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Firefly T67M New MT 3 blade prop

PostPosted:Wed Jan 13, 2021 2:17 pm
by BobDale
Hi,
Following the recent AD (EASA-EAD- 2020-0226-E) regarding cracked propeller hubs, our T67M is having a new MT 3 blade prop fitted in the next few days. Due to Covid we wont be flying it for a while. Do any others have experience of the change from 2 to 3 blades and what impact if any does it have on manifold pressure settings etc? How has performance been impacted?

Re: Firefly T67M New MT 3 blade prop

PostPosted:Wed Jan 13, 2021 2:39 pm
by PeteSpencer
When our 200HP Arrow 2-blade prop got such a severe FOD dink from a local East Anglian airfield many years ago, we replaced it with a 3 blade (Hartzell) prop which was actually cheaper than a 2 blade.

It made absolutely no difference to performance (speed/climb/fuel consumption).

It just looked better and gave better ground clearance.

However it seemed to introduce new vibration which took quite some sorting out with several returns to the shop for static and dynamic balancing.

Visitors (instructors/examiners ) continued to comment on excessive vibration, tho' I didn't notice it.

A few years ago a group member took off with the towbar attached, got two greens and landed with the nosegear jammed up.

The engine mount and 3 blade prop were replaced as part of the engine tear-down and rebuild.

The vibration disappeared......................... :wink:

Re: Firefly T67M New MT 3 blade prop

PostPosted:Wed Jan 13, 2021 3:30 pm
by Trent772
The MT pop is superb. Very smooth, great acceleration and also deceleration.

You may find it better, not to land fully fine, leave the descent setting - say 2300 rpm on for the circuit and landing - the disking can be a little harsh otherwise.

You still have plenty of go around performance btw and the prop can always be wound in if necessary on the climbout.

Keeps everything quiet for you and the neighbours as well.

Re: Firefly T67M New MT 3 blade prop

PostPosted:Wed Jan 13, 2021 9:42 pm
by T67M
Our shareoplane was the first one modified - 'SR spent two weeks in Germany for the flight test regime, and the STC was issued based upon those flight results. Overall I'd say there is no significant change other than the reduced cost of overhauls. That said, there are a few minor differences:
  • The noise has a different and (to my ears) slightly nicer sound because of the 3/4 beat of cylinders and blades compared to the 2/4 beat. Actual noise level is about the same, it's just slightly more "bassy".
  • Overall, performance is unchanged and as per the POH
  • For the first 50-100 hours, we noticed that the prop sped up by 50rpm during aerobatic down-lines - easily overcome by setting 2550RPM rather than 2600RPM. However now that we have 2 years and 350 hours on it, I've gone back to using 2600RPM with no over-speed noticed.
  • I've never had any problem landing with the prop set fully fine on 500m of tarmac or 600m of grass. I do only wind fully-fine in after I've reduced the manifold pressure on the base leg though, so the prop is already out of the governing range.
  • Take-off roll feels slightly shorter - but only by a few metres, so not really worth counting.
  • It looks much nicer!
  • It's cheaper (how MT can make a 3-blader cheaper than a Hoffman 2-blader I will never understand)

Re: Firefly T67M New MT 3 blade prop

PostPosted:Wed Jan 13, 2021 9:48 pm
by Trent772
Monster !

Aeros are fine at 2300

2500 for noise

:mrgreen: :thumleft: :pirat:

Re: Firefly T67M New MT 3 blade prop

PostPosted:Wed Jan 13, 2021 10:01 pm
by Remosflyer
Scott Perdue of the excellent FlyWire YouTube/Patreon channel did an extensive evaluation recently, replacing the 2 blade prop on his Bonanza with a 3 blade MT, conclusion was slightly better take off performance, climb and cruise figures about the same, but a big reduction in noise and vibration.


Re: Firefly T67M New MT 3 blade prop

PostPosted:Wed Jan 13, 2021 10:17 pm
by Trent772

Re: Firefly T67M New MT 3 blade prop

PostPosted:Wed Jan 13, 2021 10:18 pm
by T67M
Trent772 wrote:Monster !

Aeros are fine at 2300

2500 for noise

:mrgreen: :thumleft: :pirat:


But at 2300 you can't go to full throttle!

[For those unfamiliar with the Firefly, this is because of the "+4" rule in the POH (max MP = RPM/100 +4), so 2300RPM means a max manifold pressure of only 27 inches.]

Re: Firefly T67M New MT 3 blade prop

PostPosted:Thu Jan 14, 2021 11:56 am
by Trent772
Darn.... I forgot - you need all the power I guess :pirat:

2 up at 23/23 we climb doing aeros 8)

Re: Firefly T67M New MT 3 blade prop

PostPosted:Thu Jan 14, 2021 2:49 pm
by rikur_
Ours is in the process of the same mod ... all the research beforehand got the same feedback as above.
May be it was from the same people via different routes!!

Re: Firefly T67M New MT 3 blade prop

PostPosted:Thu Jan 14, 2021 6:30 pm
by T67M
Trent772 wrote:Darn.... I forgot - you need all the power I guess :pirat:

2 up at 23/23 we climb doing aeros 8)


But at least mine only drinks 33 litres per hour! :lol:

Re: Firefly T67M New MT 3 blade prop

PostPosted:Thu Jan 14, 2021 8:56 pm
by Trent772
As much as that...... :wink:

31lph at 23/23 while stunting

Re: Firefly T67M New MT 3 blade prop

PostPosted:Fri Jan 15, 2021 1:01 am
by neilmurg
Different a/c but, it should be slightly more drag in cruise (more blade length) but more powerful on takeoff/acceleration from slow speed (more blade) [constant speed prop], a tiny increase in ground clearance.
It sounds MUCH better than the 2 blade, looks much better (IMO).
We've had some problems with the blades being softer, more prone to dinks than the 2 blade aluminium one it replaced, and quality control on the blade root.
I'm happy with it, some members not so much.

Re: Firefly T67M New MT 3 blade prop

PostPosted:Fri Jan 15, 2021 12:46 pm
by Stu B
The Hoffmann T-67 blades are wood with a composite "skin" and a metal LE strip (which started to come away on one of ours). What are the MT T-67 blades made of? Wholly composite?

Re: Firefly T67M New MT 3 blade prop

PostPosted:Fri Jan 15, 2021 12:57 pm
by gfry
Stu B wrote:The Hoffmann T-67 blades are wood with a composite "skin" and a metal LE strip (which started to come away on one of ours). What are the MT T-67 blades made of? Wholly composite?


Depends on which prop it is. It is most likely wood with a composite skin.

Are you sure there is an STC for the T67. According to MT there is not.

https://www.mt-propeller.com/en/entw/stcs.htm