Fuel burn and things

[motzartmerv]

Farri mate... perhaps its me not being clear in my above post's, but you were a cfi weren't you??...surely you understand the effects of power on Vs???...im confused...

 

 

[farri]

Farri, the drifta is a pusha, the engine faces backwards and the prop blows over the tail section and not the wing.. The reason power reduces stalling speed (Vs) on conventional aircraft is due to the extra airflow over the inboard sections of the wing when power is applied.. ,the wing doesn't care where the extra airflow comes from..so an acft that has the engine behind the wing won't have a reduced Vs when ya add power..So, how are we going with that stalling speed??;)

Merv,

 

The fact that we have a pusher engine on the Drifter has nothing to do with a lower stall speed with power on.

 

You are correct that the air has to come from somewhere and it comes over the wing which is in front of the motor.

 

Frank.

 

 

[motzartmerv]

so are you saying that the stall speed is reduced with power on on pusher types??

 

 

[farri]

Merv,

 

I hope you`re not having me on.

 

No that`s not what I`m saying at all,you added the bit about the pusher configuration,pusher has nothing to do with the stall speed, Now I`ll try and explain it to you.

 

I`ll start by saying that my Maxair Drifter has a book figure, Vs of 36 kts at MTOW.

 

The IAS at which stall occurs,power off is 36kts, the IAS, full power on is 32kts,Why is it so?

 

We all know or should know that the stall occurs when the critical angle of attack has been exceeded,this angle is the angle of the chordline to the relative airflow and is indicated by our ASI,this has nothing at all to do with engine configuration.

 

The propeller which is basicly a rotating aerofoil is decreasing the static pressure in front of it thereby moving the aircraft foward,with power on as the critical angle of attack is reached the aircraft is actualy being held there by the propeller and the airflow over the wing,this will give a particular IAS.

 

With power off, there`s little or no thrust being created by the prop and therefore the drag is higher and a higher IAS is shown and it will always be higher than power on.

 

On the Drifter, pusher does have an effect,oposite to a tractor,on yaw from slipstream on take off,but thats another lesson,If you want it I will give it.

 

Frank.

 

 

[motzartmerv]

Frank.. no i wasn't having you on.. I understand quite well the aerodynamics involved in the stall.. My question related to the "propwash" changing the IAS Vs as apposed to no propwash on pusher types..But as youve givin me quoted IAS Vs for power on and power off on the pusher type then i realise the reduced pressure in front of ( or behind in the case of the pusher) is affecting the Vs..

 

Thanx for giving me the Vs on your drifta.. Now i have some numbers to work with..

 

 

[Guest sypkens]

Merv,

 

If you are looking for a drifter in exceptional good condition have a look at

 

Drifter 25-194 for sale - Recreational Flying

 

Regards,

 

jan

 

 

[farri]

MTOW.

 

Merv,

 

The original certified twin seat Maxair Drifter,which mine is, came out with a MTOW of 400 kg,it was later updated to 450kg however to my knowledge nothing was changed on the aircraft.

 

Frank.

 

 

[Barefootpilot]

Just to throw a spanner in the works of the stall discussion. Some aircraft don't actually achieve an Aerofoil Stall - That is the wing does not actually reach the critical angle of attack (around 16') What happens (in some aircraft! not all) is the elevator stalls or its travel limted the aircraft won't be able have the AOA increased AT SLOW SPEED which means just like an aerofoil stall the aircraft pitches down.

 

Aircraft with very "soft" stalls very often have the elevator stalls versus more agressive aircraft that actually stall the wing. Some aircraft such as the Piper Warriors have the elevator travel limited to limit the angle of attack that is achievable at slow speeds and thus stops a proper stall being developed.

 

I'm putting on my fire proof suit now and i'm ready to be shot down!

 

Adam.

 

 

[facthunter]

True.

 

This effect is the same as when the aircraft is very nose heavy. 2 ways to make it so the plane won't bite you. Nev..

 

 

[motzartmerv]

Yep, for sure.. I think the jabiru (160) fits that catagory..The only way to get it to missbehave is to get the nose right up, have some power on and then mabye you'll get the nose to drop..mabye.. if you get the thing fast (90 kts) and leave full power on and pull her right up the nose gets a bit funky then, but still nothin to right to jabiru about..

 

 

[farri]

Hi Adam Holt,

 

I certainly won`t shoot you down,this forum is for expression of opinions and debate.

 

I believe what you`re reffering to is not actually a stall,the aircraft simply can`t reach the critical angle and due to COG will simply lower the nose.

 

Trikes are designed this way and the bar with which the pilot controls the wing and AOA can only be moved forward to a point where the wing can`t reach critical AOA.

 

We also have the Canard design where the Canard on the front of the aircraft is set at a higher AOA than the main wing and this causes it to stall before the main wing has reached critical AOA,with the same result that you have described.

 

What I`ve been talking about is that a pusher engine configuration has nothing to do with the stall or IAS of the stall.

 

Cheers.

 

Frank.

 

 

[motzartmerv]

isn't that exactly what he said??:hittinghead:

 

 

[motzartmerv]

Frank..ok mate.. sounded pretty similar to me buddy..I don't appreciate the aspersions simply because i DID understand what he said..

 

cheers

 

 

[farri]

Merv,

 

You are correct on that point and I do apologise sincerely and maybe I should have worded it different,must have been having a bad hair day.

 

Thank you for pointing it out,I stand corrected.

 

Cheers.

 

Frank.

 

 

[Barefootpilot]

Ah the one thing I did forget to mention was part of the entire reason I posted! Funny how that happens.

 

With a power on stall with a pusher or a puller there will be more airflow over the tail surface and therefore it will produce a higher amount of downforce allowing the aircraft to reach a higher AOA and a slower "stall" speed. You probably all worked that out but thought I'd better put it in just incase.

 

Adam.

 

 

[motzartmerv]

Frank, no problems mate, we all have bad hair days (even those without hair).:thumb_up:. My only concern is that students or greenhorns may read our post's and if they get conflicting ideas from instructors it may add ambiguety and confusion to an already complex subject..so i think what we need to do is avoid "overinstructing" on these topics..

 

I had a look at your blog, gee mate how do you cope with living in such a beautifull area, would be wonderfull flying hey??

 

anyway, don't sweat it mate,

 

Adam, are you sure the stalling angle of attack is increased?, or perhaps its a higher nose attitude with the stalling AoA remaining the same?

 

cheers lads

 

 

[farri]

Merv,

 

Couldn`t agree more.

 

Cheers,

 

Frank.

 

 

[Barefootpilot]

Sorry there I go again! :raise_eyebrow:

 

What I mean is the AOA achieved by the aircraft with power on and therefore greater elevator authority is higher compared with no power or prop wash over the elevator. The actual stalling AOA for an aerofoil never changes.

 

Its alot easier if I could draw pictures and show you how I'm moving my hands!!!!

 

Maybe I've got it out this time!!

 

 

[Bruce Robbins]

......................... The actual stalling AOA for an aerofoil never changes.

quote]

 

Hi Boys,

 

Good discussion, even if it is has drifted a little off topic .

 

Being a little pedantic here, but want to pick you up on one point. The stalling angle of attack can, and does change. The change is not always dramatic, but it is usually discernable.

 

We all tell our students that the stalling angle of attack doesn't change because it keeps the picture simple, and reinforces that it is angle, not speed that leads to a stall.

 

However there are several factors that change the stalling angle. Most (like Reynolds Number and planform) are not usually relevant in our case, but one is.

 

* Increasing the camber of an airfoil will decrease the stalling angle.

 

Lowering the flaps increases the camber of our wing, and results in a small decrease in stalling angle. This is one of the reasons why stalls with flap can sometimes be a little more interesting.

 

As Adam pointed out, many aircraft are designed so that they are very difficult to fully stall due to lack of elevator authority keeping the wing just below the stalling angle. That same aircraft with flaps down however, may very well be able to bring the wing to the new lower stalling angle, so giving a definite stall.

 

Cheers,

 

Bruce

[Barefootpilot]

You are right Bruce in both your points this is a very big thread drift and that the AOA can change slightly. It have been a few years since I've actually had to think about it in depth but thanks for picking me up on it! I have no problem in being corrected because I know I still make mistakes!

 

Adam.

 

 

[Yenn]

Really the critical angle of attack doesn't change. What does change is the wing. The position of the flaps changes the wing profile so it is no longer the same wing with flaps as without them. I can't see that the Reynolds number will change much. It is dependent on chord and speed, and the change of chord is not much in proportion to the whole wing with flap use.

 

 

[Bruce Robbins]

Hi Ian,

 

Like I said, I'm being a little pedantic here so don't take me too seriously. In teaching someone to fly, the actual detailed mechanics of a stall are not so important as learning to recognise when you are getting close to a stall, what might happen if you do, and how to avoid being bitten. Unfortunately people are still killing themselves by stalling in from low altitude, and it should be totally avoidable.

 

Anyway, back to pedantry. I agree that Reynolds number, planform, slots, slats etc etc are usually out of our control in the aircraft we typically fly, which is why I said before "they are not usually relevant". The one that is however is camber change, and so flaps (and power settings) can change the stall behaviour significantly.

 

As an example, go for a fly in one of the LSA55 Jabs and try some stalls. No flap simple stall - benign as anything, basically not really stallable. Full flap simple stall - bit more interesting. Full flap simple stall with 2400 rpm - sudden, sharp wing drop, large nose down pitch. Do that one turning final and you won't survive.

 

As I intimated before, the average student probably doesn't really need to know why and how the plane reacts differently, just that it does.

 

Have a look at this diagram which shows Cl plotted against AoA and you can see what I mean by the stalling angle (of the same airfoil section) decreasing with flap.

 

[ATTACH]5935.vB[/ATTACH]

 

Note how the peak Cl (stalling angle) is shifting left and up.

 

Regards,

 

Bruce

 

 

 

[Guest TOSGcentral]

Aye Bruce,

 

A good, solid, simple and very practical approach to the subject.

 

May I add to it a little, and I do not think we are too far off topic as fuel burn equates to drag and flaps 'do' things to you!

 

The point that I want to bring up is negative (or reflex) flap that we are already seeing and will see a lot more of as designers try to keep balance with practical low speed handling/performance and yet retain efficient low fuel burn cruise.

 

While increasing positive flap certainly changes the stalling behaviour (particularly in terms of wing drop and aileron control effectiveness the progressing is clear and there are ample warning signs - negative flap can change the entire stall 'feel' spectrum from the pilot's point of view.

 

In my experience the negative flap stall is totally different from the clean or increasing positive flap stall in terms of warning and feel - the thing simply stops flying quite abruptly and drops out of your hands. There is none of the 'pre stall warning via feel'.

 

This points up the need for increased airmanship disciplines in the cockpit both in the operation of more complex systems aircraft and particularly in the need for good quality flight condition checking procedures.

 

This situation may be irritated by design features in recreational aircraft that have the systems but have not included some basic mechanical precautions or clearly seen visual flap setting indicators.

 

You have to consider that the move from neutral flap to reflex flap may be less than a 1/2" on a mechanical lever system. This may not be discernable by either feel or visually in a busy cockpit when things are getting interesting. The inclusion of a flap lever stop where you actively have to do something else to get from neutral to reflex is wise but may not be included.

 

While concepts such as 'Safe Speed Near the Ground' give protection in most situations they may not adequately protect you in reflex settings.

 

The bottom line is really two part - prior to take off check that you have cycled and set the flaps correctly and prior to the latter part of the circuit make a positive check that you are OUT OF REFLEX and know what flap setting you are actually in.

 

The consequences of reducing fuel burn may have a domino significance in other areas.

 

Aye

 

Tony

 

 

[Bruce Robbins]

Tony,

 

Interesting point, and one I've not thought much about. If what you've experienced is widespread behaviour in reflex flap designs then you've made a number of very valid points to incorporate into operating such aircraft.

 

Do you know if there are any aerodynamic "fixes" like stall strips, boundary layer trips, wing fences, vortex generators etc etc that may introduce some feel back into the stall?

 

Bruce

 

 

[Guest TOSGcentral]

Hi Bruce,

 

I have flown a number of types with reflex flaps but none of them had any sort of gadgets that you mention.

 

Anything like that would be self defeating as the purpose of reflex is to reduce drag and increase cruise airspeed while also reducing fuel burn. They are therefore a top end of the flight envelope thing where you are long way from the stall and in turn stall behaviour is not an issue - or should not be. I would think the devices you refer to would increase drag.

 

The stall behaviour only becomes a potential issue if the in-cockpit disciplines have slipped - eg a tired pilot who anyway is sloppy on pre-landing checks or does not do them at all. and he/she has become used to the slightly flatter flying attitude after an hour or so of cruise flight.

 

Flap management is really no part of this thread though so now I am guilty of taking us off topic.

 

Tony