Go Back  PPRuNe Forums > Aircrew Forums > Rotorheads
Reload this Page >

Robinson Safety Notice SN-25 (carb ice) - additional 1.5 in. HG MAP?

Rotorheads A haven for helicopter professionals to discuss the things that affect them

Robinson Safety Notice SN-25 (carb ice) - additional 1.5 in. HG MAP?

Old 13th Sep 2012, 09:51
  #21 (permalink)  
 
Join Date: Sep 2007
Location: I am not sure where we are, but at least it is getting dark
Posts: 356
Received 19 Likes on 9 Posts
A friend at the Robinson Safety Course just got confirmation from Tim Tucker (Chief Instructor at Robinson) that YES we can pull the additional 1.5 in. Hg (if it is there - see the bit in the SN-25 about high DA).
If this is the intent of SN-25, then another amendment of the POH is required. As it stands, manifold pressure limitations are published in section 2, and placarded in the cockpit. As long as this doesn't get amended, you can still not legally use more MAP than what the limitation chart tells you, carb heat or not.
lelebebbel is offline  
Old 13th Sep 2012, 10:21
  #22 (permalink)  
 
Join Date: Jan 2008
Location: Ecton
Age: 70
Posts: 71
Likes: 0
Received 0 Likes on 0 Posts
Lelebebbel (or something along those lines)
Correct, that has been the problem from the start. The Limitation is a BHP limitation, therefore referring to MAP as a limitation is not the best way to convey the information to the pilot (in my view).
There will be situations where given certain atmospheric conditions where a reasonable amount of carb heat has to be applied to keep the CAT needle out of the yellow will result in the MAP indication being greater than the MAP limit - I cannot fly, even if I know the actual BHP is not being exceeded.

Keep them in the green, best wishes

Dick Sanford
Dick Sanford is offline  
Old 14th Sep 2012, 07:42
  #23 (permalink)  
 
Join Date: Mar 2006
Location: UK
Age: 57
Posts: 34
Received 0 Likes on 0 Posts
So this turns into a different kettle of fish, or is that barrel of monkeys?

I'm perfectly happy with the physics and had pretty much figured this phenomenon out for myself. I also go to great lengths to make sure students understand the difference between over boosting (exceeding BHP limits through the gearbox), over pitching (pulling so much pitch that engine goes beyond full throttle so that rrpm droop ensues) and the difference that DA makes to the situation.

With the issue of the SN and, as Lelebebbel has observed, no change to POH or placards, I have to ask what is the motivation of the SN? Are we to try and teach students about something that is legally unusable? Is putting the information in a SN a hope that the flying community will run with it without Robinson having to formally change performance and perhaps certification data?

Please don't get me wrong, love the Robbos, lots of hours in 'em but this seems to be a tip and a wink rather than information we can actually use to educate pilots about their machines.

Thoughts anyone?
HeliChopter is offline  
Old 14th Sep 2012, 09:33
  #24 (permalink)  
 
Join Date: Jan 2008
Location: Ecton
Age: 70
Posts: 71
Likes: 0
Received 0 Likes on 0 Posts
I am not sure why RHC issuing a SN should be treated with suspicion? For RHC to change the required pages in the POH, which have to be FAA approved takes time. Issuing information via a SN does not. Sounds like "damned if you do, damned if you don't" RHC are stating in a publication that the pilot can take into account the reduction in engine output power when using carb' heat. Not sure where the "legally unusable" come from.

I am not sure what is meant by over boosting through the gearbox. Over boosting is usually something the pilot might do to a turbocharged or supercharged engine (you cannot over boost a normally aspirated engine as in the Robinson R22, R44) Exceeding the BHP limits will increase the drive train load per cycle thus subjecting the components to greater loads than RHC applied during their fatigue testing - the pilot becomes a test pilot.
Full throttle on your engine is a physical stop, after this point the engine will not produce any more HP.

We are not talking about the risk of exceeding the limited BHP but getting across to the pilot the effect of the application of carb' heat on the power output of the engine.
Dick Sanford is offline  
Old 14th Sep 2012, 10:37
  #25 (permalink)  
 
Join Date: Jan 2012
Location: Blackbushe City Limits
Posts: 69
Likes: 0
Received 0 Likes on 0 Posts
Confused student.

So for a R22 Beta.
OAT 25C.
CH Cold.
Max continuous: 23.75
5 min take off: 24.75
CH Hot (+1.5").
Max continuous: 25.25
5 min take off: 26.25

So its then OK to exceed the MP red line at 25.2 since the whole scale has effectively shifted? Red line is now at 25.2 + 1.5 = 26.7.

Working things backwards from the CH hot figures above to use the CAT gauge to base calculations upon, the CAT I would be looking for to support this would be off the scale, which terminates at +40C.

OAT 25C.
CH Hot (+1.5").
Max continuous: 25.25
25.25 - 23 = 2.25
2.25 * 100 = 225
225 / 3 = 75C

LOZZ is offline  
Old 14th Sep 2012, 14:37
  #26 (permalink)  
 
Join Date: Mar 2006
Location: UK
Age: 57
Posts: 34
Received 0 Likes on 0 Posts
Hi Dick,

My post was not intended to imply suspicion I promise. Merely to try and garner opinion on to what practical use this information can be put. Combined with your own excellent document the SN does explain the effect of carb heat on engine power and MAP indications.

What does not appear to be stated however is what you can do with this knowledge in the cockpit. We are obliged to fly within the published (POH) limits which do not currently reflect any allowance for using carb' heat. Happy to be corrected here.

Therefore the debate I'm inviting is "How do we put this knowledge to use?".

The document says that for a given BHP output the application of full carb' heat will increase the MAP reading by 1.5" (providing of course you don't reach full throttle first). What it doesn't say is that it's therefore legal to use an amended MAP limit when flying with full carb' heat.

Maybe that's not the intent of the SN and I'm missing the point but the questions are already being asked, see LOZZ's post above.

Perhaps amendments to the POH are in the pipeline?

Disclaimer: It doesn't seem to matter what you type it always seems to come over as confrontational on here! Really not the case here. I welcome the published information, I'm just interested in opinions on how we put it to use.
HeliChopter is offline  
Old 14th Sep 2012, 19:43
  #27 (permalink)  
Thread Starter
 
Join Date: Oct 2011
Location: Bend, OR
Age: 46
Posts: 18
Likes: 0
Received 0 Likes on 0 Posts
Thumbs up @LOZZ - YES

@LOZZ: In your example, according to Robinson Safety Course this week, yes you could exceed the redline, because the hot air reduces the air density and therefore means you are pulling less BHP, the extra 1.5 in. allow for that, even if it means exceeding the redline. You are NOT exceeding the engine BHP limit. I think Dick explained this well.

Now if you are at operating high DA you may not be able to get there and may suffer a drop in engine/rotor RPM!

Last edited by JB77UK; 14th Sep 2012 at 19:44. Reason: Mistake
JB77UK is offline  
Old 14th Sep 2012, 19:46
  #28 (permalink)  
Thread Starter
 
Join Date: Oct 2011
Location: Bend, OR
Age: 46
Posts: 18
Likes: 0
Received 0 Likes on 0 Posts
Ambiguity

@HeliChopper:

I wish Robinson would release a draft of changes to POH to forums like this to get feedback on any ambiguity. Would save themselves a ton of emails and questions at every RSC.
JB77UK is offline  
Old 14th Sep 2012, 20:27
  #29 (permalink)  
 
Join Date: Jan 2008
Location: Ecton
Age: 70
Posts: 71
Likes: 0
Received 0 Likes on 0 Posts
"Approximately 1.5 in. Hg additional MAP is required to generate maximum continuous power (MCP) or takeoff power (TOP) with full heat applied."

The above is taken from the subject SN. The intent seems to be clear to me but I would agree that it would be absolute if it was ratified in the POH limitations section, however as explained, this takes time.

I cannot and do not speak for RHC but I would think that a POH limitation section amendment is on it's way.

I for one, will be very happy to see such an amendment as those people that know me or have attended my European Robinson Flight Safety Course will know that I have been teaching/discussing/explaining the subject of Carb Icing in the Robinson Product for more years than I wish to remember.
In particular this current subject of Carb' heat -v- BHP.

I apologise if I have come across as 'confrontational' as that was not the intent and is unhelpful, however, we should be able to have a "Frank" !!!!!!!!!!!! discussion.
Dick Sanford is offline  
Old 15th Sep 2012, 07:21
  #30 (permalink)  
 
Join Date: Mar 2006
Location: UK
Age: 57
Posts: 34
Received 0 Likes on 0 Posts
Dick, No apology required, just poor grammar on my part, I should have written "whatever one writes on here....". Seems so easy to get into a bun fight.

Careful though, after the "Frank" gag you could be labled a pundit!
HeliChopter is offline  
Old 15th Sep 2012, 09:27
  #31 (permalink)  
 
Join Date: Mar 2006
Location: scotland
Posts: 212
Received 0 Likes on 0 Posts
Leave it on?

Does this mean that if I lose 1.5 inches MAP with full carb heat applied and I fly around at 2000ft above sea level most of the time that I could leave the full carb heat on permanently and just pull 1.5 extra inches so long as I don't run out of throttle which I know here in Scotland at these altitudes I won't? Why bother with the on and off and continually checking if I'm reading this correctly?
bvgs is offline  
Old 15th Sep 2012, 13:28
  #32 (permalink)  
 
Join Date: Jan 2008
Location: Ecton
Age: 70
Posts: 71
Likes: 0
Received 0 Likes on 0 Posts
bvgs:
Well, yes, you could, however why would you want to unless the conditions required you to?
The engine is less efficient, that is why we need to pull more indicated MAP, it's running rich, therefore your fuel consumption is increased.
Under certain atmospheric conditions you could be actually inducing carb' icing by having it applied. That is why you have a lock on the carb' assist system.

Regards Dick Sanford
Dick Sanford is offline  
Old 15th Sep 2012, 15:38
  #33 (permalink)  
 
Join Date: Mar 2006
Location: scotland
Posts: 212
Received 0 Likes on 0 Posts
Yes I can see how that makes sense, always tend to run with it on more than is required anyway just to be on the safe side. Question? Why when hovering at almost max weight so the mp is right up on the limit, the throttle pretty much approaching its limit does the carb temp gauge show its very cool, to the point of needing heat applied to keep it safe. I thought that the venturi would be well open and therefore the air not accelerating as quick etc etc...just wondered ? Loved your article on the icing, perhaps if I read it again I might be able to answer my own question but the pub calls for a pint
bvgs is offline  
Old 15th Sep 2012, 23:05
  #34 (permalink)  
 
Join Date: Jan 2008
Location: Ecton
Age: 70
Posts: 71
Likes: 0
Received 0 Likes on 0 Posts
bvgs:

Pleased you found my carb' icing document interesting. Yes, you will find the answer in the document, even if you are demanding your 131 BHP the butter fly valve is nowhere near fully open. The R22 Beta II is a bigger problem than an R22 Beta as the 0-360-J2A BII engine is more de-rated than the 0-320-B2C Beta engine, take a look at the butterfly angles I have presented in the document i.e. MAP -v- butterfly angle.
Please to hear that you use a little more than a little less, that is the right way to approach the setting. The manufacture has given the pilot a method of preventing the ice build up in the carb', it is the pilot's primary responsibility to prevent the engine from stopping by using it.
Depending on the flight exercise I was flying and the atmospheric conditions I very often left the car' heat in the full hot position so as not to have to disturb Blogs.

As I tell the pilots on the flight safety course; I have had the R22 engine stop on me due to carb' icing and the scary thing was, we did not know it had stopped! Work that one out!

Keep them in the green.

Regards Dick

Last edited by Dick Sanford; 15th Sep 2012 at 23:07.
Dick Sanford is offline  
Old 15th Sep 2012, 23:09
  #35 (permalink)  
 
Join Date: Sep 2007
Location: I am not sure where we are, but at least it is getting dark
Posts: 356
Received 19 Likes on 9 Posts
The CAT probe is located before the throttle valve in the R22 carburettor. The throttle position therefor shouldn't affect indicated temperature much (hence the need for the "below 18in apply carb heat" placard).
It can imagine that you would see a slight decrease in indicated CAT because more fuel is being injected at higher power.

Last edited by lelebebbel; 15th Sep 2012 at 23:11.
lelebebbel is offline  
Old 8th Dec 2023, 08:17
  #36 (permalink)  
 
Join Date: Mar 2006
Location: scotland
Posts: 212
Received 0 Likes on 0 Posts
R44 Carb Ice

I totally get the carb ice thing when we are coming into land, lowering the lever and creating a Venturi causing the air to speed up and subsequently get colder thus allowing ice to form from in the carburettor due to the moisture in the air. So all good with that. However, I was hover taxing the other day at just under 25 inches of manifold pressure (almost at full weight) and the gauge was going well into the minus. No big deal as I pulled carb heat to ensure it was above zero but of course subsequently lost some power and had to cushion crawl and use the down draft. Im just wondering why the carb gets so cold when the butterfly valve is almost fully open. Is it simply the vast amount of air being sucked in? Thanks for any insights👍🏻👍🏻
bvgs is offline  
Old 8th Dec 2023, 10:51
  #37 (permalink)  
 
Join Date: Jun 2012
Location: SE of there
Age: 43
Posts: 253
Received 48 Likes on 32 Posts
It's a lot of air being sucked in through relatively small opening. Venturi is still prety much in effect.
admikar is offline  
Old 8th Dec 2023, 10:57
  #38 (permalink)  
 
Join Date: Mar 2019
Location: Europe
Posts: 234
Received 15 Likes on 9 Posts
Originally Posted by bvgs
... Im just wondering why the carb gets so cold when the butterfly valve is almost fully open. Is it simply the vast amount of air being sucked in? Thanks for any insights👍🏻👍🏻
In order to atomise the fuel and turn it from liquid into gaseous form, energy is required (gaseous state having a higher energy than liquid state). This transfer of energy from the air to the fuel 'sucks' heat out of the air and cools it down slightly. There's far more thermodynamics than I can comprehend going on in reality, but that's one mechanism at work explained in a simple way that can help explain the temperature drop from ambient even at full power.
ApolloHeli is offline  
The following 2 users liked this post by ApolloHeli:
Old 8th Dec 2023, 12:01
  #39 (permalink)  
 
Join Date: Jul 2017
Location: UK
Posts: 457
Likes: 0
Received 104 Likes on 72 Posts
Would carb icing even occur at 24-25" ?
hargreaves99 is offline  
Old 8th Dec 2023, 14:27
  #40 (permalink)  
 
Join Date: Jun 2012
Location: SE of there
Age: 43
Posts: 253
Received 48 Likes on 32 Posts
Yes, but it is unlikely. While OP saw temp decrease, icing is not very likely at high power.
Excerpt from Wiki:
The venturi effect can reduce the air temperature by 39 K; 39 C (70 F). In other words, air at an outside temperature of 38 C (100 F), can drop to −1 C (30 F) in the carburetor. Carburetor icing most often occurs when the outside air temperature is below 21 C (70 F) and the relative humidity is above 80 percent.[1] The risk of carburettor icing is significantly increased at partial power settings (such as when power is reduced during descent), due to the cooling effect of the partly-closed throttle.[2]
admikar is offline  
The following users liked this post:

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.