PDA

View Full Version : Max TAS



mcguif0a
11-13-2008, 08:21 PM
So as a self declared kitfox newby I have gotten myself a little confused.
In a couple of locations most notably on the Kitfox website is a desciption of the series 7 being able to "cruise at over 150mph" when powered by the R914 with the 115hp. My problem is that Vne for my SS is 140mph. I assumed that the SS Vne is because beyond 140 the fabric is at risk of being damaged or turbulance at this speed could cause accelerations beyond the aloud limit.

So, is the 150mph cruise speed an old add ploy that has just stuck or were those planes really able to cruise that fast? Also was the series 7 that much different from the SS that they had a diferent Vne?
I know most of you know the answer to this one so please set me straight.

kmul
11-14-2008, 07:18 AM
turbo 912 can cruise at higher altitudes which gives a higher tas. ias remains the about the same. so you are not exceeding vne. you are just in thinner air.

jdmcbean
11-14-2008, 08:43 AM
TAS vs IAS

True Airspeed: The actual speed of an aircraft relative to the air it is flying through... the calibrated airspeed corrected for temperature and density(or compressibility)

Indicated airspeed: Used in aircraft operation as the aircraft stalling speed and structural limiting speeds such as Va or Vne are dependent on indicated airspeed. It is simply, the airspeed as shown by a differential-pressure airspeed indicator...

Typically as altitude increases IAS decreases.

jonbakerok
11-15-2008, 07:35 AM
Actually, I don't think that's the case. I believe VNE is the only V speed that is based on TRUE airspeed rather than indicated. The fact that there are fewer molecules does not diminish their relative velocity or the velocity's effect on the airframe. It's particularly significant if the VNE happens to be due to flutter issues rather than structural limits.

I don't mean to sound like some sort of expert, but there was an article in the Vans newsletter a while back on this subject that generated a lot of discussion over on VansAirforce.net.

Since the Kitfox doesn't use balanced tail surfaces, flutter could be a problem, although 150 mph is probably still low enough to avoid the issue.

darinh
11-15-2008, 10:20 AM
As one who owns a Series 7 with the 914, I am a bit skeptical of the numbers Skystar quoted. I say skeptical because I have yet to fly my Fox any higher than 10,500 with a DA of 11,400 and saw TAS of 120 mph and IAS or 98 mph at 5000 rpm and 29" MAP. Maybe you could get that extra 30 mph up in the lower flight levels, I am not sure. I know John has flown the Skystar Series 7 they claimed hit these speeds...What were your findings John? One of these days on a flight over Kings Peak, I will take mine to 14k or 15k and see what I get.

I do love the Kitfox and the 914 though!

jonbakerok
11-17-2008, 05:51 AM
http://www.vansaircraft.com/pdf/hp_limts.pdf

I should have given the reference. Van goes into great detail in the above document on why TAS is the basis of VNE, not IAS.

The fact that it's possible to exceed VNE in most planes at altitude doesn't make it a good idea. In fact, I remember reading an article once about the U-2 spy plane that mentioned that one of the difficult things about flying it was that at its cruising altitude, VNE and Vsi are very close to one another, because VNE is TAS and stall speed is IAS.

mcguif0a
11-18-2008, 08:28 AM
My original confusion came from interpreting the statement separately. I thought this series 7 was capable of making 150 mph TAS at sea level as well as having a ceiling of 25,000 feet. Doing 150TAS at 25,000 feet still seems rather far fetched but much better sense so thanks for setting me straight.

My next question is because I am a senior at Embry-Riddle studying aerospace eng. Was Vne of 140 calculated from possible g loads that could happen at 140. Was flutter a part of it. I have also heard of crashes resulting from flapperon flutter which is why the counter balances are on there.
To summarize my question... Where did a Vne of 140 come from?

jdmcbean
11-18-2008, 10:20 AM
My next question is because I am a senior at Embry-Riddle studying aerospace eng. Was Vne of 140 calculated from possible g loads that could happen at 140. Was flutter a part of it. I have also heard of crashes resulting from flapperon flutter which is why the counter balances are on there.
To summarize my question... Where did a Vne of 140 come from?

Vne established using FAR Part 23 as a reference.

There has not been any Kitfox crashes due to flaperon flutter that we are aware of. There are some Kitfox copies that have had some issues.