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(was) Most HP on a Kitfox? Which Engine

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lcfitt(at)sbcglobal.net
Guest

Posted: Sun Apr 08, 2007 3:24 pm Post subject: (was) Most HP on a Kitfox? Which Engine

This is an interesting subject of which I am by no means an expert, but all 

ears (eyes).  I have long felt that the Rotax 912 engine was a perfectly 

design for the IV and earlier model Kitfoxes.  The 912 ULS coming later and 

being a marked improvement on that.



Then the V through Seven Series airplanes came in with no real clear winner 

in engine choice.  Any one of the several choices have advantages and 

disadvantages.  Rotax - light weight, but arguably under powered.  Certified 

and auto conversions - more powerful, but significantly heavier.



My impression on reading posts over the past few years seems to indicate 

that with the larger airplanes and the more powerful engines cruise speed is 

definitely enhanced.  How much of that is due to the airframe improvements - 

electric trim, smooth cowl etc., I can only guess.  But, and correct me if I 

am wrong,  short takeoff distances and enhanced climb does not seem to be 

one of the benefits of the more powerful, heavier engines.



What I would like to see is several similar airplanes of different engine 

configurations fly together in and out of all sorts of places, all in 

exactly the same conditions.  Pilot proficiency would then be the only 

variable and we could get some real numbers.  I personally have trouble with 

stop watches and charts.  I get questions from time to time as to my cruise 

speeds.  I really don't have a good answer as it depends a lot on 

conditions.  I do, however, know how I do with respect ot the guys I fly 

with - especially on the long cross country legs - ability to keep up and 

fuel burn.  We also climb out of some interesting places and also will 

sometimes delay a climb over approaching terrain until the last moment for 

competitive excitement.  There have been times we have misjudged and been 

forced into a 360 or two.



Another problem with coming to a consesus is that most guys that make engine 

choices tend to like their choices and will defend them pretty vigorously. 

And in most cases, they have not had the opportunity to fly their airplane 

or a similar airplane with one of the other engines.  So who's to know.



Lowell



do not archive

---

davef(at)cfisher.com
Guest

Posted: Sun Apr 08, 2007 6:05 pm Post subject: (was) Most HP on a Kitfox? Which Engine

Lowell,



Numbers are all a help but really have to compare apples to apples as in 

total weights and speed mods, fairings etc that have been done.



I have seen 912 Kitfox cruise at  80s mph and other at 120 mph ........



Anyone seen a 582 powered in the high 90s or over 100 yet ?   My ASI reads 

115 in cruise but TAS is about 90

Dave



---

lcfitt(at)sbcglobal.net
Guest

Posted: Sun Apr 08, 2007 9:54 pm Post subject: (was) Most HP on a Kitfox? Which Engine

Dave,  I flew alongside Guy Buchanan.  He is in the high 90s for sure if not 

100.  During the flight I asked the others what they thought he was doing, 

the answer back - 100.



But your point is well taken.   That is why I want to fly alongside someone 

for comparison.  All the variables are there to see.  I just wonder how much 

horsepower can compensate for the other variables and how a heavy high 

horsepower airplane compares with a light lower horsepower airplane in climb 

and cruise. What we need to do is all meet at Johnson Creek in Idaho and see 

who can climb straight out and clear the ridge.   I have seen it done by a 

Kitfox.



Lowell



do not archive

---

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Mon Apr 09, 2007 5:22 am Post subject: (was) Most HP on a Kitfox? Which Engine

Lowell:

 	  Quote: 	
		  From what I've been reading here, it sounds like the heavier engines do

enjoy a marginal increase in rate of climb but at the expense to take off
	


roll.  I assume they make that take off roll faster than the light engines

but it still requires more room.  No doubt as you said the aerodynamic

improvements in the later models probably have something to do with

increased cruise speed and ROC.



Noel



[quote] --

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

Bob

Joined: 24 Oct 2006
Posts: 89
Location: Damascus, Maryland, USA

Posted: Mon Apr 09, 2007 8:36 am Post subject: Re: (was) Most HP on a Kitfox? Which Engine

Lowell -

I know that you said you're not a charts kind of guy, but if you want to give it a try, it'll answer the question for your particular airplane.  Since fuel flow is a fair indicator of horsepower, make an chart with true airspeed on the vertical axis and fuel flow on the horizontal.  Write down some numbers at a spread of throttle settings and the chart will show a curve.  Yes, the draggy Kitfox shape can blow the wings off an RV-8 with enough power, but of course then there would be one or two structural problems, right?  



If you don't have a flow-meter, use a common hand-held GPS and you can get a set of average speeds to put over the number of gallons that it takes to refill with after each flight.  If you need, I'd be more than happy to plot data you collect.

Bob

_________________
Remember that internet advice may only be worth what you pay.

Bob

Joined: 24 Oct 2006
Posts: 89
Location: Damascus, Maryland, USA

Posted: Mon Apr 09, 2007 8:47 am Post subject: Re: (was) Most HP on a Kitfox? Which Engine

Just noticed Jeff Hayes mention that the airplane flies best when light and it made me remember to add to my post.  The heavier engines typically make for a heavier airplane, and the big three issues in stall speed are WEIGHT, wing area, and coefficient of lift; two of which you can't do much about.  

While the big engines will get a little faster cruise, their weight counts against shorter takeoffs. The exception is sometimes at high density altitudes, where the smaller engine might just get you into trees at the far end of a runway. 

Bob

_________________
Remember that internet advice may only be worth what you pay.

n85ae

Joined: 14 Mar 2007
Posts: 403

Posted: Mon Apr 09, 2007 9:07 am Post subject: Re: (was) Most HP on a Kitfox? Which Engine

I think the IO-240B is a better quality engine than the 912S, but if I built

another Kitfox, I'd more likely use the 912S, or a Jabiru that the IO-240B



Weight, cost, and fuel choice being big factors.



Jeff.

 	  lcfitt(at)sbcglobal.net wrote: 	
		  This is an interesting subject of which I am by no means an expert, but all 

ears (eyes).  I have long felt that the Rotax 912 engine was a perfectly 

design for the IV and earlier model Kitfoxes.  The 912 ULS coming later and 

being a marked improvement on that.



Then the V through Seven Series airplanes came in with no real clear winner 

in engine choice.  Any one of the several choices have advantages and 

disadvantages.  Rotax - light weight, but arguably under powered.  Certified 

and auto conversions - more powerful, but significantly heavier.



My impression on reading posts over the past few years seems to indicate 

that with the larger airplanes and the more powerful engines cruise speed is 

definitely enhanced.  How much of that is due to the airframe improvements - 

electric trim, smooth cowl etc., I can only guess.  But, and correct me if I 

am wrong,  short takeoff distances and enhanced climb does not seem to be 

one of the benefits of the more powerful, heavier engines.



What I would like to see is several similar airplanes of different engine 

configurations fly together in and out of all sorts of places, all in 

exactly the same conditions.  Pilot proficiency would then be the only 

variable and we could get some real numbers.  I personally have trouble with 

stop watches and charts.  I get questions from time to time as to my cruise 

speeds.  I really don't have a good answer as it depends a lot on 

conditions.  I do, however, know how I do with respect ot the guys I fly 

with - especially on the long cross country legs - ability to keep up and 

fuel burn.  We also climb out of some interesting places and also will 

sometimes delay a climb over approaching terrain until the last moment for 

competitive excitement.  There have been times we have misjudged and been 

forced into a 360 or two.



Another problem with coming to a consesus is that most guys that make engine 

choices tend to like their choices and will defend them pretty vigorously. 

And in most cases, they have not had the opportunity to fly their airplane 

or a similar airplane with one of the other engines.  So who's to know.



Lowell



do not archive

---

smokey_bear_40220(at)yaho
Guest

Posted: Mon Apr 09, 2007 6:15 pm Post subject: (was) Most HP on a Kitfox? Which Engine

This seems to be a Kitfoxism - to always compare

unlike aircraft, or aircraft with pilot and fuel only,

since we fly alone a lot.  It is a bit apple and

oranges that way though.  We don't all weigh the same,

or carry the same fuel.  We don't all have the same

fairings, range, etc either.



A -4 at 1200 lbs is near full, or full, and a -7 at

1200 lbs is near empty.  I did most of my S-5 testing

at 1300 lbs, near empty.  How can we compare

performance that way?  I can beat the lower model's

performance, if he has to make 2 trips to carry the

load.  And above some density altitude, I might be

able to beat any 1-4 on takeoff distance with my

turbo.



In the past, manufacturers were supposed to present

performance at gross weight, not something less.  The

figures included payload that could be carried and

still achieve that performance.  Anything under weight

would obviously produce better performance, but that

is cheating those who give gross weight numbers.



The S-5,6,7 will always perform less than the 1-4 on

the same HP, payload, and the same drag reducing

treatments.  The newer ones are just heavier and use

more of a cruise wing profile, and so stall faster. 

Cruise is improved by the wing, but there still is a

price to be paid for being wider and heavier, ie.

slower cruise on the same HP.  Notice there are no 582

S-5's and later.  I don't know of many 80 HP -5,6,7's

out there either, even though they are lighter.



But if you bought the later wide bodied Foxes, you

could up the HP to equal performance, except stall

speeds, and have more elbow room.  The IV and older

still stall slower, all else the same.



But the 5 and later could stand a much larger increase

in HP.  They can overcome the difference and perform

even better on non-stall related performance.  So

takeoff speed and landing speed will be higher, but

with higher allowable HP, takeoff distance, climb, and

cruise could be better than the IV and older models. 

This is where the power to weight figure comes in as a

truth teller.



So one better measures to compare between planes would

be, what is your max payload?  What is your range at

gross?  What is your T/O and landing distance at

gross? etc.



Or we could establish a standard payload and compare

all aircraft performance with that standard load

carried.  Something like, "What is your performance

with 100 lbs of fuel, 400 lbs of people, and 100 lbs

of baggage?  Can you even carry that?  How far can you

go?"



Can we all carry 700 lbs?  Some can.  Can we all fly

500 miles with 30 minutes reserves?  Some can.



Or something like, "What payload and performance can

your plane achieve with 500 miles range?  Or what

range and performance can your plane achieve with 500

lbs of non-fuel payload?"



Otherwise one of the pilots I know (Mr T) on our list

will blow everyone away by always taking off in less

than 100 feet.  I can't, but he can not carry the load

I can, legally 800 lbs.



And when you see Cubs in competition, there aren't

many 65 HP Cubs beating the high HP Cubs in the short

field area.



You can lower the lbs per HP by lighter weight - same

HP, or by increasing the HP more than the weight. 

Where higher HP can't compete is in $/mile or $/hour.



But if you want to take off over 50' obsticles at

7,000 ft DA carrying you and your loved one and

camping gear, a turbo'ed KF with excess HP will do it

where the lighter weight one will not.



Bottom line:  Mission dictates and the rest is

bragging rights.  If we want to compare, we need a

standard to compare to.



Just my opinion,



Kurt S.  S-5

       

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rjdaugh

Joined: 30 Aug 2006
Posts: 195

Posted: Mon Apr 09, 2007 8:19 pm Post subject: (was) Most HP on a Kitfox? Which Engine

Kurt says:  "A -4 at 1200 lbs is near full, or full, and a -7 at

1200 lbs is near empty."



Kurt,

I about fell off my chair when I read that.  But I guess what I am about to

relate just confirms what you say.  



My Series 5 with a Series 7 firewall forward is near full at 1200 lbs.  I

weigh 200, my wife weighs 125, - on our cross country flights we usually

have 50 - 60 lbs of luggage and 23 gal of fuel and still weigh less than

1300 lbs. - Which is a good thing since my fox is sport plane compliant.

Most of my flying is with passengers and at less than 1200 lbs.  One time I

had a 300 lb passenger and I couldn't have full fuel, but so far, I haven't

felt the 1320 gross weight is much of a handicap.



I have Grove gear and big heavy tires and a heavy pneumatic tail wheel but

still have an empty weight of 776 lbs with the 912S engine.



Randy



.           

--

_________________
Randy
Kitfox 5/7 912S
Black Hills, South Dakota

Bob

Joined: 24 Oct 2006
Posts: 89
Location: Damascus, Maryland, USA

Posted: Tue Apr 10, 2007 8:52 am Post subject: Re: (was) Most HP on a Kitfox? Which Engine

Here's a way to visualize the power and drag curves and compare airplanes that isn't that hard. Kurt is right in that as people and pilots we tend to compare what we know, and it is like the old proverb about the three blind guys describing the elephant to each other.  I've got fancy aero books, but the David Rogers series of Beech Society papers was really well done and his graphs were easily understandable without the math.  This is a usable reference collection in one place! http://web.usna.navy.mil/~dfr/technical_flying.html



The visualization of the current thread is Figure 1 in "Weight Effects Part 1."  As the airplane gets less drag, the right end of the smile-shaped curve would move to the right.  As we can stall slower, the left end moves to the left.  An earlier and more draggy Kitfox wing would shift the whole smiley-curve to the left. Why care?  Well, for a start, the curve can give best climb and other performance for your particular airplane! So, if you've got lots of data from boring initial flight testing, here's a good use!



Note: new sets of curves are needed for different density altitudes.  The turbine guys who learned before everything was computerized are used to digging through whole books of performance plots to set up for cruise.



As we go up in density altitude, the hump-shaped curve gets lower, unless you have a turbo that keeps up the output.  However, the smile-shaped plot also shows how increasing power can be offset by the weight of a heavier engine, which was what I referred to in my earlier post (inadvertently putting speed over power). Any of us can make such a set of charts, and in addition to being able to have a set of clear speeds to use for best climb and most efficient cruise, we can compare efficiencies with others.



Most of us have probably seen Dave Fischer’s videos of his incredibly short take-offs, so I’ll use him as an example.  The smiley portion of a plot for his plane would probably be low  (largely due to lack of weight) and fairly flat (his reported high cruise speed), shifted to the left (his low stall).  His hump-shaped power curve would be high in relation to the smiley-curve, and the distance between curves would be the excess power of a healthy engine at low density altitude.  Voila, thinking of the curves in this fashion, you can visualize airplane performance.  (Of course, I feel cramped in the early airplane cockpits, but the charts don’t show that.   )  From what Deke’s posted, I'd say that his Series 5 is better set up for cruise and wouldn’t be as preferable in a really short strip, as Deke’s smile and hump-shaped curves would be shifted to the right of Dave’s (again, using the same density altitude).  



As for gathering data, it's a pretty rare Kitfox that has a gauge calibrated in "horsepower" (lame engineering joke) and horsepower is affected by numerous things.  While they won’t be exact, the engine makers have posted curves that relate rpm or fuel consumption (typically as bsfc, or pounds/hour) to power.  Again, not exact, but close enough to construct very usable sets of curves.  Here are a couple of examples:



For 582:  http://www.kodiakbs.com/2intro.htm

For 912S: http://www.kodiakbs.com/4intro.htm



Bob



ps -the problem of comparing airplanes is old and the CAFE foundation came up with an efficiency measure that worked well but unfortunately is a bit heavy on math and charts that were not really intuitive.

http://cafefoundation.org/v2/pdf_pav/PAV.CAFE.Formula.Deriv.7.7.pdf

See also how they did it in a sample report: http://cafefoundation.org/v2/pdf/rv-9a.pdf

_________________
Remember that internet advice may only be worth what you pay.

lcfitt(at)sbcglobal.net
Guest

Posted: Tue Apr 10, 2007 10:15 am Post subject: (was) Most HP on a Kitfox? Which Engine

I also got a jolt when I read:



 	  Quote: 	
		  A 4 at 1200 lbs is near full, or full, and a -7 at

 1200 lbs is near empty.  I did most of my S-5 testing

 at 1300 lbs, near empty.


	


This is a fact as you state it, but the only difference in the two at 1200 

lbs is the thickness of the structural tubing, some aerodynamic cleanliness, 

the empty weight and the payload.  The wing that carries the load is the 

same.  Agreed that HP is the mitigating factor.



I also agree entirely with your concluding statement:



 	  Quote: 	
		  Bottom line:  Mission dictates and the rest is

 bragging rights.  If we want to compare, we need a

 standard to compare to.


	


My only reason to suggest a performance side by side is to get people of a 

like minded mission profile together and put together some comparison data 

that all will agree to.



A bit reluctant to say this, but since the demise of the Model IV and older, 

the mission can only be determined now by engine choice, airframe is no 

longer a factor, unless buying used and I suspect that hurts the factory to 

an extent.  Not everyone wants big and fast.



Lowell

smokey_bear_40220(at)yaho
Guest

Posted: Tue Apr 10, 2007 10:21 pm Post subject: (was) Most HP on a Kitfox? Which Engine

Right Randy.



For you at Sport plane weights, 1200 is "near" full. 

But there you are with over 500# payload and still

over 200# more you could carry to the aircraft's 1550

limit, if you had chosen it.  And I don't know that

the IV and earlier planes would be packing that all in

safely, or performing well like the V and later with

that same load.



Limited to Sportplane weight in a V and you have a

bigger structural buffer then the earlier planes, but

possibly a bigger payload too, with the available

weight above 1200.  Your gross weight limit might go

up more than your empty weight did.  For me at 1300#,

not a Sportplane, I felt like I had a lot of room to

go.



Really, they are 2 different planes - the IV and

earlier vs the V and later.  As Lowell says, lacking

the IV kits now limits our choices to the heavier

mission only, and just engine choices primarily.  The

same size wing will be carrying a bigger load and thus

stall faster.



It is a bit like trying to compare the Cherokee 180

and the Cherokee 6, or the C-182 vs the C-210.  Really

different planes, different engines, and different

missions.  Same wings?



We each picked what we wanted for our mission.  I know

I had a hard time deciding between the IV and the V. 

Each had tradeoffs.



Lowell always does good with his side-by-side

comparisons.  It takes out the instrument errors and

weather related differences.  But we need to either

compare IV and earlier only, or V and later only, or

have them use a standard mission/load to really know. 

Otherwise heavy guys like me will always seem to lose.

 ;-(



But the question we started with was about putting in

enough HP to achieve the best performance of all.  You

can put more HP in the V and later than the IV and

earlier by a bunch.  Enough that you can carry a big

load and still claw your way out of a short field at

high altitude.  This means you need to accellerate to

a faster speed in the same distance pulling more

weight.



Same wing?  Only one way to do it.  More HP.  Without

more HP, the extra empty weight of the V and later is

aerodynamic dead weight.  We don't need dead weight,

but more HP is not dead and can overcome the

difference.



So the wing loading and power loading dictate in the

end, if you can carry the same or greater weight

after.



Sounds like something someone with a spreadsheet and a

lot of time could put together???  Don't have that

time right now myself.  Best I can do is say that they

are different planes and more HP will work.  But

really good comparisons of different planes need

standards.



Kurt S.  S-5/NSI turbo



--- Randy  Daughenbaugh <rjdaugh(at)rapidnet.com> wrote:



 	  Quote: 	
		   Kurt says:  "A -4 at 1200 lbs is near full, or full,

 and a -7 at 1200 lbs is near empty."

 

 Kurt,

 I about fell off my chair when I read that.  But I

 guess what I am about to

 relate just confirms what you say.  

 

 My Series 5 with a Series 7 firewall forward is near

 full at 1200 lbs.  I

 weigh 200, my wife weighs 125, - on our cross

 country flights we usually

 have 50 - 60 lbs of luggage and 23 gal of fuel and

 still weigh less than

 1300 lbs. - Which is a good thing since my fox is

 sport plane compliant.

 Most of my flying is with passengers and at less

 than 1200 lbs.  One time I

 had a 300 lb passenger and I couldn't have full

 fuel, but so far, I haven't

 felt the 1320 gross weight is much of a handicap.

 

 I have Grove gear and big heavy tires and a heavy

 pneumatic tail wheel but

 still have an empty weight of 776 lbs with the 912S

 engine.

 

 Randy


	




       

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