Aerodynamics/Engineering Questions
Thread Starter
Join Date: Dec 2012
Location: Aberdare, Wales
Age: 31
Posts: 174
Likes: 0
Received 0 Likes
on
0 Posts
Aerodynamics/Engineering Questions
Hey I'm a newbie, interested in helicopters, but my questions are mainly aerodynamics and engineering related.
I would be so grateful if there is anyone who would be interested in responding.
My first question is to do with how a helicopter's weight and or load is carried (non-dynamically).
In other words, if you are lifting a helicopter (take a Bell 206 for example) off the ground by its rotorhead (I understand there are straps used for this) how is the weight of the helicopter transmitted?
My understanding is:
- All of the helicopter's weight can be suspended beneath the rotorhub, meaning that the rotorhub must be of sufficient strength to carry the full weight of the helicopter which in the case of a fully loaded Long Ranger could be around 1800kgs?
- Everything connected to the rotorhub must also be able to carry this weight including the main rotor driveshaft?
- The main rotortransmission must also be capable of carrying the entire aircraft weight?
And this is the bit where I get stuck - the main transmission is bolted onto the roof of the helicopter, but what happens after that?
In the case of the 206 there seems to be a honeycomb layer which surrounds the cabin, are the main transmission mounts just bolted onto a mounting point on the roof or is there a frame within the honeycomb (such as a steel frame within the helicopter's fuselage) to which it is connected?
If there isn't a steel frame surrounding the main cabin to which the transmission mountings are attached, how come the transmissions mounts don't just tear away from the cabin roof (under extreme circumstances).
I'm sorry because I realise such a question must sound very stupid.
I am sorry.
I would be so grateful if there is anyone who would be interested in responding.
My first question is to do with how a helicopter's weight and or load is carried (non-dynamically).
In other words, if you are lifting a helicopter (take a Bell 206 for example) off the ground by its rotorhead (I understand there are straps used for this) how is the weight of the helicopter transmitted?
My understanding is:
- All of the helicopter's weight can be suspended beneath the rotorhub, meaning that the rotorhub must be of sufficient strength to carry the full weight of the helicopter which in the case of a fully loaded Long Ranger could be around 1800kgs?
- Everything connected to the rotorhub must also be able to carry this weight including the main rotor driveshaft?
- The main rotortransmission must also be capable of carrying the entire aircraft weight?
And this is the bit where I get stuck - the main transmission is bolted onto the roof of the helicopter, but what happens after that?
In the case of the 206 there seems to be a honeycomb layer which surrounds the cabin, are the main transmission mounts just bolted onto a mounting point on the roof or is there a frame within the honeycomb (such as a steel frame within the helicopter's fuselage) to which it is connected?
If there isn't a steel frame surrounding the main cabin to which the transmission mountings are attached, how come the transmissions mounts don't just tear away from the cabin roof (under extreme circumstances).
I'm sorry because I realise such a question must sound very stupid.
I am sorry.
Dont worry, lad. No need for apologies.
Sometimes a picture is worth a thousand words
Huey diagram
As with most conventional aircraft, the airframe consists of mostly aluminum and titanium frames, beams, longerons, and stringers - all of which comprise the internal "skeleton" aka airframe. It is the layout of this structure that is designed to distribute the load from the rotor system. The nomex/honeycomb core skin you mention can serve as a shear panel between the airframe structure.
Sometimes a picture is worth a thousand words
Huey diagram
As with most conventional aircraft, the airframe consists of mostly aluminum and titanium frames, beams, longerons, and stringers - all of which comprise the internal "skeleton" aka airframe. It is the layout of this structure that is designed to distribute the load from the rotor system. The nomex/honeycomb core skin you mention can serve as a shear panel between the airframe structure.
Last edited by Senior Pilot; 11th Dec 2012 at 18:11. Reason: Photo too large
Thread Starter
Join Date: Dec 2012
Location: Aberdare, Wales
Age: 31
Posts: 174
Likes: 0
Received 0 Likes
on
0 Posts
Thank you, especially for the diagram.
From the diagram the frame that Shawn Coyle mentions doesn't seem very obvious. Are the beams of the frame not very thick?
The engine must be a massive weight to carry. How is that load connected so that it can be carried by the main transmission? Is there a beam running from the main transmission and runs back to carry the engine?
From the diagram the frame that Shawn Coyle mentions doesn't seem very obvious. Are the beams of the frame not very thick?
The engine must be a massive weight to carry. How is that load connected so that it can be carried by the main transmission? Is there a beam running from the main transmission and runs back to carry the engine?
Join Date: Aug 2011
Location: Ventura Ca U.S.A.
Posts: 222
Likes: 0
Received 0 Likes
on
0 Posts
The honeycomb stiffens the skin on a 206. The lifting beam is of aluminum fittings & angles, with aluminum bulkheads fore & aft. Find a 206 in a shop with the plastics removed, Lots to see.
Thread Starter
Join Date: Dec 2012
Location: Aberdare, Wales
Age: 31
Posts: 174
Likes: 0
Received 0 Likes
on
0 Posts
Hillberg, thank you for your response. Are you saying that the helicopter's aluminium fittings and the honeycomb structure together with the aluminum and titanium frames, beams, longerons, and stringers mentioned by SansAnhedral, combine to turn the fuselage into a frame as opposed to having a solid metal frame contained within the fuselage?
There is not straight forward answer to this question. Each manufacturer has there own and in many cases different approaches to the question of exactly how the rotor systems lifting loads are transmitted to the airframe. As examples, Sikorsky utilizes thrust bearings in the main gear box to transfer the flight to the main gear box housing. The weight of the airframe is supported by the housing. Eurocopter utilizes struts from a thrust bearing on the main rotor shaft of the AS-350 to react the flight loads. The struts then support the airframe. On the Bell 206 the airframe is supported by a lift link from the bottom of the main gear box that attaches directly to a beam in the airframe structures. Additionally, the 206 has soft mounts that allow limited motion between the airframe and the main gear box. These soft mounts are attached to the honey comb roof structure but do not provide any substantial lift. There may be others on this site that can provide additional detailed information on some of the other helicopter models.
Join Date: Aug 2011
Location: Ventura Ca U.S.A.
Posts: 222
Likes: 0
Received 0 Likes
on
0 Posts
Sorry the Bell 206 has an aluminum roof beam with angles, from the gear box you have two mounting legs off the sides of the gear box that attach to mounts bolted throuth the roof to the angles, A 3rd soft mount aft on the roof attaches to the gearboxes spike mount fitting .
On the 205 the mounts are soft rubber to aluminum frames & a lift link takes the main lifting loads on the bottom of the Gearbox case.
The sikorsky have steel tube legs attached directly to the airframe fittings (S-58)
On the 205 the mounts are soft rubber to aluminum frames & a lift link takes the main lifting loads on the bottom of the Gearbox case.
The sikorsky have steel tube legs attached directly to the airframe fittings (S-58)
Thank you for the clarification on the 206. I have found that the older I get the more I have to learn. Having only minimal experience in the S-58 (Wessex 5), I didn’t realize that Sikorsky utilized that system in that model. However, the MGB is the primary lift structure in the CH-53A/D/E, CH-54, H-60 series, and S-76 series. I believe that the RAH-66 Comanche utilized a system similar to the AH-64 and maybe the S-58 where the mast through struts to the airframe supports the airframe.
Just as an aside to the discussion, the C/MH-53E aircraft single point lift system attaches the hook system and ultimately the sling load directly to the bottom of the MGB. In this case, the airframe provides no support of the external load. This was not the case in the earlier CH-53A/D series where the lifting hook was mounted in the floor structure.
Thread Starter
Join Date: Dec 2012
Location: Aberdare, Wales
Age: 31
Posts: 174
Likes: 0
Received 0 Likes
on
0 Posts
Dear Hillberg/Jack Carson
You have both answered many questions for which I am most grateful. Thank you so much.
Thank you for this answer as it now really begins to clear things up for me.
Thank you again. When I saw both a Jet Ranger and Long Ranger in the hangar I couldn't understand how the lifting loads were transmitted through the airframe because I only saw what I suppose were the soft mounts and in my mind I imagined that maybe if the pilot pulled too much power these might rip right off the roof/upper honeycomb section of the fuselage (unless as I began to think) the transmission mountings were connected to a frame embedded within the honeycomb. The existence of a "lift link" answers this question.
Thank you. This answers additional questions that I had but also raises some more.
Some additional questions:
1. In helicopters which do not have a link between the MGB and the belly hook isn't there a risk of "warping" to the fuselage?
2. Is it true that in the Eurocopter PUMA there is a pole which connects the gearbox to the lifting hook?
3. Why doesn't every helicopter have this arrangement for lifting in order to reduce stress on the airframe?
4. A helicopter used in constant lifting work which does not have a link between the MGB and the helicopter's hook is bound to have more airframe stress than a helicopter which has a direct link between the two?
5. From everything you are saying it seems that in many helicopters the main transmission gearbox (as you have said) is the principal component which connects all the helicopter's upper dynamic (lifting) components to the fuselage. I suppose this is why gearboxes are such hefty pieces of industrial engineering?
6. If this is so, when then is there a high magnesium content in the manufacture of gearboxes when, so I have been told, this makes the gearbox vulnerable to things such as salt water (apparently)?
7. What is a thrust bearing please?
And finally:
8. By putting the helicopter's weight onto a dynamic component directly, such as the main rotor shaft, (as opposed to the MGB) would this cause the loads to increase on that component?
9. Does this mean that the AS-350 rotorshaft must be bigger and stronger?
10. By putting the lifting loads onto a rotating shaft, does this cause any increase in control forces?
I beg your forgiveness if these are too many questions.
You have both answered many questions for which I am most grateful. Thank you so much.
On the Bell 206 the airframe is supported by a lift link from the bottom of the main gear box that attaches directly to a beam in the airframe structures.
Additionally, the 206 has soft mounts that allow limited motion between the airframe and the main gear box. These soft mounts are attached to the honey comb roof structure but do not provide any substantial lift.
Just as an aside to the discussion, the C/MH-53E aircraft single point lift system attaches the hook system and ultimately the sling load directly to the bottom of the MGB.
Some additional questions:
1. In helicopters which do not have a link between the MGB and the belly hook isn't there a risk of "warping" to the fuselage?
2. Is it true that in the Eurocopter PUMA there is a pole which connects the gearbox to the lifting hook?
3. Why doesn't every helicopter have this arrangement for lifting in order to reduce stress on the airframe?
4. A helicopter used in constant lifting work which does not have a link between the MGB and the helicopter's hook is bound to have more airframe stress than a helicopter which has a direct link between the two?
5. From everything you are saying it seems that in many helicopters the main transmission gearbox (as you have said) is the principal component which connects all the helicopter's upper dynamic (lifting) components to the fuselage. I suppose this is why gearboxes are such hefty pieces of industrial engineering?
6. If this is so, when then is there a high magnesium content in the manufacture of gearboxes when, so I have been told, this makes the gearbox vulnerable to things such as salt water (apparently)?
7. What is a thrust bearing please?
And finally:
Eurocopter utilizes struts from a thrust bearing on the main rotor shaft of the AS-350 to react the flight loads.
9. Does this mean that the AS-350 rotorshaft must be bigger and stronger?
10. By putting the lifting loads onto a rotating shaft, does this cause any increase in control forces?
I beg your forgiveness if these are too many questions.
Thread Starter
Join Date: Dec 2012
Location: Aberdare, Wales
Age: 31
Posts: 174
Likes: 0
Received 0 Likes
on
0 Posts
Maybe the questions above were too many!
I have a different subject and it is to do with maintaining blade rpm in autorotation.
I have read that in order to sustain autorotation the very end part of the blade must have negative pitch and with some helicopters when the collective lever is fully down you can see what appears to be negative pitch at the very end of the blade as if the blade is twisted from the root - maybe the Gazelle and MD500 series are examples?
But on other types like the old big Bells like the 214ST and even the Huey Cobra (2 blade version) when the collective lever is fully down there does not appear to be any negative pitch at the end of the blade?
Can someone explain this?
I have a different subject and it is to do with maintaining blade rpm in autorotation.
I have read that in order to sustain autorotation the very end part of the blade must have negative pitch and with some helicopters when the collective lever is fully down you can see what appears to be negative pitch at the very end of the blade as if the blade is twisted from the root - maybe the Gazelle and MD500 series are examples?
But on other types like the old big Bells like the 214ST and even the Huey Cobra (2 blade version) when the collective lever is fully down there does not appear to be any negative pitch at the end of the blade?
Can someone explain this?
The blades are twisted (it's called washout) to equalise lift along the length of the blade - remember, lift is mostly about V squared which is very low at the root of the blade and in the region of 400 kts at the tips.
If you didn't have the washout, the tip section would produce an inordinate amount of lift which would cause bending stresses along the blade and at the hinges.
Maintaining Nr in autorotation is a function of rigging the blades such that at minimum pitch on the ground (MPOG) the amount of lift (and therefore drag) allows the mid-section of the blade to operate at an AoA that gives a forward component of rotor thrust, thus keeping the blades driving round. The tips and the root are producing more drag so when you have the optimum rigging, the blades driving and dragging forces are in balance and Nr is constant.
As for load hooks, many helos have them attached to the skin at several strong points (usually where the the lifting frames discussed earlier are).
Since the Puma was a. French and b. plastic, they used the load pole from the bottom of the MRGB. This did have the advantage that there wasn't a great mass of hook and cables under the aircraft when you didn't need to lift loads.
If you didn't have the washout, the tip section would produce an inordinate amount of lift which would cause bending stresses along the blade and at the hinges.
Maintaining Nr in autorotation is a function of rigging the blades such that at minimum pitch on the ground (MPOG) the amount of lift (and therefore drag) allows the mid-section of the blade to operate at an AoA that gives a forward component of rotor thrust, thus keeping the blades driving round. The tips and the root are producing more drag so when you have the optimum rigging, the blades driving and dragging forces are in balance and Nr is constant.
As for load hooks, many helos have them attached to the skin at several strong points (usually where the the lifting frames discussed earlier are).
Since the Puma was a. French and b. plastic, they used the load pole from the bottom of the MRGB. This did have the advantage that there wasn't a great mass of hook and cables under the aircraft when you didn't need to lift loads.
Thread Starter
Join Date: Dec 2012
Location: Aberdare, Wales
Age: 31
Posts: 174
Likes: 0
Received 0 Likes
on
0 Posts
Types of Take Off
I was told by a pilot I spoke to at Cardiff Heliport that in the old days helicopters would take off from oil rigs by moving forward and building up airspeed instead of the current vertical maximum performance take off used today.
I would be very grateful if someone were able to explain the vertical style take off because I see that it is also used onshore (as in the video below).
What are the benefits of this type of take off, does it bring the helicopter near to its limit of engine temperature and torque?
What about the loss of translational lift?
It is true that in the flight manual for the Squirrel that it says that for a normal take off you should speed to 60 knots before climbing?
I would be very grateful if someone were able to explain the vertical style take off because I see that it is also used onshore (as in the video below).
What are the benefits of this type of take off, does it bring the helicopter near to its limit of engine temperature and torque?
What about the loss of translational lift?
It is true that in the flight manual for the Squirrel that it says that for a normal take off you should speed to 60 knots before climbing?
The vertikal T/O at rigs are (supposed to be) Cat A take off's.
Moste two-engined helicopters wont be able to stay in hover if one engine quits. Cat A take off's is supposed to make it possible to return to the pad/take off site if needed. The vertical climb(or in fact often a bit backwards) will be performed until the height over the surroundings is good enough for a diving speed pickup if one engine qiuts in that moment. With enough speed the heli can stay airborne with one engine shut down.
If the heli has only one engine you wont stay airborne if ot quits anyhow. The nosing forward and speed build up is to reduce the time when yo have to low speed and/or altitude for a proper autorotation.
Moste two-engined helicopters wont be able to stay in hover if one engine quits. Cat A take off's is supposed to make it possible to return to the pad/take off site if needed. The vertical climb(or in fact often a bit backwards) will be performed until the height over the surroundings is good enough for a diving speed pickup if one engine qiuts in that moment. With enough speed the heli can stay airborne with one engine shut down.
If the heli has only one engine you wont stay airborne if ot quits anyhow. The nosing forward and speed build up is to reduce the time when yo have to low speed and/or altitude for a proper autorotation.
Thread Starter
Join Date: Dec 2012
Location: Aberdare, Wales
Age: 31
Posts: 174
Likes: 0
Received 0 Likes
on
0 Posts
Thank you, now I understand better.
So when on an oil rig the Cat A take off stops the helicopter from touching the sea in case one engine fails because it has more height?
The videos I have seen make it look hard for a helicopter to land back on a rig helipad as soon as it moves forward because there seems to be very little sight of the pad.
In the video below what is happening in the hover before they go up?
Also, if one engine stops at around 38 secs in the video I don't think they can land again on the pad can they? How will they see clearly what they are doing?
So when on an oil rig the Cat A take off stops the helicopter from touching the sea in case one engine fails because it has more height?
The videos I have seen make it look hard for a helicopter to land back on a rig helipad as soon as it moves forward because there seems to be very little sight of the pad.
In the video below what is happening in the hover before they go up?
Also, if one engine stops at around 38 secs in the video I don't think they can land again on the pad can they? How will they see clearly what they are doing?
Last edited by HeliStudent; 8th Jan 2013 at 11:15.
Didnt notice anything strange in hover before take off. Its a good idea though, to make sure everything is OK with green values before taking off.
This was the first hit on google for cat A (may be better ones), but it'll tell you about the cat A take of: http://www.mdhelicopters.com/helicop...operations.pdf
For the 38s mark on the video, didn't look at it very carefully but you have to keep the pad in sight for a landing until the height is sufficient for completing the take off on only one engine. You dont leave the eyemarks before you have made the decision ( Take-Off-Decision ) to complete the take off anyhow.
The needed height is of course type depending, and differs from time to time due to outside air temparerature, helicopter weight, air pressure and wind velocity.
For the one engined heli as a AS350 you could google for a diagram of 'dead mans curve' or autorotation diagram. Will show you where the best combination of height vs speed for a 'safe' auto if the engine stops working.
This was the first hit on google for cat A (may be better ones), but it'll tell you about the cat A take of: http://www.mdhelicopters.com/helicop...operations.pdf
For the 38s mark on the video, didn't look at it very carefully but you have to keep the pad in sight for a landing until the height is sufficient for completing the take off on only one engine. You dont leave the eyemarks before you have made the decision ( Take-Off-Decision ) to complete the take off anyhow.
The needed height is of course type depending, and differs from time to time due to outside air temparerature, helicopter weight, air pressure and wind velocity.
For the one engined heli as a AS350 you could google for a diagram of 'dead mans curve' or autorotation diagram. Will show you where the best combination of height vs speed for a 'safe' auto if the engine stops working.
Last edited by AAKEE; 8th Jan 2013 at 13:10.
