F21b 1750# Gw

[tabranch]

I am trying to determine what aircraft changes were made to allow an increase in Gross Weight from 1500# to 1750#. The factory has recently told me that it did not include a change in strut size. In mid 1984 I replaced the struts on an F21 and received larger front ones that measured 3.875"x1.345. This was about the same time that the F21B was introduced at 1750# GW and I always believed that the strut size increase went with the GW increase. Can someone please tell me about the design changes related to GW increase?

[jgerard]

Re: F21b 1750# Gw

fuel tank relocation and spar bushings?

[dennispip]

Re: F21b 1750# Gw

Go to the home page on this site. On the left side, click on the Technical Resources heading and then click on the Frequently Asked Questions area.

Scroll down to the model F19 section and you will see what they say is the difference between the A and B.

Essentially, it says that they put a metal skin on the underbelly and raised the baggage allowance to 200 pounds. This then raised the Gross Weight to 1750#. I assume the metal skin was under the floor of the large baggage compartment and it needed to be beefed up to support that much cargo. The baggage area is certainly quite large and could accomodate that much "stuff". I know the B had two 21 gallon wing tanks and I guess the A model did as well. The original F21 model (no A or B designator) may have had a different configuration of the fuel tanks.

Hope that helps! Forrest could probably tell you more if you get in touch with him.

[Forrest Barber]

Re: F21b 1750# Gw

No real difference except as noted in the faq section just a re check of engineering. All struts are same ( same part#) they have three different sizes that can be used. The TC # 1A9 says it completely go to it for reference. Big tanks and increased baggage was about it.

[tabranch]

Re: F21b 1750# Gw

Close but no cigar! There were structural changes made doesn't anybody have a clue?

[Forrest Barber]

Re: F21b 1750# Gw

What do you mean?

[Robert Bradbury]

Re: F21b 1750# Gw

Close but no cigar! There were structural changes made doesn't anybody have a clue?

Well...if your going to make a comment like that back it up. We're all about
learning so please do spread the wealth of knowledge you are withholding.

My guess is the front and rear spars were a little bigger and a tube was welded underneath the seat to add compresssion strength ??

[tabranch]

Re: F21b 1750# Gw

Well...if your going to make a comment like that back it up. We're all about
learning so please do spread the wealth of knowledge you are withholding.

My guess is the front and rear spars were a little bigger and a tube was welded underneath the seat to add compresssion strength ??

I was looking for some factual information. Not Guesses. Such items as spar size and bushing and attach bracket and cage reinforcement. I had hoped that someone might actually know. The TCD does not include any engineering info! The FAA would not approve a weight increase with only a baggage floor change as there were additional stress tests done on at least the wings.. according to an eye witness.

[Forrest Barber]

Re: F21b 1750# Gw

just catching up again. The ship had already been built to 1750 structually ( overbuilt) ..... the engine increase to 118 hp from 100 hp allowed the increase. It had not been done with the F-21 due to alck of funds.
Actually the ships could have gone higher, the rear spar was the stopping point at that point in history. The "engineering" was done by Darrell Romick , google him sometime.....

[Robert Bradbury]

Re: F21b 1750# Gw

I was looking for some factual information. Not Guesses. Such items as spar size and bushing and attach bracket and cage reinforcement. I had hoped that someone might actually know. The TCD does not include any engineering info! The FAA would not approve a weight increase with only a baggage floor change as there were additional stress tests done on at least the wings.. according to an eye witness.

It's been so long that I cannot specificaly remember what I did and what I was told at the time, around mid 90's but, I know I increased the width and
depth of both the front and rear spars. The front original as just measured is
5 5/8 X 3/4 and the rear is 4 3/8 x 5/8. I think I went 5 3/4 x 7/8 and 4 1/2
x 3/4 on the rear. Upgraded the spar bushings to the larger ones and also
upgraded the fittings to the newer material.

As for the fuse all I added was a singe tube between the upper seat cross member tube and the lower fuse cross member tube. This was a weak point found during load testing at 1750LB as I recall and the weakness was in compression .

Anyway, I fly floats and quite often at 1700 ++ without an issue to date. I have about 300 hrs on the plane so far, tho none this year sadly

If I die from a structural failure I'll let everyone know