MikeGazdik
Senior Airman
Does the prop design change or alter the torque affect? Such as if a plane was switched from a 3 blade to a 4 blade propeller.
A question about propeller torque during takeoffs
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- #22
Yes I had to look closer my books and cutaway drawings. And still there are no evidence of separate aileron trim tab. So I would imagine that the whole aileron is working as a trim.
FLYBOYJ
"THE GREAT GAZOO"
It probably will as not only do you have another lifting surface, but that extra blade adds weight increasing torque inertia.
evangilder
"Shooter"
I can remember looking down at the rudder pedals while sitting in the back seat of the T-6 and seeing the constant motion of slight corrections to keep the plane on center during take-off and landing. Sweet to fly, but they can be very unforgiving on landing, but not as bad as the Stearman.
drgondog
Major
The amount of torque applied by engine to the prop (2, 3 or 4 or 5 blade) is the amount of torque applied by the engine/prop system to the airframe.
While the thrust may vary on the prop efficiency and the pitch, the torque applied to the prop is the same torque that must be absorbed by the airframe.
Joe - I need to ponder on prop inertial effects to the above but believe that is only a factor wrt torque in the acceleration/deceleration of the rotational speed to and from zero to the selected rpm. The max torque of the engine should be the limiting and design factor
FLYBOYJ
"THE GREAT GAZOO"
Ok, but I could tell you this much - you could feel the difference between a 2 bladed and 3 bladed Bonanza with the same engine intalled - I've always felt it was partially due to the weight of that third blade (which could add up to another 50 pounds).
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- #27
http://www.free-online-private-pilot-ground-school.com/images/trim_tabs.gif
I just found this pic and the description says that
"Figure 16: The movement of the elevator is opposite to the direction of movement of the elevator trim tab." Obviously when the trim tab is neutral it doesn't move with the aileron or the stabilator.
Does the both aileron and rudder works in the same way?
I just found this pic and the description says that
"Figure 16: The movement of the elevator is opposite to the direction of movement of the elevator trim tab." Obviously when the trim tab is neutral it doesn't move with the aileron or the stabilator.
Does the both aileron and rudder works in the same way?
drgondog
Major
I'm scratching my head now. I've flown the V-35 and owned an A-36 and trying to remember the differences. IIRC the V-35 was both lighter and lighter on the controls than the A-36, and the V-35 had distinct dutch roll in cruise but seemed more responsive overall. I cannot remember whether the V-35 I flew had two blades or not. The A-36 was definitely three blades.
The extra weight would increase the prop rotational inertia but I can't figure out why that would affect the 'torque' response of engine/airframe and prop system to each other unless we look at interim throttle settings. Maybe we get a slighly lower rpm for same throttle because of the increased inertia of the heavier prop and for the same rpm/different throttle setting the torque supplied (and absorbed) is higher for the heavier prop.
If we ignore an airframe attached to the engine mounts and view it as a test stand anchored, then at max torque (i.e take off) that the engine is capable of - will be applied to the mounts as compression on one side and tension on the other - trying to rotate the stand opposite of the prop rotation.
The amount of propeller 'in-plane drag' is equal to and opposite of the torque supplied by the engine/shaft combination..so, as the rotational moment of the prop system increases it should show up as lower rpm for same torque applied?? I am wandering a little bit here in what should be simple engineering but the only time I fooled with 'props' of any kind in my industry experience was at Bell and that was more in the structural analysis side - not blade performance.
Joe - are you comparing a three blade A-36 to a two blade V-35 or within same model type like V-35 with two different props?
FLYBOYJ
"THE GREAT GAZOO"
Two V-35s. I worked with a few folks who had STCs that put 3 blade props on them. During the take off roll you were really honking on the right rudder with the 3 blade conversion from what I remember.
drgondog
Major
A trim tab acts to alter (slightly) the camber of the airfoil surface no matter where it is (rudder, elevator, aileron) and designed to produce small changes in forces to the airfoil to accommodate changes to flight profiles such as rpm, throttle settings etc as well as even altitude/speed changes like in a dive.
What it does is alter the force applied (slightly) one way or the other to the airframe through the flight control surface it is attached to.
If you are holding a stick which in turn is maintaining the elevator in a certain position (or on auto pilot) and change the trim tab setting on the elevator - the elevator doesn't move, the trim tab does, altering the aerodynamic forces on the horizontal stabilizer, and in turn the pitch of the airframe.. ditto Rudder except that alters the response of the airframe in yaw.
There are exceptions to the general rule of Hold main surface constant/alter trim to couple the rudder/rudder trim or elevator/elevator trim in a different and opposite way for certain conditions (usually dangerous) of flight.
drgondog
Major
OK. The reason I didn't fully get this is that I didn't really see very much difference transitioning from the V-35 to the A-36 but I suppose there was a difference that I attributed more to the different weight and tail config than the torque. Had I flown one then the other in succession I probably would have noticed.
The explanation seems then to be the engine applies more torque to overcome the higher prop intertia at same rpm and that it takes a slightly higher throttle setting for same rpm.
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- #32
FLYBOYJ
"THE GREAT GAZOO"
Makes sense - I do know that one of these guys had the Feds nix a field approval when he wanted to put an IO-470 (I don't remember which model) in with a 3 bladed prop. The PMI we were dealing with was worried about torque and the effects on the engine mounts. He wanted to have a bunch of engineering analysis and flight testing done prior to approval. In the end the guy with this aircraft stayed with the IO-470 and a two bladed prop. The FAA hard at work!
drgondog
Major
My 'guess' is that the engine mounts were designed to 2x ultimate for the two blade and should be adequate for the higher torque engine/prop config - but I would err on the side of caution and get an 'official opinion' from Beech, particularly if it was my ass!
FLYBOYJ
"THE GREAT GAZOO"
Yep - I think the aircraft owner was on that line of thinking. It's funny though, McCauley wanted to help us only if they could retain all the flight test data - we figured they were going to get "free data" and then apply for the STC - this pissed off the Feds as well.
Funny thing about torque on another aircraft - We take care of the twin otters operated by USAFA - recently we upgraded the PT6s to -34s and the aircraft are continually flown to their max. allowable torque. The STC that installed these engines said nothing about engine mounts - we've we noticed that although they have no specified time life, the engine mounts are lasting about 1/3 of what we've seen before. The mount mfg. told us no one ever contacted them about analysis on their mounts with a -34 engine.
drgondog
Major
Joe - what standards are applied during inspections to determine 'near end of life' for engine mounts in an Otter?
From a structural perspective I sure would want both visual and penetrant inspections of the bolts but what about the engine and airframe mounts?
Are they degrading at attach points/engine and Mount or mount to fuselage or ??
FLYBOYJ
"THE GREAT GAZOO"
On condition - no splits or tears on the rubber - this came from DeHavilland.
It's all in the rubber - the structure is perfect and we have done NDI around the area - the folks from the mount mfg looked at some of the failed mounts - they felt the mouts were being operated to their limits - there is also a bushing in the middle of the rubber - never had any deformation or problems there and they and the bolts are changed every time we change a mount - it seems the rubber it taking al the load.
drgondog
Major
At least two implications (or three actually). One is material related if the rubber/elastomeric mounts have changed say in manufacturing process rendering it more susceptible to 'fatigue' due to the vibration.
Two, is the frequency input of the new engines is somehow applying 'more' compression cycles on the rubber (i.e 'fatigue' due to more repeat loads per engine hour).
Third and last is appreciable increase in the load applied to the rubber - but this one would also suggest to me that more Shear is somehow being applied to the rubber instead of compression type loads...
Does the bushing lay slightly below the surface of the rubber mount (seems like it should to allow the rubber mount to compress slightly and create a preload tension on the mount bolts).
I would suspect something related to increased shear loads on the mounts - which theoretically would occur only if the new engine also had significantly more thrust?
FLYBOYJ
"THE GREAT GAZOO"
Below is a pic of what they look like - The mount mfg who analyzed some of the mounts did say that there were manufacturing imperfections in some of the mounts we sent him. There's another company who has PMA on these parts and of course the prime mfg says theirs is superior (we had a mix from both sources). The bushing is actually in the center of the mounts (see photo).
I think you're correct on all accounts Bill - our birds are flown hard daily and most of the time they're taking off straight to Vx with the torque meter at the max. permissible torque. Talking to an FBO that does a lot of twin otter work, he thinks we're the only ones seeing this because of the way we fly. When hauling jumpers the mandate is to get in the air and climb ASAP, throw the jumpers out and get down ASAP for another load - sometimes this goes on 10 hours a day.
We do trend monitoring on these aircraft and will know when there is an over torque, we download engine data at the end of the flight day. Other otter operators told me they would fire any pilots who consistently fly close to max torque limits. In our environment it's the norm!
BTW - the newer engines are putting out 750shp, the PT6-27 650shp, so we are lookin at 100 shp.
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drgondog
Major
It all makes sense now
