"Full Throttle" height
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"Full Throttle" height
Hi, Forumites:
Has anyone come across, or heard of the expression, "Full Throttle" height? A friend has read this expression in relation to RR Merlin engines, for example the Full Throttle height for a Spitfire MkIX was 19,000 ft....
Tia
MACR
Has anyone come across, or heard of the expression, "Full Throttle" height? A friend has read this expression in relation to RR Merlin engines, for example the Full Throttle height for a Spitfire MkIX was 19,000 ft....
Tia
MACR
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If full throttle is required to give a certain manifold pressure at any selected r.p.m., then the altitude at which this occurs is the full-throttle height for that condition of r.p.m. and boost.
I found that from flitting around on google, and also this: Spitfire IX Trials at +25 boost
I believe the term only applies to supercharged engines, but may well be wrong. Hope that helps until someone with a better understanding comes along!
Regards,
Spr K, (retd)
PS: I am NOT a test pilot, or involved with testing... Just trying to help
I found that from flitting around on google, and also this: Spitfire IX Trials at +25 boost
I believe the term only applies to supercharged engines, but may well be wrong. Hope that helps until someone with a better understanding comes along!
Regards,
Spr K, (retd)
PS: I am NOT a test pilot, or involved with testing... Just trying to help
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Full throttle height (also referred to as "Critical Altitude") is the height at which any increase in altitude will see a reduction in manifold pressure, airflow through the engine and brake horse power. The altitude is specific to a particular MAP and RPM combination, and will be specified for a particular condition ie maximum rated power, maximum cruise power, maximum combat power etc
Full throttle height (also referred to as "Critical Altitude") is ..
Sorry - wrong.
Full throttle height refers to a supercharged engine. It is the height where the throttle will be fully open for a given set power (rpm and MAP).
Critical altitude refers to a turbocharged engine. It is the height, with the throttle in any position, where the wastegate becomes fully closed. In this application, whether ground boosted or altitude boosted, the throttle will be fully open at sea level take off conditions, and so full throttle height cannot apply.
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Sorry - wrong.
What they taught us at Pensacola many decades ago. Bolding mine.
The effect of altitude is to reduce the engine airflow and power output and supercharging is necessary to maintain high power output at high altitude. Since the basic engine is able to process air only by the basic volume displacement, the function of the supercharger is to compress the inlet air and provide a greater weight of air for the engine to process. Of course, shaft power is necessary to operate the engine driven supercharger and a temperature rise occurs through the supercharger compression. The effect of various forms of supercharging on altitude performance is illustrated in figure 2.17.
The unsupercharged-or naturally aspirated-engine has no means of providing a manifold pressure any greater than the induction system inlet pressure. As altitude is increased with full throttle and a governed RPM, the airflow through the engine is reduced and BHP decreases. The first forms of supercharging were of relatively low pressure ratio and the added airflow and power could be handled at full throttle within detonation limits. Such a “ground boosted" engine would achieve higher output power at all altitudes but an increase in altitude would produce a decrease in manifold pressure, airflow, and power output.
More advanced forms of supercharging with higher pressure ratios can produce very large engine airflow. In fact, the typical case of altitude supercharging will produce such high airflow at low altitude operation that full throttle operation cannot be utilized within detonation limits. Figure 2.17 illustrates this case for a typical two-speed engine driven altitude supercharging installation. At sea level, the limiting manifold pressure produces a certain amount of BHP. Full throttle operation could produce a higher MAP and BHP if detonation were not the problem. In this case full throttle operation is unavailable because of detonation limits. As altitude is increased with the supercharger or “blower” at low speed, the constant MAP is maintained by opening the throttle and the BHP increases above the sea level value because of the reduced exhaust back pressure. Opening the throttle allows the supercharger inlet to receive the same inlet pressure and produce the same MAP. Finally, the increase of altitude will require full throttle to produce the constant MAP with low blower and this point is termed the “critical altitude” or “full throttle height.” If altitude is increased beyond the critical altitude, the engine MAP, airflow, and BHP decrease.
The critical altitude with a particular supercharger installation is specific to a given combination of MAP and RPM. Obviously, a lower MAP could be maintained to some higher altitude or a lower engine speed would produce less supercharging and a given MAP would require a greater throttle opening. Generally, the most important critical altitudes will be specified for maximum, rated, and maximum cruise power conditions.
A change of the blower to a high speed will provide greater supercharging but will require more shaft power and incur a greater temperature rise. Thus, the high blower speed can produce an increase in altitude performance within the detonation limitations. The variation of BHP with altitude for the blower at high speed shows an increase in critical altitude and greater BHP than is obtainable in low blower. Operation below the high blower critical altitude requires some limiting manifold pressure to remain within detonation limits. It is apparent that the shift to high blower is not required just past low blower critical altitude but at the point where the transition from low blower, full throttle to high blower, limit MAP will produce greater BHP. Of course, if the blower speed is increased without reducing the throttle opening, an “overboost” can occur.
Since the exhaust gases have considerable energy, exhaust turbines provide a source of supercharger power. The turbosupercharger (TBS) allows control of the supercharger speed and output to very high altitudes with a variable discharge exhaust turbine (VDT). The turbosupercharger is capable of providing the engine airflow with increasing altitude by increasing turbine and supercharger speed. Critical altitude for the turbosupercharger is usually defined by the altitude which produces the limiting exhaust turbine speed.
The minimum specific fuel consumption of the supercharged engine is not greatly affected by altitudes less than the critical altitude. At the maximum cruise power condition, specific fuel consumption will decrease slightly with an increase in altitude up to the critical altitude. Above critical altitude, maximum cruise power cannot he maintained but the specific fuel consumption is not adversely affected as long as auto lean or manual lean power can be used at the cruise power setting.
Is speed dependent as well due ram recovery at the intake. Consider combat aircraft carrying underwing stores ie drop, tanks, bombs etc
Last edited by Brian Abraham; 17th Jan 2011 at 04:43.
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I know you gave us a whopping in the Ashes LM, but no need to take your bat and ball and retire. Just not cricket old man.
FTH versus Critical may be down to different usages on either side of the pond.
major seversky | american aircraft | american public | 1942 | 2045 | Flight Archive
Note that all three aircraft mentioned are supercharged rather than turbo. FTH or Critical Altitude very rarely get a mention in flight manuals eg
P-51 the British manual mentions FTH in passing, no mention of anything in the American
B-29 only mention is a line on performance graphs indicating "Wastegate closed"
P-38 one sentence saying critical altitude is when the wastegate closes
Beaufighter no mention
Lancastrian no mention
B-24 only refers to "Altitude", though it is referring to what is FTH or Critical Altitude
Merlin 130 as fitted to the DH Hornet only refers to "Altitude", as with the B-24. Merlin chart reproduced below. Test bed figures.
-----------------------------------------------Boost
-----------------------Gear--BHP----RPM---lb/sq. in--Altitude
Max. T-O power-------MS--1,670--3,000---18---------SL
Max. Combat power---MS--2,030--3,000---25-------1,250
-------------------------FS--1,890--3,000---25------13,750
Max. Climbing power--MS--1,420--2,850----12-----11,000
-------------------------FS--1,350--2,850---12------22,000
Max. cruising power---MS--1,205--2,650-----9------11,000
-------------------------FS--1,190--2,850-----9------25,000
Once again we strike language difficulties. Gear refers to supercharger speed, and MS equates to low blower and FS high blower.
Edited to add, see P-51 Mustang Performance for the usage of the term Critical Altitude with respect to testing the P-51 (supercharger) by Army Air Forces Material Command,Wright Field, Dayton, 18 May 1943. You will see the same used in the tests by the same organization on the P-38 (turbos) P-38 Performance Trials
On the facts presented it would seem Critical Altitude = Full Throttle Height.
FTH versus Critical may be down to different usages on either side of the pond.
major seversky | american aircraft | american public | 1942 | 2045 | Flight Archive
"Our main fighters," he writes, "the Kittyhawk and Tomahawk and the Airacobra, are stymied by engine shortcomings above the critical altitude of 16,000ft., though British and Axis aircraft function above 30,000ft.
P-51 the British manual mentions FTH in passing, no mention of anything in the American
B-29 only mention is a line on performance graphs indicating "Wastegate closed"
P-38 one sentence saying critical altitude is when the wastegate closes
Beaufighter no mention
Lancastrian no mention
B-24 only refers to "Altitude", though it is referring to what is FTH or Critical Altitude
Merlin 130 as fitted to the DH Hornet only refers to "Altitude", as with the B-24. Merlin chart reproduced below. Test bed figures.
-----------------------------------------------Boost
-----------------------Gear--BHP----RPM---lb/sq. in--Altitude
Max. T-O power-------MS--1,670--3,000---18---------SL
Max. Combat power---MS--2,030--3,000---25-------1,250
-------------------------FS--1,890--3,000---25------13,750
Max. Climbing power--MS--1,420--2,850----12-----11,000
-------------------------FS--1,350--2,850---12------22,000
Max. cruising power---MS--1,205--2,650-----9------11,000
-------------------------FS--1,190--2,850-----9------25,000
Once again we strike language difficulties. Gear refers to supercharger speed, and MS equates to low blower and FS high blower.
Edited to add, see P-51 Mustang Performance for the usage of the term Critical Altitude with respect to testing the P-51 (supercharger) by Army Air Forces Material Command,Wright Field, Dayton, 18 May 1943. You will see the same used in the tests by the same organization on the P-38 (turbos) P-38 Performance Trials
On the facts presented it would seem Critical Altitude = Full Throttle Height.
Last edited by Brian Abraham; 19th Jan 2011 at 04:02.
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Full throttle Height
Don't want to fall out with anyone on this. However, Brian Abraham is completely correct in post #5.
However, how FTH is obtained will depend on the method of throttle lever to throttle butterfly (or Corliss valve) connection.
With a directly connected (i.e. no compensating system) throttle system, the pilot will be responsible for ensuring the engine is not overboosted in the climb, as he (or indeed she) will need to gradually move the throttle lever forward to compensate for decreasing ambient pressure with altitude. At the height where the throttle lever is fully forward and the manifold pressure is maintained at the selected MAP, that altitude is the Full Throttle Height for the selected MAP. Any further increase in altitude will result in decreasing MAP and therefore BHP.
For an automatic compensating system, the pilot puts the throttle lever fully forward at a low altitude (engine limitations allowing) or to a limiting MAP (e.g. max continous) and the manifold pressure will be automatically limited to that selected MAP. As full throttle height is reached, the throttle butterfly or Corliss valve will be fully open. Any increrase in altitude above FTH will be noted by a gradual decrease in MAP.
As can be seen, FTH is completely variable dependent on selected MAP. However, quoted manufacturers figures normally relate to Maximum Take Off Power (often with time limit imposed), METO (maximum except take off power) or Maximum Continous, and give figures for low blower speed and high blower speed (or in Rolls Royce parlance, Moderate Speed and Full Speed).
And as an aside, FTH is not always in excess of 10,000 ft. For low-altitude engines, such as the Rolls Royce Griffon 58, FTH low gear @ max rpm of 2750 and 67" MAP was 2000 ft. FTH high gear @ max rpm of 2750 and 81" MAP was 3000 ft.
For eMACaRe's Spit mk IX with a 60 series Rolls Royce Merlin, FTH of 19,000 ft in high gear sounds about right.
In a simple nutshell, for supercharged engines (including turbosupercharged) the altitude reached where the throttle butterfly or Corliss valve can open no more is FTH for the selected MAP.
Hope this helps.
camlobe
However, how FTH is obtained will depend on the method of throttle lever to throttle butterfly (or Corliss valve) connection.
With a directly connected (i.e. no compensating system) throttle system, the pilot will be responsible for ensuring the engine is not overboosted in the climb, as he (or indeed she) will need to gradually move the throttle lever forward to compensate for decreasing ambient pressure with altitude. At the height where the throttle lever is fully forward and the manifold pressure is maintained at the selected MAP, that altitude is the Full Throttle Height for the selected MAP. Any further increase in altitude will result in decreasing MAP and therefore BHP.
For an automatic compensating system, the pilot puts the throttle lever fully forward at a low altitude (engine limitations allowing) or to a limiting MAP (e.g. max continous) and the manifold pressure will be automatically limited to that selected MAP. As full throttle height is reached, the throttle butterfly or Corliss valve will be fully open. Any increrase in altitude above FTH will be noted by a gradual decrease in MAP.
As can be seen, FTH is completely variable dependent on selected MAP. However, quoted manufacturers figures normally relate to Maximum Take Off Power (often with time limit imposed), METO (maximum except take off power) or Maximum Continous, and give figures for low blower speed and high blower speed (or in Rolls Royce parlance, Moderate Speed and Full Speed).
And as an aside, FTH is not always in excess of 10,000 ft. For low-altitude engines, such as the Rolls Royce Griffon 58, FTH low gear @ max rpm of 2750 and 67" MAP was 2000 ft. FTH high gear @ max rpm of 2750 and 81" MAP was 3000 ft.
For eMACaRe's Spit mk IX with a 60 series Rolls Royce Merlin, FTH of 19,000 ft in high gear sounds about right.
In a simple nutshell, for supercharged engines (including turbosupercharged) the altitude reached where the throttle butterfly or Corliss valve can open no more is FTH for the selected MAP.
Hope this helps.
camlobe
It's really quite simple...
Full throttle height - the height an aircraft can reach whilst maintaining the maximum permitted MAP for the engine. Above FTH the MAP will start to drop because the supercharger can no longer compensate for decreasing ambient air pressure to maintain that maximum permitted MAP. Example below is for a P-40-N, FTH is marked with the asterisk *
Other aircraft/engine operational settings will change the FTH but MAP will always start to drop above FTH even though the throttle is wide open. The aircraft will climb above FTH of course but from then on it is a losing battle between the supercharger and ever-decreasing atmospheric pressure. Service ceiling is, I believe, the height reached when rate of climb is down to 100 fpm and absolute ceiling the height reached when rate of climb becomes zero - again, see below.
Other aircraft/engine operational settings will change the FTH but MAP will always start to drop above FTH even though the throttle is wide open. The aircraft will climb above FTH of course but from then on it is a losing battle between the supercharger and ever-decreasing atmospheric pressure. Service ceiling is, I believe, the height reached when rate of climb is down to 100 fpm and absolute ceiling the height reached when rate of climb becomes zero - again, see below.
