (C) below zero made climbing the fuselage a little difficult for the 60+ old man and there was no ladder about to help   I hope I will come back to this subject once again soon.   

Cheers

Michael

Edited August 10, 2018 by KRK4m
a typo found

[KRK4m]

On 2/24/2018 at 3:52 PM, KRK4m said:

To be sure I should also measure the top fuselage distance between intake lip and front of the cockpit canopy. But heavy snowfall today and 7 degrees (C) below zero made climbing the fuselage a little difficult for the 60+ old man and there was no ladder about to help   I hope I will come back to this subject once again soon.   

"Soon" proved to last almost half a year as finally yesterday I had a day off enabling us (JWM and yours truly) to visit the Museum Fishbeds again (this time with a ladder on the car roof). Contrary to previous measurements circumstances the weather was quite different yesterday with 35 degrees (C) above zero, but I hope that these 76 degrees (F) of difference have no impact on the fuselage dimensions

And the August verdict exactly follows the one from February - the distance between top of the intake lip REAR end and the lower (forward) end of armoured windshield is 207cm for all the "big mouth" variants (i.e. PF, PFM, M, MF and Bis). As the 99cm distance between the rear edge of steel intake lip and front u/c bay forward edge is the same for the M/MF and Bis there is NO DIFFERENCE in front fuselage tilt angle. Contrary - as the PF/PFM feature the same dimension along the nose top and 101cm on the underside - the nose of these older variants is some 1.2 deg "more horizontal" than in the later variants.

BTW the forward (both framing and all three glass panels) part of canopy in Bis is totally different when compared to M/MF one. Curious!

Cheers

Michael   

Edited August 10, 2018 by KRK4m
purely cosmetics

[KRK4m]

On 8/10/2018 at 6:47 PM, KRK4m said:

BTW the forward (both framing and all three glass panels) part of canopy in Bis is totally different when compared to M/MF one. Curious!

Here are the photos (by my brother - JWM here) of the MiG-21M and -21bis cockpit canopies.

 

 

The differences are easily visible

Cheers

Michael

 

[exdraken]

Wow!

Thanks for this wealth of info here!!

And great to have physical access to a variety of MiGs!!