Transient Torque Spikes in Helicopters
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- Опубликовано: 12 сен 2024
- Welcome back to Helicopter Lessons in 10 Minutes or Less! I'm Jacob and this video topic is one that was requested quite a few times. The topic being Transient Torque Spikes. That said, be sure to leave your comments and video requests in the comments below. Hit like and subscribe as well.
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Transient torque spikes are aerodynamic phenomenon that occur when left or right (lateral) cyclic is applied. It is different than the operating limits labelled "torque transients" in operator's manuals that limit time at certain torque levels. These spikes, both up and down in torque value occur due to aerodynamic forces acting on the rotor disk because engine control units (ECU's) are trying to maintain a constant rotor RPM. Here's what it looks like: If I want to turn left, I need more lift on the right side of my rotor disk to do so. But, considering gyroscopic precession/ phase lag, the input has to be made 90 degrees prior. So to increase lift on the right, the increase in pitch is made over the tail. This simultaneously decreases lift on the left side of the rotor disk by making the input over the nose. The aircraft begins to turn to the left and torque increases.
This increase occurs because of the increase in lift and drag over the aft half of the disk where induced flow is greatest. This momentarily slows the rotor down. Aircraft equipped with ECU's, Full Authority Digital Electronic Units (FADEC's), Droop Compensators, or whatever brains in the engines are always trying to maintain constant rotor RPM. So when a decrease is sensed, they dump more fuel in the engine to bring the rotor back to say, 100%. The fuel is usually regulated by some sort of Hydromechanical Fuel Control (HMU). This surge in fuel to recover the rotor RPM is what causes a spike in torque. Turning to the left increases torque. Conversely, turning to the right decreases torque. Right cyclic increases pitch over the nose an decreases pitch over the tail. Gyroscopic precession/phase lag has its 90 degree delay effect and increases lift on the left side of the disk. The rotor momentarily increases in RPM. So the brains of the engine reduce fuel flow to try to get the rotor back down to 100%.
Consider the Running Man analogy. If he's maintaining a constant speed (constant rotor RPM) and you toss him a 20 lb weight and keep running. If he wants to maintain the same speed, his heart rate will go up (increase in torque/fuel flow) because more is demanded. Thats like turning to the left. I'm demanding more and I'll see a spike up in torque. Now the running man throws the weight down and maintains the same speed. That's like turning right. His heart rate will go down because the load demanded has been decreased.
All of this is important to know if you are aggressively maneuvering the helicopter and don't want to exceed a torque limit or "over torque" an aircraft. That said, there are factors that amplify these torque spikes.
1. Rate of Movement: how quickly are you moving the cyclic? Faster means bigger fluctuations.
2. Rate of Magnitude: a 1" cyclic displacement compared to a 4" displacement. More displacement means bigger spikes and dips in torque, especially full L to full Right or alternating Left Right Left.
3. Power Applied: Compare 60% to 95% at the start of maneuvering. The higher the initial power setting , the higher the torque spike will be.
4. Airspeed: Faster speeds generally mean more induced flow on the rear half which causes bigger fluctuations.
5. Weight: Heavier aircraft means more coning in the rotor disk which will make these spikes more pronounced.
So how do you compensate? think of string theory. Imagine a string is tied from the collective to the cyclic and your job is to keep it taut. If you turn left, lower the collective to keep the imaginary string tight. If you turn right, increase collective. These movements generally keep you compensating throughout the maneuvers.
That wraps up this subject. Thanks for watching. Be sure to hit like, subscribe, and leave a comment. As always I'm Jacob. Safe Flying.
Bro we have missed you, glad to have you back and posting!
By the way, as weird as it feels, thank you for taking off what could be a distracting watch in your recordings. Cheers
It’s actually a memorial band for one of my buddies that died when we were in combat. But I don’t like to have too much identifiable info in my videos.
@@helicopterlessonsin10minut10 I see. Thank you for the clarification. It's important to maintain a level of anonymity online as youtube content creators. Well, again, as always, thanks for sharing the great content and look forward to more. Stay safe brother.
If you still need suggestions please make a video about how clouds are formed and when and why it starts raining (temperature/dewpoint). Thanks in advance. Really enjoying your lessons. It always teaches me something new!
Thanks for the video brother. I'm currently in CC and this is helpful thank you!
Thank you sir. Great explanation.
great analogy on the string theory
I keep re-watching docs on the evolution / history of the helicopter (specifically the mechanical innovations that made stability & control possible). I don't know why I find it so interesting. If you're looking for subjects, maybe a shortlist of those discoveries...
Great Video as always. I did find the drawing a little confusing with arrows demonstrating phase lagged pitch changes going in the opposite direction to the rotor but I still learnt something new and please keep up these excellent videos. 👍
first of all thanks for sharing these kind of knowledge such a great video as usual keep it up👍🏽
if you don’t mind to share with us the subject if power available and required and the importance of it, also the consequences if not taking the matter seriously.
Sure thing. I’ll add it to the list.
You are the best! I finally understand!! Thank you so much!
23 thousand subscribers! Wow. I mean, it's deserved in this case, but I had no idea until just now. Impressive.
Nicely done !
Sir, there are many Digital Combat Simulator helicopter pilots of which I am one who thanks you very much for these explanations, especially VRS. the program is soon to have the Apache, we currently have the Hind, Ka-50, MI-8, and the UH-1. feel free to expand your discussions to include combat tactics involving attack, and combat search and rescue pickups in hot LZ's. Thanks!
I appreciate the feedback. Unfortunately I cannot openly disclose combat tactics used by US military helicopters. Doing so would reduce the effectiveness of those tactics.
I am a DCS AH-64 Pilot and i deeply appreciate these vids i discovered these after flying around by myself for like 1 ½ months and alot of the stuff you get a feel for by trial and error but these scientific explanations of helicopter physics/aerodynamics are unparalleled in helping my understanding of WHY the things that are happening are happening
Excellent. Thank you.
Could you do a video on climbing and descending turns and airspeed increases and decreases in level flight?
I wonder: How much of these lessons content is simulated in dcs apache early access?
Nice video ✊🏾
Can you talk about your tricks in Fuel calculations in minutes
Sure thing. Although fuel flow varies significantly by aircraft. I’ll see what I can do.
Thanks!
Thanks bro 👍
what about co-axial helicopters
As someone who flies a piston i'm going to ignore all that but save the video for future reference!!
Flying pistons you will be more susceptible to the trq changes, this is all reversed for clockwise turning rotors. Your governor or left hand throttle (for real Flying) is making the compensation for the increase in drag on the rotor, left / right pedal inputs will have a similar effect, this is why I almost always climb mountains or ridges on my left with turn out abilities to my might in the event of an engine malfunction or t-r. Fly Safely!!!
Thanks for these videos! I'm a little confused... does that mean that forward or aft cyclic does not affect torque changes? It feels like those would as well as the relative air speed is greatest on the right side and least on the left side. The increased/decreased AOA then affects the speed of the rotor blade which would require more/less engine power to compensate... no?
The forward and aft cyclic would affect rotor coming and conservation of angular momentum. So a forward cyclic would slow the rotor and increase torque. An aft cyclic would accelerate the rotor and reduce torque. It’s generally not as prevalent as lateral cyclic inputs though and a slightly different aerodynamic principle. But still noteworthy
can you explain why the helicopter (counterclockwise) during a hover climbing when you applying left pedal and descending when you applying right pedal? by the way faa said the opposite
It really depends on the type of helicopter, specifically if it has a governor or Electronic Control Unit to maintain rotor RPM. In a counterclockwise rotating system, you need left pedal to counteract the torque effect. Adding left pedal demands more power if you want to maintain the same altitude. This is caused by the increase in drag and therefore torque required. So if you have a fuel governor in the system, there would be an increase in torque for a left turn a drop in torque for a right turn. Without a fuel governing system the aircraft would descend in a left turn because the increase in demand would slow down the drive system of the main and tail rotor. I hope this helps
Thanks for your time
I'm pilot on bell GX 407 and its ECU fuel system even that its climbing when applying left pedal I tired to know the reason and i failed so i need your help if you know
@@nighthunter1601 In a counter rotating rotor helicopter like the Bell 407, a left pedal turn in the hover will accompany the rotor rotation, effectively adding a few rpm to it's rotation through the air, thus the climb. A right pedal turn will create a descent. The rotor rpm indicated in the cabin is relative to the airframe, but what affects lift is the rotor rpm through air.
Question. I mainly been flying r44 cadet, the governor on those aircraft would that be considered a fadec system? it seems to be a fairly elementary system that responds to changes in rpm and mechanically moves the throttle.
I’m not an expert on R44’s but I believe their governor isn’t considered a FADEC. Governors simply try to maintain constant rotor RPM while FADEC systems adjust for optimum engine efficiency in any environment. FADEC systems tend to be more computer controlled and monitor far more variables like air density, engine temperature, engine pressure, etc. Simple governors are cheaper and easier to put into helicopters without huge cost increases to install or maintain.
@@helicopterlessonsin10minut10 thanks for the explanation
Pl explain the concept of inherent sideslip in helicopter
I’ll add it to the list.
@@helicopterlessonsin10minut10 thanks pl do the needful as soon as possibe
I'm confused on why the input is made 90° prior due to gyroscopic procession. I thought it was 90° later?
The input is made 90 degrees prior because the effect of gyroscopic precession means that effects manifest 90 degrees later. So if I want to turn left, I need to increase the pitch over the tail so it manifests 90 degrees later over the right half of the disk.
@@helicopterlessonsin10minut10 Ah. I understand now. Thank you.
Can you speak about stabilizer
Sure. Are you talking about the vertical or horizontal stabilizer on the helicopter?
@@helicopterlessonsin10minut10 horizontal stabilizer.. Thank you
I outline stabilizers more in my Types of Airfoils and Autorotation videos. But short answer is the horizontal stabilizer provides a comfortable fuselage attitude in forward flight as well as in autorotations.
How does an over-torque situation physically present itself?
You recognize it by knowing the limits of your aircraft. In some aircraft data can be logged and pulled by maintenance personnel. When an over torque has occurred the engine must be completely inspected for cracks and wear.
Like stated above, some aircraft have data logs that track “exceedances.” But an over torque condition isn’t damage to the engine but rather the drive train. At some point the engine provided more torque than the drive train components (drive shafts, gearboxes, transmissions) could handle. This requires inspections and possible replacement of parts if damaged to a certain extent.
@@helicopterlessonsin10minut10 But is an over-torque theoretical, meaning it went above a specified percentage for a period of time, such as a reactor going to 104%, when in reality that number is actually derated for a factor of safety? Or does it mean the torque actually went over the factor of safety and caused plastic deformation of the components? Basically, does an over-torque condition actually over-torque the components or does it simply exceed a specified limit that was established for a factor of safety? If a gear can physically handle 2000 lb ft of torque before plastic deformation, they would establish a factor of safety of say 2.5 and therefore 800 lb ft would be considered 100% torque.
@@UncleDiddles are you familiar with SN curves? Because that would answer your own question. The issue is the number of cycles at a high magnitude of stress that causes issues. By the time you've got plastic deformation you are falling out of the sky.
@@otm646 Specifically what I am stating. The term over-torque is ambiguous. Is it referring to a theoretical fatigue cycle life under a safety factor, or is it referring to actual plastic deformation. They could state over-torque is any condition over 1000 lb ft, however the fatigue life or yield strength of the components could be 3000 lb ft, thus its over-torqued if the limit is a FOS of 3 but its not actually over-torqued in anyway whatsoever.
love the merits, but the pen scratching sounds make me sick, unfortunately :(
Sorry about that distraction.
I am tell when your Sharpie is getting dry lol
What are the implications of electric motors for reducing this problem?
My Tesla has extraordinary acceleration. This is because, first, I don’t have wait for fuel to be dumped into the engine and for it to start burning; and second, the torque depends much less on RPM. I get instant torque without having to depend on - or wait for - RPM.
Would a helicopter pilot’s need to focus on torque and RPM range become less critical?
Could electric motors be a boon to helicopter safety?
Can a helicopter experience transient torque spike in a straight take-off leg. Without those banks being in play.? By the way thanks for all your videos . Best wishes.
Not quite sure I understand your question. Transient torque spikes are present with lateral cyclic as a result of more induced flow on the back half of the disk. If you have any airspeed you can have this affect with left or right cyclic.
At take off during transition speed .. holding onto the same collective setting I keep noticing a torque rise and purposefully I need to reduce collective slightly everytime to maint the takeoff torque .. I was thinking maybe that is also related to what you are teaching us here.. anyways .. thanks for the clarification . Happy Landings
It's 102% for us
It varies by aircraft but is generally around 100% plus or minus 3%
@@helicopterlessonsin10minut10 what's the rotor speed on the Apache helicopter ?
Unfortunately I cannot talk about specifics of the Apache.
@@helicopterlessonsin10minut10 what's the best manner, for going to the hover from the ground on a heavy helicopter ?
*Moistness intensifies*