Multi-engine Vmc vs. increased horsepower

robertb

robertb

Pre-takeoff checklist
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robertb
Hello,
First, I am not a multi-engine pilot. But, I've been flying with a buddy in his Cessna 340A with the RAM STCed engines. The STC increases the horsepower from 310 to 335, I believe. If an engine fails, and the remaining engine has more power than the plane was originally designed, would Vmc increase, decrease, or stay the same when the higher powered engines are installed? I poked around but didn't find any info on how the indicated V speeds get adjusted when the STC is installed.

My thought is that Vmc would need to increase since the running engine is able to produce a larger yawing moment yet there was no increase in the rudder authority.

While on the topic, would Vyse change when the STC is installed? My guess is that Vyse would not change.

These thoughts came into my head on a recent flight and I just got curious.
 
robertb said:
The STC increases the horsepower from 310 to 335, I believe. If an engine fails, and the remaining engine has more power than the plane was originally designed, would Vmc increase, decrease, or stay the same when the higher powered engines are installed?
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VMC will increase!

There will be more power and torque to create a yawing tendency and the size of the rudder remains the same.
 
RyanB said:
VMC will increase!

There will be more power and torque to create a yawing tendency and the size of the rudder remains the same.
Click to expand...
thats assuming there was no excess rudder deflection available and/or they maxed out the allowable pedal force.
 
Consider CFR Sec 23.149 and Criteria for Establishing Vmc.
If first factor, 'maximum available takeoff power' has been increased, as mentioned above ... expectation is Vmc increases.

With respect to title, 'Multi-engine Vmc vs. increased horsepower', note increased engine HP may not be complete answer.
For C340 turbo-charged engines at sea level, m.p. limited for takeoff (to avoid over-boost).
i.e. Vmc criteria depends on STC increase max 'takeoff' power.
MauleSkinner said:
AFM Supplement for the STC
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:yeahthat::thumbsup:
 
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For the 335hp engines RAM also added Vortex Generators on the wings and vertical stabilizer. If I remember correctly, it actually lowered the VMC by 10K even with the higher HP.
 
Same general reason early BE33 critters reportedly run out of rudder on the slightest crosswind on takeoff when they swap to IO550. No buck no buck rogers. Gotta increase the pressure area by dimension or by throw.
 
Plano Pilot said:
For the 335hp engines RAM also added Vortex Generators on the wings and vertical stabilizer. If I remember correctly, it actually lowered the VMC by 10K even with the higher HP.
Click to expand...

That is what I was planning to say. If the engine swap was the only modification of the STC, logic says more power means higher Vmc. But if other changes were made such as vortex generators, stall strips, or other little tricks, there may not be an impact.
 
My C414 with original 310hp engines had an AFM listed Vmc of 84kcas. Now with a RAM VI STC which includes vortex generators now with 335hp engines it has a Vmc of 79kcas. FYI Vyse blue line is 103kcas.
 
VGs are what sets these improvements apart. Typically, the STC for the VGs also include an adjustment of the travel of the rudder to provide additional deflection.

On my 421C, I gained about 110lbs of load increase and a decrease in stall and Vmc speed of 7 knots.

On my 310Q, I gained 75 lbs of load and decreased my stall speed and Vmc speed by 3 kts.
 
Thanks all for the info! I'll see if I can read through the AFM next time I take a flight in the plane and have some time to kill. It will be interesting to see if the STC incorporates both the engine modification as well as VGs and rudder adjustments. I assume that info would be in the STC's installation documents.
 
From RAM -

  • Two RAM Overhauled TSIO-520 Engines and installation package
  • Two New ScimitarPlus Propellers with new spinners
  • RAM Propeller Sync System with McCauley MC-1 Governors
  • Shadin Digiflo-L Electronic Fuel Management System
  • RAM Vortex Generators
  • Gross Weight Increase (+400 lbs. Useful Load)
  • Engine & Airframe STC
 
k9medic said:
VGs are what sets these improvements apart.
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Yes, including VGs on the vertical stabilizer to delay a stall of this vertically mounted wing.
Vmc in a multi-engine aircraft is all about maximizing the (horizontal) lift from the vertical stabilizer and rudder without stalling.

- Martin
 
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