*OT* Variable Pitch Propellers *OT*

[warchild]

Who says you can't use flaps gliding?

Welll, in the specific case of the P-61 the operation manual warns against it.. The full wing flaps n that thing create so much drag that you have to use power to stay in the air.. The version we're building here isnt quite that bad, but it isnt far off..

 

[warchild]

That's how military planes typically approach an airport to land. They come in low and fast right over the runway, then turn hard 180 degrees to enter the down wind leg, pulling lots of G and reducing power. All those Gs bleed off speed. The goal is to be at gear lowering speed as you come out of the 180 degree turn and stabilize on down wind. Then they finish off the landing in the "normal" way. If there are more than one plane in the formation, they each make the hard turn at some number of seconds of interval, in order to space them out enough...

THAT would explain why a lot of P-61 pilots did chandelles as they turned to final.. So maybe i'm not so crazy as i thought i was and maybe this thing simply really does hate to slow down once its moving.. It's smaller than a B-25, weighs 7000 pounds more and has those two monster radials on it.

 

[fliger747]

Turbine aircraft are flown in a somewhat different manner as they are not prone to cylinder shock cooling from rapid power reduction.

An aircraft such as the P61 is actually pretty slick aerodyanmically. What will initially slow down the plane a lot is drag from the propellers at higher RPM's and low MP. In some of the versions that I have worked with it is not unusual to see -500 HP developed each side!

When dealing with prop pitch issues it is critical to use a utility that shows both actual prop pitch at any setting as well as efficency.

Cheers: T

 

[warchild]

yup.. i'm finding on the widow that coming back on both throttle and spc levers causes the most efficient slow down.. But you guys have helped a lot here...

 

[PRB]

Now to confirm the answer to the question “which fixed pitch prop would you choose for high speed flight?”, we find this. See the pic? That is the “Speed Spitfire”, or the “Schneiderised” Spitfire. In 1937 the Supermarine guys decided to modify a Spitfire for the purpose of breaking the world speed record. They never actually made the attempt, but the modified plane is shown here. Among the modifications to this plane were (from the book Spitfire, a Test Pilot's Story, by Jeffrey Quill):

“...The main changes apart from the engine were the fitting of shorter-span wings; a main coolant radiator of substantially less area to satisfy the increased cooling requirement; a long streamlined Perspex windscreen with no clear view front panel; a four-bladed fixed-pitch wooden propeller of greatly increased pitch angle; ...”

And if you look at the photo, the prop is almost feathered!

I don't know why I should be surprised at this, having just noted above that in the A2A P-51D, the air brake effect of max RPM is significant...

 

[Brett_Henderson]

A thing to ponder:

That is in effect, a cruise prop. If they did their math correctly; the point at which the engine cannot spin the prop faster, will be the RPM where the engine's HP peaks.

During the takeoff roll, and as it accelerates, the RPMs will be held well below where peak HP is generated. Now of course, with that much engine, it's moot... but per this discussion, it's worth noting that takeoff/climb performance will be greatly reduced.

This is where the CSP shines. It allows the engine to work in its peak HP RPM, regardless of MP, or airspeed... or keeps the RPMs in a less abusive RPM, for cruising... and so on..

Also.. it's worth mentioning that the prop-controlled, max-RPM for a "conventional" airplane engine is rarely where the engine's HP peaks. If you were to put the average Lycoming or Continental on a dyno; you'd find that the HP peaks well into 3000RPM range. There are two factors in play, when deciding what max-RPM a CSP is set to.

1) Prop blade-tip speed
2) Engine life

So, even though an engine could generate more HP; whatever the CSP max-RPM might be, it's considered the max HP RPM.

Back in the day, a common cheat for getting your High-Performance endorsemnet (airplane over 200HP), was to take something like a Piper Arrow, or Cessna Cardinal ( both rated at 200HP), and then adjust the CSP to allow for a slightly higher, max-RPM.. yielding 200+ HP

 

[fliger747]

Once upon a time (like 30 years ago) I flew a fleet of Cessna 206's in an Alaskan bush village. One of the planes was a much peppier performer than the others, and had a Robertson STOL kit as well. The Robertson kit was not so popular due to lateral control issues at slow speed, but it was a zippy plane.

In for one of it's 100 hr inspections metal was found in the oil screen. Further investigation found that the tach was off a couple of hundred RPM and she was turning almost 3000 RPM and making (for a while) much more than 300 hp, and the cruise powers were off as well.

Some large engines may actually develop max HP at slightly below the max RPM figure. A case in point is the difference between the R2800 B and C models. The C is really quite a different engine in many ways. One of the places they got a couple of hundred extra HP was a refinement of the oil scavengeing sysem. At High RPM the B was wasting a lot of power just splashing oil around the case. With the new system, a higher gear reduction ratio, new cylinders etc the engine was able to put out more power and operate at a higher effective RPM for Takeoff and WEP.

For the above fixed pitch spitfire the prop would have been designed for a quite narrow effective speed range, one at which the engineers would have hoped the airframe and powerplant could maintain. Takeoff and initial climb performace might have been pretty miserable. Sort of like having a race car with only a top gear.

T

 

[Brett_Henderson]

Some large engines may actually develop max HP at slightly below the max RPM figure

The question would be... Why would the prop be designed and set up to ever let the engine get above max HP RPM ?

With most GA, there's no guessing.. there is always more HP to be had.. but why would you ever want a big radial reving higher than where the most HP is generated ?


Now..on occasion I've heard that you'll stumble across a small airplane (mostly experimentals).. where the highest airspeed can be reached, at slightly less than max-RPM.. but this would have to do with prop/airframe dynamics.. or a prop not perfectly suited for the engine/airframe.. or even an engine so worn that the power-curve is distorted.

 

[warchild]

ya gotta realise that the Spit was a military plane, crewed by military people.. If the engine was destroyed, so what?? it would get replaced, ut more importantly, it went ut every day and was put to risk of being completely destroyed alone wih its pilot anyway, so why not go for it?? the CO wasnt usually a mechanic so he wasnt always that hard to convince about why something was a "good" idea.. the crew wanted to find out what would happen, and they found a way to do it. didnt matter what happened to the plane. it was expendable.. Most things in war are like that..

 

[SkippyBing]

Most things in war are like that..

Errr the Spit in question was flying in '37, two years before war in Europe broke out. They were just trying to break a speed record.
The pitch they've chosen isn't surprising, when the aircraft gets up to speed the prop will be taking the minimum number of turns per unit distance advanced. With a CSP it would look like that once you were going fast enough, it may help to think of it as a screw driving into a plank of wood, the greater the pitch the further into the piece of wood it'll go per turn. But to work well you have to get up to the right speed in the first place otherwise it'll be slipping and working at less than optimum efficiency.

 

[fliger747]

For a one off racer, easier to carve a fixed pitch prop rather than have Dowty Rotol cough up a special constant speed. Also lighter and even production Spits didn't have Constant Speed props, two pitch selectables if I remember correctly.

HP/RPM, the answer is in THRUST. These engines had geared props so tip speed was not generally an issue. With Augmentation such as water methanol injection, higher MP's could be utilized as well as the higher RPM. Use of a higher RPM to produce nearly the same HP reduced internal cylinder pressures, another important consideration. Also the engine torque curve more closely matched the desired climb settings, overall generally a more important operating area.

T

 

[SkippyBing]

production Spits didn't have Constant Speed props, two pitch selectables if I remember correctly.

I believe they gained constant speed units sometime in 1940/41 after the two pitch selectables proved less helpful vs ME-109s!