A flutter test is a test of vibration of the aircraft. A "flight flutter test", as the name suggests, is a test of vibration in the airframe during flight. This usually happens when the airplane gathers speed, usually nearing its maximum theoretical speed. This in effect is done by gaining significant elevation and diving the plane so it gathers speed.
The vibration is tested on various parts of the plane and compared against theoretical limits. If the plane vibrates too much, it risk getting ripped apart in mid-air. Flight flutter testing is a significant and hi-risk test for most airplanes.
Flutter involves the interaction between aerodynamics, elastic and inertia forces that come into play on structures that results in an unwanted (sometimes dangerous) oscillation (vibration).
INS Hansa (Goa) In the final phase of its tests before formal commissioning, India's indigenous light combat aircraft Tejas went past its ultimate speed of 1,350 KMPH over the Goa skies and clocked the fastest speed ever, a top IAF officer said on Tuesday.
"The aircraft went past its ultimate speed of 1350 kmph on December 7 over the skies in Goa after take off from the naval air station INS Hansa," Commander Rohit Varma, project director (flight test), National flight test centre, told reporters here.
"This is the fastest speed ever achieved by an Indian- made fighter aircraft," he said. The aircraft also passed flight flutter test diving from an altitude of four kilometers to almost sea level at 900 feet. "Tejas has already passed high-altitude tests in Leh, the desert rigours in Rajasthan and now it has proved its worth over the maritime space in Goa," Varma said.
INS Hansa Base (Goa), Dec 9 (PTI) The aircraft dived towards the sea as the pilot lifted his hands off the control stick for five seconds. In such a short duration, it had plummeted from four kilometres above sea level to just 900 metres before the pilot hit to the throttle again to take to the skies.
The 'flight flutter test', a mandatory procedure for fighter crafts, was conducted off the Goa coast on Tejas in maritime environment by the agencies, which are testing this technology, a senior Indian Air Force officer said today.
So the tejas achieved a speed of mach 1350 kmph at sea level off goa during this flutter test. Some people have misunderstood it to be a dive to gather enough momentum to achieve max sea level speed to overcome the induced drag due to the shortcommings pointed out by the cemilac team. But this interpretation also seems wrong.
So there is no reason to believe that this 1350 kmph achieved at sea level from a 4km dive cannot be achieved in level flight.
Read the report carefully. The pilot took his hands off the handle and aircraft was diving free from the height.
So the pilot was not pushing the stick in the dive to overcome any drag that stops the fighter from reaching its top sea level speed in a normal level flight at sea level to overcome any design deficiency.
An interesting graph from the presentation-
Originally Posted by sudhir007
http://www.aero.iisc.ernet.in/kartik/INCAST_sudha5.pdf
this graph depicts the step by step opening of the flight envelope of tejas. Without understanding this process many people say that tejas is unable to achieve it's topspeed due to design faults and drag issues and it is inferior to grippen and even mig-21.
So the achieved stop speeds at sea level and mach 1.6 does not correspond to the design specs. It corresponds to the partially opened flight envelope specs only.
Extrapolating the curves will show that suitable engine even mk-1 can have much higher speeds than achieved within the partially opened flight envelope.
Observe the lower most curve in the graph. That is the Calibrated Airspeed (CAS) curve. It suggests that the LCA has been designed for a max CAS of 1354 kmph which the reporters mistook for the max speed of the LCA. CAS has nothing to do with the max speed of an air craft and it is impossible to estimate the max speed of an aircraft from its max design CAS.
So what is CAS?
Calibrated airspeed (CAS) is the speed shown by a conventional airspeed indicator after correction for instrument error and position error. Most EFIS displays also show CAS. At high speeds and altitudes, calibrated airspeed is further corrected for compressibility errors and becomes equivalent airspeed (EAS).
When flying at sea level under International Standard Atmosphere conditions (15°C, 1013 hPa, 0% humidity) calibrated airspeed is the same as equivalent airspeed and true airspeed (TAS). If there is no wind it is also the same as ground speed (GS). Under any other conditions, CAS may differ from the aircraft's TAS and GS.
Calibrated airspeed in knots is usually abbreviated as KCAS, while indicated airspeed is abbreviated as KIAS.
Practical applications of CAS
CAS has two primary applications in aviation:
"¢ for navigation, CAS is traditionally calculated as one of the steps between indicated airspeed and true airspeed;
"¢ for aircraft control, CAS (and EAS) are the primary reference points, since they describe the dynamic pressure acting on aircraft surfaces regardless of density altitude, wind, and other conditions. EAS is used as a reference by aircraft designers, but
EAS cannot be displayed correctly at varying altitudes by a simple (single capsule) airspeed indicator. CAS is therefore a standard for calibrating the airspeed indicator such that CAS equals EAS at sea level pressure and approximates EAS at higher altitudes.
Calibrated airspeed - Wikipedia, the free encyclopedia
In short what it basically means is - a conventional airspeed indicator in the LCA will show a true airspeed under International Standard Atmosphere conditions (15°C, 1013 hPa, 0% humidity) upto a speed of 1354 kmph. Beyond this speed the CAS and TAS for the LCA will start diverging at ISA conditions.
The graph also proves that the LCA has already been tested at Mach 1.4 at an altitude of 1000 m. Further, it can be noted that the LCA is capable of achieving Mach 1.6 at altitudes of about 7000 m at CAS.
Because of the restricted flight envelope , the curves in the graph have been truncated after Mach 1.6. But if one extrapolates the CAS curve, it can be observed that the LCA will touch speeds of upto Mach 2 at altitudes approaching 10000 m. The MKI can achieve a top speed of about Mach 2.3 at an altitude of 11000 m.
However, what is interesting to note is that the MKI has a max ground level speed of only 1350 kmph. The LCA on the other hand has achieved that speed at sea level that too in Goa.
The location is significant because the weather conditions in Goa vary drastically from ISA conditions of 15°C, 1013 hPa and 0% humidity. The density of air in Goa at sea level will be higher than the density of air at ISA conditions at ground level
taht too with alesser powered GE-F404 engine.
