Certify a straight tail 172 for IFR?

[poadeleted20]

When my wife and I spoke about this topic, we decided to follow the recommendations, to drop the idea to get a good looking straight tail 172 and to get a newer 172 with a vacuum pump instead.
That was yesterday.

After some more digging into this topic, particularly in the Cessna 170 forum, where quite a few people happily fly IFR with a venturi vacuum, I am however not so sure anymore. Word is that venturi systems are, other than mechanical pumps which seem to be prone to failure, pretty much bullet proof. Icing would be the only concern, these were however not the conditions a 170/172 should be operated in anyway. It is also said that the icing of a venturi cannot be compared to the icing of a carburetor, as no gasoline is being evaporated. The only real advantage of vaccum pumps would be that the vacuum driven instruments are fully functional before take off, so that one could take off into low IFR conditions, what would however again not be something one wants to do in a 170 / 172. To go from a venturi system to a vacuum pump would therefore mean to give up a very reliable system, with well defined limitations (no take off into low IFR, no flights into icing conditions) for a system which is pretty likely to fail, what could happen anytime and which offers very little, if any, benefits over a venturi system under real-world-recreational-travel-IFR conditions.

Frankly, I am confused.
On one hand, we would rather make concession regarding the plane we actually want, in order to be safe(r). On the other hand, the arguments of the venturi advocates sound very reasonable and I don’t want to invest into a system which is less reliable and which advantages are actually almost irrelevant for us!?

There's nothing wrong with sticking with the venturi system as long as you understand those "well defined limitations" and discipline yourself to stay within them -- which shouldn't be hard for a "German guy". :wink2:

Alles klar? Gute flieg'.

 

[poadeleted20]

Sneaking parts out of your client's airplanes in your lunch box?

Process, not procurement. And Johnny's was a big lunch box -- I prefer just going to Panera's.

You're going to really need to grease the wheels at the FSDO since it'll take the whole staff to type up the 60 lbs of paperwork.

Actually, that's about what the paperwork on my plane weighs, given the dozen or more STC's and field approved alterations as well as the fact that it's built from the parts of half a dozen different Tigers and Cheetahs. Good news is that paperwork's also been getting done "one piece at a time" for the last 15 years.

 

[jesse]

Process, not procurement. And Johnny's was a big lunch box -- I prefer just going to Panera's.
Actually, that's about what the paperwork on my plane weighs, given the dozen or more STC's and field approved alterations as well as the fact that it's built from the parts of half a dozen different Tigers and Cheetahs. Good news is that paperwork's also been getting done "one piece at a time" for the last 15 years.

Uh huh....

Well, I left the Navy back in sixty nine
An' went to PIC workin' on instrument ratings
The first year they had me teach'n in Bonanzas.

Every day I'd watch them beauties roll by
And sometimes I'd hang my head and cry
'Cause I always wanted me one that was all glass.

One day I devised myself a plan
That should be the envy of most any man
I'd sneak parts from their planes in a lunchbox in my hand
Now gettin' caught meant gettin' fired
But I figured I'd have it all by the time I retired
I'd have me a Bonanza worth at least a hundred grand.

I'd get it one piece at a time
And it wouldn't cost me a dime
You'll know it's me when I come through your town
I'm gonna fly around in style
I'm gonna drive everybody wild
'Cause I'll have the only one there is around.

So the very next day when I stumbled in
With my big lunchbox and with help from my friends
I left that day with a lunch box full of wires
I've never considered myself a thief
But that rich doctor wouldn't miss just one little piece
Especially if I strung it out over several years.

The first day I got me an air data computer
And the next day I got me a magnetometer and a transponder
Then I got me a proper steering nosewheel and all the chrome
The little things I could get in my big lunchbox
Like nuts, an' bolts, and all three tires.
But the big stuff we snuck out in my Nates's giant ****ing camper.

Now, up to now my plan went all right
'Til we tried to put it all together one night
And that's when we noticed that something was definitely wrong.

The prop was fixed pitch
And the motor turned out to be a 550
And when we tried to put in the bolts all the holes were gone.

So we drilled it out so that it would fit
And with a little bit of help with a HaasAir adapter kit
We had that engine runnin' just like a song
Now the tip tanks' was another sight
We had one on the left and none on the right
But when we pulled out the switch it all worked ok.

The back end looked kinda funny too
But we put it together and when we got through
Well, that's when we noticed that we half a v tail and half a straight
About that time my wife walked out
And I could see in her eyes that she had her doubts
But she opened the door and said "Honey, take me for a spin."

So we flew to the FSDO to get the paperwork
And I headed her right on down
I could hear everybody laughin' for airports around
But up there at the FSDO they didn't laugh
'Cause to type it up it took the whole staff
And when they got through the logbook weighed six hundred pounds.

I got it one piece at a time
And it wouldn't cost me a dime
You'll know it's me when I come through your airport
I'm gonna fly around in style
I'm gonna drive everybody wild
'Cause I'll have the only one there is around.

Ugh! Yeah, Geico
This is the LEVY
In the better than a RV Ronanza Come on!

 

[poadeleted20]

Brilliant!

 

[Fearless Tower]

James and I might not agree with everything, but I agree with the above, although I'd probably replace naive with a little friendlier word

Except what the OP is describing is EXACTLY what I used to do in my 170. You guys are trying to make a bigger issue than really exists, assuming the OP understands the limits ( which it sounds like he does).

If there was any question about conditions potentially becoming more than I was comfortable doing with the vintage equipment, I left the 170 on the ground and rented a better equipped airplane for the trip.

 

[peerlesscowboy]

Except what the OP is describing is EXACTLY what I used to do in my 170. You guys are trying to make a bigger issue than really exists, assuming the OP understands the limits ( which it sounds like he does).

If there was any question about conditions potentially becoming more than I was comfortable doing with the vintage equipment, I left the 170 on the ground and rented a better equipped airplane for the trip.

IOW, you only flew your 170 IFR when you could have just went VFR instead?

 

[Tom-D]

Well, if a vacuum pump can't be installed,

It can be, I have done several, it is simply a matter of swapping the starter adaptor to run the -D version angle drive starter, then you can run a vac pump on the rear of the starter adaptor. just like the later Cessna 172s with the 0-300-D,
you can do this on a field approval.

 

[Fearless Tower]

IOW, you only flew your 170 IFR when you could have just went VFR instead?

Nope. I don't know about you, but I consider filing IFR to pop through a 500-1000' layer in temps well above freezing to be safer than trying to scud run under a 1000-1200' ceiling.

 

[Tom-D]

Process, not procurement. And Johnny's was a big lunch box -- I prefer just going to Panera's.
Actually, that's about what the paperwork on my plane weighs, given the dozen or more STC's and field approved alterations as well as the fact that it's built from the parts of half a dozen different Tigers and Cheetahs. Good news is that paperwork's also been getting done "one piece at a time" for the last 15 years.

he'd get no data tag, so it's E/AB

 

[JHW]

some of these comments are right out of the twilight zone. There is no more unreliable cource of vacuum than a dry pump. A venturi is failproof as long as it doesn't ice. That's not difficult to avoid, don't fly in clouds or precip in the cold. No different than flying with an unheated pitot. I've flown plenty of IMC with venturis but I refuse to fly behind a dry pump. Many people fed up with their dry pumps install backup vacuum sources. One backup you see fairly often is a small venturi on the belly. With a little thought you can even put that backup venturi in the exhaust stream so it's heated.

 

[German guy]

Well, venturis might soon not be an issue for us anymore.

I just learned that our yesterday's offer for a very nice 1966 172G has been accepted! Even though we would have preferred a straight tail, she is in such a nice condition, that we couldn't let her pass. Assuming the pre-buy will provide satisfactory results, she will soon be ours - including a vacuum pump, 6-pack panel, 2 nav coms (1 with glideslope), a VFR GPS and a renewed IFR check!

With a Garmin 650, we should have a very nice and capable setup.

 

[JHW]

Well, venturis might soon not be an issue for us anymore.

I just learned that our yesterday's offer for a very nice 1966 172G has been accepted! Even though we would have preferred a straight tail, she is in such a nice condition, that we couldn't let her pass. Assuming the pre-buy will provide satisfactory results, she will soon be ours - including a vacuum pump, 6-pack panel, 2 nav coms (1 with glideslope), a VFR GPS and a renewed IFR check!

With a Garmin 650, we should have a very nice and capable setup.

that sounds to me like it's a very nice setup just the way it is. Nothing wrong with buying a plane and just flying it for awhile without pouring a mountain of money into it.

 

[peerlesscowboy]

Well, venturis might soon not be an issue for us anymore.

I just learned that our yesterday's offer for a very nice 1966 172G has been accepted! Even though we would have preferred a straight tail, she is in such a nice condition, that we couldn't let her pass. Assuming the pre-buy will provide satisfactory results, she will soon be ours - including a vacuum pump, 6-pack panel, 2 nav coms (1 with glideslope), a VFR GPS and a renewed IFR check!

With a Garmin 650, we should have a very nice and capable setup.

Sounds like a good move :wink2:
BTW, if and/or when you install the 650 the second navcom & the VFR GPS might have some trade-in value.

 

[poadeleted20]

he'd get no data tag, so it's E/AB

The data tag was on the fuselage which was the basis of this project, and yes, an FAA Airworthiness Inspector has looked at it, not just the IA who signed off the alterations.

 

[Tom-D]

The data tag was on the fuselage

But it will not be.

which was the basis of this project, and yes, an FAA Airworthiness Inspector has looked at it, not just the IA who signed off the alterations.

He'd never get the Data tag, simply because he stole the aircraft parts to build the aircraft. he'd not be able to steal the data tag because they are not kept where he could get his hands on them, especially with out all the paper work that it requires to make the aircraft a certified production model.
He'd have no AWC, or data tag, he would then be required to go with a E/AB. to get any paper.

 

[jesse]

The data tag was on the fuselage which was the basis of this project, and yes, an FAA Airworthiness Inspector has looked at it, not just the IA who signed off the alterations.

He'd never get the Data tag, simply because he stole the aircraft parts to build the aircraft. he'd not be able to steal the data tag because they are not kept where he could get his hands on them, especially with out all the paper work that it requires to make the aircraft a certified production model.
He'd have no AWC, or data tag, he would then be required to go with a E/AB. to get any paper.

lol I think you two are discussing two entirely different things.

 

[timwinters]

lol I think you two are discussing two entirely different things.

And that surprises you how?

 

[Tom-D]

lol I think you two are discussing two entirely different things.

Probably, I'm still with " he went right up to the factory and picked it up"

The buyer of the early 172 has already bought a late model. To me that ended the thread.

 

[poadeleted20]

He'd never get the Data tag, simply because he stole the aircraft parts to build the aircraft. he'd not be able to steal the data tag because they are not kept where he could get his hands on them, especially with out all the paper work that it requires to make the aircraft a certified production model.
He'd have no AWC, or data tag, he would then be required to go with a E/AB. to get any paper.

Well, the facts tell us you're wrong, because the data tag is most definitely where it belongs on my plane.

Probably, I'm still with " he went right up to the factory and picked it up"

The buyer of the early 172 has already bought a late model. To me that ended the thread.

You clearly have confused a bit of Jesse's humor with the actual genesis of my plane.

 

[Henning]

Please educate me on what is not 'reliable' about a venturi vacuum system?

They ice up the same way a carb does, only not when quite as warm since there's no evaporation to increase the cooling. If it's a heated venturi, no worries, but I haven't seen one.

 

[Henning]

You really think someone could get $125K for a 1950-something Cessna 172 with a run-out engine and a G500/GTN650 panel? Sure, if this is a plane the OP will keep for the next 20 years, maybe investment in avionics of half again the plane's purchase price makes sense, but I'm just not seeing the real value of the G500 over an Aspen being worth $12K unless the OP is doing a lot of heavy IFR flying, and maybe not even then

What does selling price have to do with operational parameters and values? Ever since I was a kid in the car business this has always confused me. For the owner that wants to use the machine, resale value is the most insignificant value. Pay for what you want to use, and use it up to your best advantage. That's how an owner/operator gets their value from a machine, what it DOES for them. Are you going to sell the Chiger? If not what difference does resale value make in your relationship with your plane?

If you want to operate in IFR using modern technology, the cheapest way to do that is to buy an old IFR airplane with the oldest technology available and a clean airframe with a mid time engine and put in the panel you want to fly. If you're comfortable with 75 year old technology and believe it will provide you with everything you need to be safe, fine, stick with it. To me it's shear stupidity to give up the advantage that billions upon billions of tax dollars have been spent to develop, but whatever, if people choose money or ego over maximum advantage when trying to survive, that's their choice to make.

 

[poadeleted20]

They ice up the same way a carb does, only not when quite as warm since there's no evaporation to increase the cooling.

Not quite. Venturis ice up the way the wing ices up, that is, due to visible moisture freezing onto a surface chilled by subfreezing OAT's. Carburetor throats ice up due to the temperature drop across the throat causing nonvisible moisture to freeze on the throat, and it happens at OAT's up to 70F or so.

 

[Tom-D]

Well, the facts tell us you're wrong, because the data tag is most definitely where it belongs on my plane.

Who was talking about your aircraft prior to your post 82
?


You clearly have confused a bit of Jesse's humor with the actual genesis of my plane.

Had you stayed on subject in post 82, there wouldn't be any confusion.

 

[Henning]

Not quite. Venturis ice up the way the wing ices up, that is, due to visible moisture freezing onto a surface chilled by subfreezing OAT's. Carburetor throats ice up due to the temperature drop across the throat causing nonvisible moisture to freeze on the throat, and it happens at OAT's up to 70F or so.

Does the drop in pressure through a venturi cool the air? Yes or no answer only.

 

[Tom-D]

Does the drop in pressure through a venturi cool the air? Yes or no answer only.

no, understand evaporative cooling as the fuel converts from a liquid to a gas.

 

[poadeleted20]

Does the drop in pressure through a venturi cool the air? Yes or no answer only.

I'll answer that question when you tell me if you have stopped beating your wife (yes or no answer only).

 

[Henning]

I'll answer that question when you tell me if you have stopped beating your wife (yes or no answer only).

I don't have a wife, ergo your question is moot, while mine is not. Shall we return to the relevant question?

 

[poadeleted20]

I don't have a wife, ergo your question is moot, while mine is not. Shall we return to the relevant question?

Since you refuse to answer yes or no to that question (which to be honest cannot be fairly answered yes or no any more than the one you asked), I will do the same.

Temperature goes up initially as it enters the throat (there is some compression), then goes down later in the expansion area. However, absent really bizarre conditions, the amount of net temperature drop is not enough to create the same effect as carb icing and block the passage of air through the venturi -- see Tom's mention of the contribution of evaporative cooling in addition to the pressure/temp effects. However, from a structural icing standpoint, that thing is a pretty much worst case ice collector due to the small radii of its various leading edges, and that is the actual icing problem one has with vacuum venturis.

 

[Tom-D]

Decompression of a gas will cause a drop in temp. That is the whole principal of refrigeration. but if a gas is compressed and then allowed to decompress before it can stabilize its temperature, there will be no cooling or heating.

If you compress a gas the temperature will rise, this is the theory of a diesel engine when the air in the cylinder is compressed it heats thus when it reaches the ignition temp of the fuel the fuel will be injected and burned.

Want to see evaporative cooling? watch a Dr. burn off a wart. the liquid N2, is evaporating so fast the area around the N2 is turned to frost. thus freezing the wart. Wart suffered frost bite and it's gone.

 

[Clark1961]

Decompression of a gas will cause a drop in temp. That is the whole principal of refrigeration. but if a gas is compressed and then allowed to decompress before it can stabilize its temperature, there will be no cooling or heating.

If you compress a gas the temperature will rise, this is the theory of a diesel engine when the air in the cylinder is compressed it heats thus when it reaches the ignition temp of the fuel the fuel will be injected and burned.

Want to see evaporative cooling? watch a Dr. burn off a wart. the liquid N2, is evaporating so fast the area around the N2 is turned to frost. thus freezing the wart. Wart suffered frost bite and it's gone.

So what exactly is the pressure profile in the venturi?

 

[Clark1961]

Since you refuse to answer yes or no to that question (which to be honest cannot be fairly answered yes or no any more than the one you asked), I will do the same.

Temperature goes up initially as it enters the throat (there is some compression),

How is there flow from the low pressure outside the venturi to the higher pressure you claim is inside the venturi?

 

[Tom-D]

So what exactly is the pressure profile in the venturi?

If I understand what you are asking, very little, it is the velocity of the air moving thru the Venturi that creates the vac pressure, not pressure in the Venturi.

If I have an 11 inch venturi, it that? how long it is or how much Hq it will develop?

 

[Clark1961]

If I understand what you are asking, very little, it is the velocity of the air moving thru the Venturi that creates the vac pressure, not pressure in the Venturi.

If I have an 11 inch venturi, it that? how long it is or how much Hq it will develop?

Will you agree that fluids flow only from areas of high pressure to areas of low pressure?

 

[Tom-D]

How is there flow from the low pressure outside the venturi to the higher pressure you claim is inside the venturi?

That's not what he said.

The Venturi works on ram air, flowing the the venturi.

 

[Clark1961]

That's not what he said.

The Venturi works on ram air, flowing the the venturi.

That's exactly what he said.

 

[Tom-D]

Will you agree that fluids flow only from areas of high pressure to areas of low pressure?

that whole theory is driven by Ram air pressure, it is going to flow, no problem. the theory of this type of vac system is bace on the ram air pressure causing a velocity at the smallest portion of the venturi.
There is a rise and fall of pressure, but it is insignificant in the operation of the system.

 

[Tom-D]

That's exactly what he said.

Then what is your problem?

 

[Clark1961]

that whole theory is driven by Ram air pressure, it is going to flow, no problem. the theory of this type of vac system is bace on the ram air pressure causing a velocity at the smallest portion of the venturi.
There is a rise and fall of pressure, but it is insignificant in the operation of the system.

I didn't ask you what is your theory of a venturi, I asked you if you agreed that fluids only flow from areas of high pressure to areas of low pressure. We'll get into velocity once we agree on the pressure profile within the venturi.

 

[Tom-D]

I didn't ask you what is your theory of a venturi, I asked you if you agreed that fluids only flow from areas of high pressure to areas of low pressure. We'll get into velocity once we agree on the pressure profile within the venturi.

Pressure doesn't have any thing to do with the theory of a venturi.

This isn't a class in weather theory.

 

[Clark1961]

Pressure doesn't have any thing to do with the theory of a venturi.

This isn't a class in weather theory.

Okay, we're done here if you can't admit that a fluid only flows from high pressure to low. I detest your intellectual dishonesty in this discussion.